fix(chromium): harden outbound URL handling

pkg/gotenberg/outbound.go

@@ -68,11 +68,20 @@ func IsPublicIP(addr netip.Addr) bool {
 // The returned slice can be used to pin a subsequent dial to a specific IP
 // and prevent DNS rebinding between this validation and the connect.
 func ResolveAndCheckPublic(ctx context.Context, host string) ([]netip.Addr, error) {
+	return resolveHost(ctx, host, false)
+}
+
+// resolveHost resolves host and returns the addresses. When checkPublic is
+// true, each resolved address must pass [IsPublicIP] or the call returns
+// [ErrNonPublicIP]. When false, non-public addresses are accepted and
+// returned; the caller must pin the dial to them to retain rebind
+// protection even when the public-IP filter is off.
+func resolveHost(ctx context.Context, host string, allowPrivate bool) ([]netip.Addr, error) {
 	if host == "" {
 		return nil, errors.New("empty host")
 	}
 	if addr, err := netip.ParseAddr(host); err == nil {
-		if !IsPublicIP(addr) {
+		if !allowPrivate && !IsPublicIP(addr) {
 			return nil, fmt.Errorf("%q: %w", addr, ErrNonPublicIP)
 		}
 		return []netip.Addr{addr}, nil
@@ -84,34 +93,62 @@ func ResolveAndCheckPublic(ctx context.Context, host string) ([]netip.Addr, erro
 	if len(addrs) == 0 {
 		return nil, fmt.Errorf("resolve %q: no addresses returned", host)
 	}
-	for _, a := range addrs {
+	if !allowPrivate {
-		if !IsPublicIP(a) {
+		for _, a := range addrs {
-			return nil, fmt.Errorf("%q resolves to non-public address %q: %w", host, a, ErrNonPublicIP)
+			if !IsPublicIP(a) {
+				return nil, fmt.Errorf("%q resolves to non-public address %q: %w", host, a, ErrNonPublicIP)
+			}
 		}
 	}
 	return addrs, nil
 }
 
-// outboundDecision is the result of validating an outbound URL. It is
+// OutboundDecision is the result of validating an outbound URL via
-// stashed in the request context by [outboundRoundTripper] so that
+// [DecideOutbound]. Callers use it to dial the destination either directly
-// [secureDialContext] can either bypass the IP check (allow-list match) or
+// (operator-approved allow-list match, Bypass true) or via [DialPinned] so
-// pin the dial to the IPs that were resolved at validation time.
+// that the connect targets the IPs resolved at validation time. Passing
-type outboundDecision struct {
+// the decision to the dialer closes the window between validation and
-	// bypass is true when an allow-list pattern matched the URL. In that
+// connect that DNS rebinding exploits.
-	// case the operator has explicitly opted into the destination and the
+type OutboundDecision struct {
-	// dial should proceed without an IP check.
+	// Bypass is true when an allow-list pattern matched the URL. The
-	bypass bool
+	// operator has explicitly opted into the destination; the caller
+	// should dial directly without an additional IP check.
+	Bypass bool
 
-	// pinned holds the IPs resolved by [ResolveAndCheckPublic] for the URL
+	// Pinned holds the IPs resolved by [ResolveAndCheckPublic] for the URL
-	// host. The dial should be pinned to one of these to prevent DNS
+	// host. The caller should dial one of these via [DialPinned] to
-	// rebinding between validation and connect.
+	// prevent DNS rebinding between validation and connect.
-	pinned []netip.Addr
+	Pinned []netip.Addr
 }
 
-// outboundDecisionKey is the context key under which an [outboundDecision]
+// outboundDecisionKey is the context key under which an [OutboundDecision]
 // is stored.
 type outboundDecisionKey struct{}
 
+// decideConfig carries optional settings for [DecideOutbound] and
+// [FilterOutboundURL]. See [DecideOption] for how callers configure it.
+type decideConfig struct {
+	allowPrivateIPs bool
+}
+
+// DecideOption customizes how [DecideOutbound] and [FilterOutboundURL]
+// validate a URL. Options are applied in order and layered on top of the
+// defaults.
+type DecideOption func(*decideConfig)
+
+// WithAllowPrivateIPs disables the public-IP filter on the resolved host.
+// DNS is still resolved and the returned [OutboundDecision] still carries
+// the pinned IPs, so the caller retains rebind protection. Only the
+// "non-public address" rejection is lifted.
+//
+// Use this for Chromium deployments behind private networks (Docker
+// Compose, Kubernetes with ClusterIP services) where legitimate
+// sub-resources resolve to RFC1918 or loopback addresses. The regex
+// allow-list and deny-list still apply.
+func WithAllowPrivateIPs(allow bool) DecideOption {
+	return func(c *decideConfig) { c.allowPrivateIPs = allow }
+}
+
 // httpLikeScheme reports whether scheme is one of http, https, ws, or wss.
 // Only these schemes go through the IP-based public-address check; data,
 // blob, file, and other schemes are filtered by the regex layer alone.
@@ -123,14 +160,40 @@ func httpLikeScheme(scheme string) bool {
 	return false
 }
 
-// decideOutbound parses rawURL, runs the regex allow/deny lists against the
+// DecideOutbound parses rawURL, runs the regex allow/deny lists against the
 // normalized form, and (when no allow-list match) resolves the host and
 // rejects any non-public address. It returns the resulting
-// [outboundDecision] which the caller can stash in a context for the dial.
+// [OutboundDecision] so the caller can pin the dial to the IPs that were
-func decideOutbound(ctx context.Context, rawURL string, allowList, denyList []*regexp2.Regexp, deadline time.Time) (outboundDecision, error) {
+// resolved here and skip a second DNS lookup later. This closes the DNS
+// rebinding window that affects callers that only receive an error from
+// [FilterOutboundURL].
+//
+// The semantics match [FilterOutboundURL]:
+//
+//  1. The URL is parsed and its scheme and host lowercased.
+//  2. allowList and denyList apply against the normalized form with OR
+//     semantics. The deny-list always applies.
+//  3. For http, https, ws, and wss, the host is resolved and every
+//     resolved address must pass [IsPublicIP]. An allow-list match
+//     bypasses the IP check and the returned decision carries Bypass
+//     true. Otherwise the decision carries Pinned with the resolved
+//     addresses.
+//
+// Callers that dial the destination themselves must honor Bypass and
+// Pinned: bypassed URLs dial the hostname directly (operator opt-in);
+// pinned URLs must dial one of Pinned via [DialPinned].
+//
+// Options customize behavior. [WithAllowPrivateIPs] for example disables
+// the non-public-address rejection while keeping DNS pinning.
+func DecideOutbound(ctx context.Context, rawURL string, allowList, denyList []*regexp2.Regexp, deadline time.Time, opts ...DecideOption) (OutboundDecision, error) {
+	cfg := decideConfig{}
+	for _, opt := range opts {
+		opt(&cfg)
+	}
+
 	parsed, err := url.Parse(rawURL)
 	if err != nil {
-		return outboundDecision{}, fmt.Errorf("parse URL %q: %w", rawURL, ErrFiltered)
+		return OutboundDecision{}, fmt.Errorf("parse URL %q: %w", rawURL, ErrFiltered)
 	}
 	parsed.Scheme = strings.ToLower(parsed.Scheme)
 	parsed.Host = strings.ToLower(parsed.Host)
@@ -145,9 +208,9 @@ func decideOutbound(ctx context.Context, rawURL string, allowList, denyList []*r
 			ok, err := clone.MatchString(normalized)
 			if err != nil {
 				if time.Now().After(deadline) {
-					return outboundDecision{}, context.DeadlineExceeded
+					return OutboundDecision{}, context.DeadlineExceeded
 				}
-				return outboundDecision{}, fmt.Errorf("'%s' cannot handle '%s': %w", clone.String(), normalized, err)
+				return OutboundDecision{}, fmt.Errorf("'%s' cannot handle '%s': %w", clone.String(), normalized, err)
 			}
 
 			if ok {
@@ -157,7 +220,7 @@ func decideOutbound(ctx context.Context, rawURL string, allowList, denyList []*r
 		}
 
 		if !allowMatched {
-			return outboundDecision{}, fmt.Errorf("'%s' does not match any expression from the allowed list: %w", normalized, ErrFiltered)
+			return OutboundDecision{}, fmt.Errorf("'%s' does not match any expression from the allowed list: %w", normalized, ErrFiltered)
 		}
 	}
 
@@ -168,38 +231,38 @@ func decideOutbound(ctx context.Context, rawURL string, allowList, denyList []*r
 		ok, err := clone.MatchString(normalized)
 		if err != nil {
 			if time.Now().After(deadline) {
-				return outboundDecision{}, context.DeadlineExceeded
+				return OutboundDecision{}, context.DeadlineExceeded
 			}
-			return outboundDecision{}, fmt.Errorf("'%s' cannot handle '%s': %w", clone.String(), normalized, err)
+			return OutboundDecision{}, fmt.Errorf("'%s' cannot handle '%s': %w", clone.String(), normalized, err)
 		}
 
 		if ok {
-			return outboundDecision{}, fmt.Errorf("'%s' matches the expression from the denied list: %w", normalized, ErrFiltered)
+			return OutboundDecision{}, fmt.Errorf("'%s' matches the expression from the denied list: %w", normalized, ErrFiltered)
 		}
 	}
 
 	if allowMatched {
-		return outboundDecision{bypass: true}, nil
+		return OutboundDecision{Bypass: true}, nil
 	}
 
 	if !httpLikeScheme(parsed.Scheme) {
-		return outboundDecision{}, nil
+		return OutboundDecision{}, nil
 	}
 
 	host := parsed.Hostname()
 	if host == "" {
-		return outboundDecision{}, fmt.Errorf("URL %q has no host: %w", rawURL, ErrFiltered)
+		return OutboundDecision{}, fmt.Errorf("URL %q has no host: %w", rawURL, ErrFiltered)
 	}
 
-	addrs, err := ResolveAndCheckPublic(ctx, host)
+	addrs, err := resolveHost(ctx, host, cfg.allowPrivateIPs)
 	if err != nil {
 		if errors.Is(err, ErrNonPublicIP) {
-			return outboundDecision{}, fmt.Errorf("'%s' targets a non-public address: %w", normalized, ErrFiltered)
+			return OutboundDecision{}, fmt.Errorf("'%s' targets a non-public address: %w", normalized, ErrFiltered)
 		}
-		return outboundDecision{}, fmt.Errorf("validate '%s' host: %w", normalized, err)
+		return OutboundDecision{}, fmt.Errorf("validate '%s' host: %w", normalized, err)
 	}
 
-	return outboundDecision{pinned: addrs}, nil
+	return OutboundDecision{Pinned: addrs}, nil
 }
 
 // FilterOutboundURL validates that rawURL is acceptable for an outbound
@@ -224,13 +287,13 @@ func decideOutbound(ctx context.Context, rawURL string, allowList, denyList []*r
 // An allow-list match bypasses the IP check, allowing operators to opt
 // into specific internal destinations via --*-allow-list flags. The
 // deny-list always applies and cannot be bypassed by an allow-list match.
-func FilterOutboundURL(ctx context.Context, rawURL string, allowList, denyList []*regexp2.Regexp, deadline time.Time) error {
+func FilterOutboundURL(ctx context.Context, rawURL string, allowList, denyList []*regexp2.Regexp, deadline time.Time, opts ...DecideOption) error {
-	_, err := decideOutbound(ctx, rawURL, allowList, denyList, deadline)
+	_, err := DecideOutbound(ctx, rawURL, allowList, denyList, deadline, opts...)
 	return err
 }
 
 // outboundRoundTripper is an [http.RoundTripper] that validates each request
-// URL via [decideOutbound] and stashes the resulting [outboundDecision] in
+// URL via [DecideOutbound] and stashes the resulting [OutboundDecision] in
 // the request context so that [secureDialContext] can pin the dial or
 // bypass the IP check as appropriate. Because the http.Client invokes
 // RoundTrip again for each redirect hop, this also re-validates redirect
@@ -248,7 +311,7 @@ func (rt *outboundRoundTripper) RoundTrip(req *http.Request) (*http.Response, er
 		deadline = time.Now().Add(30 * time.Second)
 	}
 
-	decision, err := decideOutbound(req.Context(), req.URL.String(), rt.allowList, rt.denyList, deadline)
+	decision, err := DecideOutbound(req.Context(), req.URL.String(), rt.allowList, rt.denyList, deadline)
 	if err != nil {
 		return nil, err
 	}
@@ -279,7 +342,7 @@ func NewOutboundHttpClient(timeout time.Duration, allowList, denyList []*regexp2
 	}
 }
 
-// secureDialContext consumes the [outboundDecision] stashed in ctx by
+// secureDialContext consumes the [OutboundDecision] stashed in ctx by
 // [outboundRoundTripper]. When the decision is to bypass (allow-list
 // match), it dials directly. When the decision contains pinned IPs, it
 // dials each in turn until one connects. When no decision is present (the
@@ -291,12 +354,12 @@ func secureDialContext(ctx context.Context, network, addr string) (net.Conn, err
 		return nil, fmt.Errorf("split host:port %q: %w", addr, err)
 	}
 
-	if decision, ok := ctx.Value(outboundDecisionKey{}).(outboundDecision); ok {
+	if decision, ok := ctx.Value(outboundDecisionKey{}).(OutboundDecision); ok {
-		if decision.bypass {
+		if decision.Bypass {
 			return outboundDialer.DialContext(ctx, network, addr)
 		}
-		if len(decision.pinned) > 0 {
+		if len(decision.Pinned) > 0 {
-			return dialPinned(ctx, network, decision.pinned, port)
+			return DialPinned(ctx, network, decision.Pinned, port)
 		}
 	}
 
@@ -304,12 +367,15 @@ func secureDialContext(ctx context.Context, network, addr string) (net.Conn, err
 	if err != nil {
 		return nil, err
 	}
-	return dialPinned(ctx, network, addrs, port)
+	return DialPinned(ctx, network, addrs, port)
 }
 
-// dialPinned dials each addr in turn until one connects, returning the
+// DialPinned dials each addr in turn until one connects, returning the
-// first successful connection or the last error.
+// first successful connection or the last error. Callers pass the Pinned
-func dialPinned(ctx context.Context, network string, addrs []netip.Addr, port string) (net.Conn, error) {
+// slice from [OutboundDecision] so that the dial targets exactly the IPs
+// that [DecideOutbound] resolved and validated, preventing DNS rebinding
+// between validation and connect.
+func DialPinned(ctx context.Context, network string, addrs []netip.Addr, port string) (net.Conn, error) {
 	var lastErr error
 	for _, a := range addrs {
 		conn, err := outboundDialer.DialContext(ctx, network, net.JoinHostPort(a.String(), port))

pkg/gotenberg/outbound_test.go

@@ -306,3 +306,27 @@ func TestResolveAndCheckPublic_HostResolvesToPublic(t *testing.T) {
 		t.Fatalf("expected [1.1.1.1], got: %v", addrs)
 	}
 }
+
+func TestDecideOutbound_AllowPrivateIPs_DenyListStillApplies(t *testing.T) {
+	withStubResolver(t, func(host string) ([]netip.Addr, error) {
+		t.Fatalf("unexpected DNS lookup for %q", host)
+		return nil, nil
+	})
+
+	// Denial must still win over WithAllowPrivateIPs(true). Gherkin
+	// coverage exercises flag on/off against the IP check but not the
+	// interaction with the regex deny-list; keep this primitive test for
+	// that specific combination.
+	deny := []*regexp2.Regexp{regexp2.MustCompile(`^http://evil\.`, 0)}
+
+	_, err := DecideOutbound(
+		context.Background(),
+		"http://evil.local/",
+		nil, deny,
+		time.Now().Add(5*time.Second),
+		WithAllowPrivateIPs(true),
+	)
+	if !errors.Is(err, ErrFiltered) {
+		t.Fatalf("deny-list must still win with WithAllowPrivateIPs(true), got: %v", err)
+	}
+}

pkg/modules/chromium/browser.go

@@ -44,6 +44,7 @@ type browserArguments struct {
 	// Tasks specific.
 	allowList         []*regexp2.Regexp
 	denyList          []*regexp2.Regexp
+	allowPrivateIPs   bool
 	clearCache        bool
 	clearCookies      bool
 	disableJavaScript bool
@@ -57,15 +58,17 @@ type chromiumBrowser struct {
 	ctxMu              sync.RWMutex
 	isStarted          atomic.Bool
 
-	arguments browserArguments
+	arguments    browserArguments
-	fs        *gotenberg.FileSystem
+	fs           *gotenberg.FileSystem
+	pinningProxy *pinningProxy
 }
 
 func newChromiumBrowser(arguments browserArguments) browser {
 	b := &chromiumBrowser{
-		initialCtx: context.Background(),
+		initialCtx:   context.Background(),
-		arguments:  arguments,
+		arguments:    arguments,
-		fs:         gotenberg.NewFileSystem(new(gotenberg.OsMkdirAll)),
+		fs:           gotenberg.NewFileSystem(new(gotenberg.OsMkdirAll)),
+		pinningProxy: newPinningProxy(arguments.allowList, arguments.denyList, arguments.allowPrivateIPs),
 	}
 	b.isStarted.Store(false)
 
@@ -136,6 +139,25 @@ func (b *chromiumBrowser) Start(logger *slog.Logger) error {
 		opts = append(opts, chromedp.ProxyServer(b.arguments.proxyServer))
 	}
 
+	// Default: route Chromium through the internal pinning proxy so that
+	// Chromium never performs its own DNS lookup for the navigation URL
+	// or any sub-resource. The proxy resolves and validates each URL
+	// once per request and dials the pinned IP, closing the DNS
+	// rebinding window between Gotenberg's validation and Chromium's
+	// connect.
+	//
+	// Skip when the operator has configured their own egress proxy or
+	// custom host-resolver mappings: those deployments take
+	// responsibility for outbound safety themselves and routing through
+	// an internal proxy would override their configuration.
+	if b.arguments.proxyServer == "" && b.arguments.hostResolverRules == "" {
+		err = b.pinningProxy.Start(logger)
+		if err != nil {
+			return fmt.Errorf("start pinning proxy: %w", err)
+		}
+		opts = append(opts, chromedp.ProxyServer(b.pinningProxy.URL()))
+	}
+
 	// See https://github.com/gotenberg/gotenberg/issues/524.
 	opts = append(opts, chromedp.WSURLReadTimeout(b.arguments.wsUrlReadTimeout))
 
@@ -236,6 +258,15 @@ func (b *chromiumBrowser) Stop(logger *slog.Logger) error {
 	b.userProfileDirPath = ""
 	b.isStarted.Store(false)
 
+	// Stop the pinning proxy after Chromium shutdown so that any
+	// in-flight requests Chromium issues during teardown complete. The
+	// Stop call is a no-op when the proxy was not started (operator
+	// configured --chromium-proxy-server or --chromium-host-resolver-rules).
+	err := b.pinningProxy.Stop(logger)
+	if err != nil {
+		logger.ErrorContext(context.Background(), fmt.Sprintf("stop pinning proxy: %s", err))
+	}
+
 	return nil
 }
 
@@ -338,7 +369,7 @@ func (b *chromiumBrowser) do(ctx context.Context, logger *slog.Logger, url strin
 
 	// We validate the "main" URL against our allowed / deny lists, and
 	// against the IP-based outbound URL guard. See [gotenberg.FilterOutboundURL].
-	err := gotenberg.FilterOutboundURL(ctx, url, b.arguments.allowList, b.arguments.denyList, deadline)
+	err := gotenberg.FilterOutboundURL(ctx, url, b.arguments.allowList, b.arguments.denyList, deadline, gotenberg.WithAllowPrivateIPs(b.arguments.allowPrivateIPs))
 	if err != nil {
 		return fmt.Errorf("filter URL: %w", err)
 	}
@@ -359,6 +390,7 @@ func (b *chromiumBrowser) do(ctx context.Context, logger *slog.Logger, url strin
 	listenForEventRequestPaused(taskCtx, logger, eventRequestPausedOptions{
 		allowList:           b.arguments.allowList,
 		denyList:            b.arguments.denyList,
+		allowPrivateIPs:     b.arguments.allowPrivateIPs,
 		allowedFilePrefixes: options.AllowedFilePrefixes,
 		extraHttpHeaders:    options.ExtraHttpHeaders,
 	})

pkg/modules/chromium/chromium.go

@@ -447,6 +447,7 @@ func (mod *Chromium) Descriptor() gotenberg.ModuleDescriptor {
 			fs.String("chromium-proxy-server", "", "Set the outbound proxy server; this switch only affects HTTP and HTTPS requests")
 			fs.StringSlice("chromium-allow-list", []string{}, "Set the allowed URLs for Chromium using regular expressions - supports multiple values")
 			fs.StringSlice("chromium-deny-list", []string{`^file:(?!//\/tmp/).*`}, "Set the denied URLs for Chromium using regular expressions - supports multiple values")
+			fs.Bool("chromium-allow-private-ips", false, "Accept sub-resources that resolve to private, loopback, or link-local addresses. Intended for operators running Gotenberg inside a private network (Docker Compose, Kubernetes ClusterIP); the regex allow-list and deny-list still apply")
 			fs.Bool("chromium-clear-cache", false, "Clear Chromium cache between each conversion")
 			fs.Bool("chromium-clear-cookies", false, "Clear Chromium cookies between each conversion")
 			fs.Bool("chromium-disable-javascript", false, "Disable JavaScript")
@@ -495,6 +496,7 @@ func (mod *Chromium) Provision(ctx *gotenberg.Context) error {
 
 		allowList:         flags.MustRegexpSlice("chromium-allow-list"),
 		denyList:          flags.MustRegexpSlice("chromium-deny-list"),
+		allowPrivateIPs:   flags.MustBool("chromium-allow-private-ips"),
 		clearCache:        flags.MustBool("chromium-clear-cache"),
 		clearCookies:      flags.MustBool("chromium-clear-cookies"),
 		disableJavaScript: flags.MustBool("chromium-disable-javascript"),

pkg/modules/chromium/events.go

@@ -25,6 +25,7 @@ import (
 
 type eventRequestPausedOptions struct {
 	allowList, denyList []*regexp2.Regexp
+	allowPrivateIPs     bool
 	allowedFilePrefixes []string
 	extraHttpHeaders    []ExtraHttpHeader
 }
@@ -52,7 +53,7 @@ func listenForEventRequestPaused(ctx context.Context, logger *slog.Logger, optio
 					return
 				}
 
-				err := gotenberg.FilterOutboundURL(ctx, e.Request.URL, options.allowList, options.denyList, deadline)
+				err := gotenberg.FilterOutboundURL(ctx, e.Request.URL, options.allowList, options.denyList, deadline, gotenberg.WithAllowPrivateIPs(options.allowPrivateIPs))
 				if err != nil {
 					logger.WarnContext(ctx, err.Error())
 					allow = false

pkg/modules/chromium/pinning_proxy.go

@@ -0,0 +1,324 @@
+package chromium
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"io"
+	"log/slog"
+	"net"
+	"net/http"
+	"net/netip"
+	"sync"
+	"time"
+
+	"github.com/dlclark/regexp2"
+
+	"github.com/gotenberg/gotenberg/v8/pkg/gotenberg"
+)
+
+// pinningProxy is a loopback-bound HTTP/1.1 forward and CONNECT proxy
+// placed between Chromium and the outbound network. It runs the same
+// allow/deny/IP-public validation as [gotenberg.FilterOutboundURL] on
+// every request and dials the destination using the IPs resolved at that
+// moment. Routing Chromium through this proxy eliminates the Chromium-side
+// DNS lookup that otherwise opens a DNS rebinding window between
+// Gotenberg's validation and Chromium's TCP connect.
+//
+// The proxy is transparent to the caller. HTTPS sub-resources tunnel
+// through CONNECT with Chromium performing its own TLS handshake using
+// the original hostname, preserving SNI and certificate validation.
+type pinningProxy struct {
+	allowList []*regexp2.Regexp
+	denyList  []*regexp2.Regexp
+
+	// decide resolves and validates a URL. Tests may override it.
+	decide func(ctx context.Context, rawURL string, allowList, denyList []*regexp2.Regexp, deadline time.Time) (gotenberg.OutboundDecision, error)
+
+	// dialPinned dials the pinned IPs for a decision. Tests may override
+	// it to connect to a stub upstream regardless of decision.
+	dialPinned func(ctx context.Context, network string, addrs []netip.Addr, port string) (net.Conn, error)
+
+	// dialBypass dials the destination hostname directly (operator
+	// allow-list opt-in). Tests may override it.
+	dialBypass func(ctx context.Context, network, addr string) (net.Conn, error)
+
+	listener net.Listener
+	server   *http.Server
+	wg       sync.WaitGroup
+
+	logger  *slog.Logger
+	started bool
+	mu      sync.Mutex
+}
+
+// newPinningProxy returns a pinning proxy configured with the given
+// allow/deny lists. When allowPrivateIPs is true, the proxy skips the
+// public-IP filter while still pinning resolved IPs to the dial. The
+// returned proxy is not yet listening; call Start.
+func newPinningProxy(allowList, denyList []*regexp2.Regexp, allowPrivateIPs bool) *pinningProxy {
+	return &pinningProxy{
+		allowList: allowList,
+		denyList:  denyList,
+		decide: func(ctx context.Context, rawURL string, allow, deny []*regexp2.Regexp, deadline time.Time) (gotenberg.OutboundDecision, error) {
+			return gotenberg.DecideOutbound(ctx, rawURL, allow, deny, deadline, gotenberg.WithAllowPrivateIPs(allowPrivateIPs))
+		},
+		dialPinned: gotenberg.DialPinned,
+		dialBypass: func(ctx context.Context, network, addr string) (net.Conn, error) {
+			dialer := &net.Dialer{Timeout: 10 * time.Second}
+			return dialer.DialContext(ctx, network, addr)
+		},
+	}
+}
+
+// Start binds the proxy to 127.0.0.1 on an ephemeral port and serves in a
+// background goroutine. Bind failures return an error; the caller must
+// not proceed to start Chromium with --proxy-server.
+func (p *pinningProxy) Start(logger *slog.Logger) error {
+	p.mu.Lock()
+	defer p.mu.Unlock()
+
+	if p.started {
+		return errors.New("pinning proxy already started")
+	}
+
+	l, err := net.Listen("tcp", "127.0.0.1:0")
+	if err != nil {
+		return fmt.Errorf("bind pinning proxy: %w", err)
+	}
+
+	p.listener = l
+	p.logger = logger.With(slog.String("logger", "pinning-proxy"))
+	p.server = &http.Server{
+		Handler: http.HandlerFunc(p.serveHTTP),
+		// Guard against slow header attacks. Body reads are controlled
+		// per-handler.
+		ReadHeaderTimeout: 15 * time.Second,
+		ErrorLog:          slog.NewLogLogger(p.logger.Handler(), slog.LevelWarn),
+	}
+
+	p.wg.Go(func() {
+		serveErr := p.server.Serve(l)
+		if serveErr != nil && !errors.Is(serveErr, http.ErrServerClosed) {
+			p.logger.ErrorContext(context.Background(), fmt.Sprintf("pinning proxy serve: %s", serveErr))
+		}
+	})
+
+	p.started = true
+	p.logger.DebugContext(context.Background(), fmt.Sprintf("pinning proxy listening on %s", l.Addr()))
+	return nil
+}
+
+// Stop shuts the proxy down and waits for in-flight handlers to complete.
+// Safe to call on a non-started proxy.
+func (p *pinningProxy) Stop(logger *slog.Logger) error {
+	p.mu.Lock()
+	if !p.started {
+		p.mu.Unlock()
+		return nil
+	}
+	srv := p.server
+	p.started = false
+	p.mu.Unlock()
+
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer cancel()
+
+	shutdownErr := srv.Shutdown(ctx)
+	p.wg.Wait()
+
+	if shutdownErr != nil {
+		return fmt.Errorf("shutdown pinning proxy: %w", shutdownErr)
+	}
+	logger.DebugContext(context.Background(), "pinning proxy stopped")
+	return nil
+}
+
+// URL returns the proxy URL suitable for Chromium's --proxy-server flag.
+// Returns an empty string when the proxy is not listening.
+func (p *pinningProxy) URL() string {
+	p.mu.Lock()
+	defer p.mu.Unlock()
+	if p.listener == nil {
+		return ""
+	}
+	return "http://" + p.listener.Addr().String()
+}
+
+func (p *pinningProxy) serveHTTP(w http.ResponseWriter, req *http.Request) {
+	if req.Method == http.MethodConnect {
+		p.handleConnect(w, req)
+		return
+	}
+	p.handleForward(w, req)
+}
+
+// handleConnect handles HTTPS (and any other CONNECT) tunnels. Chromium
+// issues CONNECT host:port; the proxy validates the host, dials the
+// pinned IP, and splices the client socket with the upstream socket.
+// Chromium then negotiates TLS end-to-end with the original hostname in
+// SNI.
+func (p *pinningProxy) handleConnect(w http.ResponseWriter, req *http.Request) {
+	_, port, err := net.SplitHostPort(req.Host)
+	if err != nil {
+		http.Error(w, "bad CONNECT target", http.StatusBadRequest)
+		return
+	}
+
+	deadline, ok := req.Context().Deadline()
+	if !ok {
+		deadline = time.Now().Add(30 * time.Second)
+	}
+
+	// The validation URL uses https:// so that http-like scheme checks
+	// apply in [gotenberg.DecideOutbound]. The scheme does not influence
+	// the CONNECT handling beyond filtering.
+	decision, err := p.decide(req.Context(), "https://"+req.Host, p.allowList, p.denyList, deadline)
+	if err != nil {
+		p.logger.WarnContext(req.Context(), fmt.Sprintf("CONNECT blocked for '%s': %s", req.Host, err))
+		http.Error(w, "CONNECT blocked", http.StatusForbidden)
+		return
+	}
+
+	var upstream net.Conn
+	switch {
+	case decision.Bypass:
+		upstream, err = p.dialBypass(req.Context(), "tcp", req.Host)
+	case len(decision.Pinned) > 0:
+		upstream, err = p.dialPinned(req.Context(), "tcp", decision.Pinned, port)
+	default:
+		err = errors.New("no pinned addresses and not bypassed")
+	}
+	if err != nil {
+		p.logger.WarnContext(req.Context(), fmt.Sprintf("CONNECT dial failed for '%s': %s", req.Host, err))
+		http.Error(w, "upstream dial failed", http.StatusBadGateway)
+		return
+	}
+	defer upstream.Close()
+
+	hj, ok := w.(http.Hijacker)
+	if !ok {
+		http.Error(w, "hijack unsupported", http.StatusInternalServerError)
+		return
+	}
+	client, _, err := hj.Hijack()
+	if err != nil {
+		p.logger.ErrorContext(req.Context(), fmt.Sprintf("hijack CONNECT: %s", err))
+		return
+	}
+	defer client.Close()
+
+	_, err = client.Write([]byte("HTTP/1.1 200 OK\r\n\r\n"))
+	if err != nil {
+		p.logger.WarnContext(req.Context(), fmt.Sprintf("write CONNECT ack: %s", err))
+		return
+	}
+
+	// Splice bytes in both directions until either side closes.
+	var splice sync.WaitGroup
+	splice.Add(2)
+	go func() {
+		defer splice.Done()
+		_, _ = io.Copy(upstream, client)
+		if cw, ok := upstream.(interface{ CloseWrite() error }); ok {
+			_ = cw.CloseWrite()
+		}
+	}()
+	go func() {
+		defer splice.Done()
+		_, _ = io.Copy(client, upstream)
+		if cw, ok := client.(interface{ CloseWrite() error }); ok {
+			_ = cw.CloseWrite()
+		}
+	}()
+	splice.Wait()
+}
+
+// handleForward handles plain HTTP requests sent to the proxy as absolute
+// URIs (GET http://host/path). The proxy revalidates the URL, then
+// forwards the request via a transport that dials the pinned IP.
+func (p *pinningProxy) handleForward(w http.ResponseWriter, req *http.Request) {
+	if req.URL == nil || req.URL.Scheme == "" || req.URL.Host == "" {
+		http.Error(w, "absolute URL required", http.StatusBadRequest)
+		return
+	}
+
+	deadline, ok := req.Context().Deadline()
+	if !ok {
+		deadline = time.Now().Add(30 * time.Second)
+	}
+
+	decision, err := p.decide(req.Context(), req.URL.String(), p.allowList, p.denyList, deadline)
+	if err != nil {
+		p.logger.WarnContext(req.Context(), fmt.Sprintf("forward blocked for '%s': %s", req.URL, err))
+		http.Error(w, "request blocked", http.StatusForbidden)
+		return
+	}
+
+	outReq := req.Clone(req.Context())
+	outReq.RequestURI = ""
+	stripHopByHopHeaders(outReq.Header)
+
+	transport := &http.Transport{
+		// Build a fresh transport per request. The decision contains the
+		// pinned IPs to dial; reusing a transport across requests would
+		// leak the decision's closure across unrelated targets.
+		DisableKeepAlives: true,
+		Proxy:             nil,
+		DialContext: func(ctx context.Context, network, addr string) (net.Conn, error) {
+			_, port, splitErr := net.SplitHostPort(addr)
+			if splitErr != nil {
+				return nil, fmt.Errorf("split forward addr %q: %w", addr, splitErr)
+			}
+			switch {
+			case decision.Bypass:
+				return p.dialBypass(ctx, network, addr)
+			case len(decision.Pinned) > 0:
+				return p.dialPinned(ctx, network, decision.Pinned, port)
+			default:
+				return nil, errors.New("no pinned addresses and not bypassed")
+			}
+		},
+	}
+	defer transport.CloseIdleConnections()
+
+	resp, err := transport.RoundTrip(outReq)
+	if err != nil {
+		p.logger.WarnContext(req.Context(), fmt.Sprintf("forward RoundTrip failed for '%s': %s", req.URL, err))
+		http.Error(w, "upstream error", http.StatusBadGateway)
+		return
+	}
+	defer resp.Body.Close()
+
+	copyHeaders(w.Header(), resp.Header)
+	stripHopByHopHeaders(w.Header())
+	w.WriteHeader(resp.StatusCode)
+	_, _ = io.Copy(w, resp.Body)
+}
+
+// Per RFC 7230 section 6.1.
+var hopByHopHeaders = []string{
+	"Connection",
+	"Keep-Alive",
+	"Proxy-Authenticate",
+	"Proxy-Authorization",
+	"Proxy-Connection",
+	"Te",
+	"Trailer",
+	"Transfer-Encoding",
+	"Upgrade",
+}
+
+func stripHopByHopHeaders(h http.Header) {
+	for _, name := range hopByHopHeaders {
+		h.Del(name)
+	}
+}
+
+func copyHeaders(dst, src http.Header) {
+	for k, vs := range src {
+		for _, v := range vs {
+			dst.Add(k, v)
+		}
+	}
+}

pkg/modules/chromium/pinning_proxy_test.go

@@ -0,0 +1,485 @@
+package chromium
+
+import (
+	"bufio"
+	"context"
+	"errors"
+	"fmt"
+	"io"
+	"log/slog"
+	"net"
+	"net/http"
+	"net/http/httptest"
+	"net/netip"
+	"net/url"
+	"strings"
+	"sync/atomic"
+	"testing"
+	"time"
+
+	"github.com/dlclark/regexp2"
+
+	"github.com/gotenberg/gotenberg/v8/pkg/gotenberg"
+)
+
+func testLogger() *slog.Logger {
+	return slog.New(slog.NewTextHandler(io.Discard, nil))
+}
+
+func mustParseURL(t *testing.T, raw string) *url.URL {
+	t.Helper()
+	u, err := url.Parse(raw)
+	if err != nil {
+		t.Fatalf("parse %q: %v", raw, err)
+	}
+	return u
+}
+
+// newRawTCPServer starts a TCP server on 127.0.0.1:0 that calls handle for
+// every accepted connection. It returns the listener address and a cleanup
+// function.
+func newRawTCPServer(t *testing.T, handle func(net.Conn)) (string, func()) {
+	t.Helper()
+	l, err := net.Listen("tcp", "127.0.0.1:0")
+	if err != nil {
+		t.Fatalf("listen: %v", err)
+	}
+
+	go func() {
+		for {
+			conn, err := l.Accept()
+			if err != nil {
+				return
+			}
+			go handle(conn)
+		}
+	}()
+
+	return l.Addr().String(), func() { _ = l.Close() }
+}
+
+// newProxyForTest returns a pinning proxy whose decide and dial functions
+// are set to test stubs. The proxy is started on a loopback ephemeral
+// port and stopped during test cleanup.
+func newProxyForTest(t *testing.T, p *pinningProxy) string {
+	t.Helper()
+	err := p.Start(testLogger())
+	if err != nil {
+		t.Fatalf("start pinning proxy: %v", err)
+	}
+	t.Cleanup(func() {
+		_ = p.Stop(testLogger())
+	})
+	return p.URL()
+}
+
+func TestPinningProxy_Forward_Pinned_Success(t *testing.T) {
+	upstream := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if r.Host != "example.com" {
+			t.Errorf("upstream expected Host=example.com, got %q", r.Host)
+		}
+		_, _ = fmt.Fprint(w, "hello-from-upstream")
+	}))
+	t.Cleanup(upstream.Close)
+	upstreamURL := mustParseURL(t, upstream.URL)
+
+	var decideCalls atomic.Int32
+	p := newPinningProxy(nil, nil, false)
+	p.decide = func(_ context.Context, _ string, _, _ []*regexp2.Regexp, _ time.Time) (gotenberg.OutboundDecision, error) {
+		decideCalls.Add(1)
+		return gotenberg.OutboundDecision{Pinned: []netip.Addr{netip.MustParseAddr("127.0.0.1")}}, nil
+	}
+	p.dialPinned = func(ctx context.Context, network string, _ []netip.Addr, _ string) (net.Conn, error) {
+		return net.Dial(network, upstreamURL.Host)
+	}
+	proxyURL := newProxyForTest(t, p)
+
+	client := &http.Client{
+		Transport: &http.Transport{
+			Proxy: http.ProxyURL(mustParseURL(t, proxyURL)),
+		},
+		Timeout: 5 * time.Second,
+	}
+
+	resp, err := client.Get("http://example.com/")
+	if err != nil {
+		t.Fatalf("GET via proxy: %v", err)
+	}
+	defer resp.Body.Close()
+
+	body, err := io.ReadAll(resp.Body)
+	if err != nil {
+		t.Fatalf("read body: %v", err)
+	}
+	if resp.StatusCode != http.StatusOK {
+		t.Fatalf("status = %d, want 200", resp.StatusCode)
+	}
+	if string(body) != "hello-from-upstream" {
+		t.Fatalf("body = %q, want %q", body, "hello-from-upstream")
+	}
+	if got := decideCalls.Load(); got != 1 {
+		t.Fatalf("decide called %d times, want 1", got)
+	}
+}
+
+func TestPinningProxy_Forward_BlockedByDecide(t *testing.T) {
+	p := newPinningProxy(nil, nil, false)
+	p.decide = func(_ context.Context, _ string, _, _ []*regexp2.Regexp, _ time.Time) (gotenberg.OutboundDecision, error) {
+		return gotenberg.OutboundDecision{}, fmt.Errorf("nope: %w", gotenberg.ErrFiltered)
+	}
+	p.dialPinned = func(_ context.Context, _ string, _ []netip.Addr, _ string) (net.Conn, error) {
+		t.Fatal("dialPinned must not be called when decide returns an error")
+		return nil, errors.New("unreachable")
+	}
+	proxyURL := newProxyForTest(t, p)
+
+	client := &http.Client{
+		Transport: &http.Transport{
+			Proxy: http.ProxyURL(mustParseURL(t, proxyURL)),
+		},
+		Timeout: 5 * time.Second,
+	}
+
+	resp, err := client.Get("http://blocked.example/")
+	if err != nil {
+		t.Fatalf("GET via proxy: %v", err)
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode != http.StatusForbidden {
+		t.Fatalf("status = %d, want 403", resp.StatusCode)
+	}
+}
+
+func TestPinningProxy_Forward_Bypass(t *testing.T) {
+	upstream := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		_, _ = fmt.Fprint(w, "bypassed")
+	}))
+	t.Cleanup(upstream.Close)
+	upstreamURL := mustParseURL(t, upstream.URL)
+
+	var bypassCalls atomic.Int32
+	p := newPinningProxy(nil, nil, false)
+	p.decide = func(_ context.Context, _ string, _, _ []*regexp2.Regexp, _ time.Time) (gotenberg.OutboundDecision, error) {
+		return gotenberg.OutboundDecision{Bypass: true}, nil
+	}
+	p.dialBypass = func(_ context.Context, network, _ string) (net.Conn, error) {
+		bypassCalls.Add(1)
+		return net.Dial(network, upstreamURL.Host)
+	}
+	p.dialPinned = func(_ context.Context, _ string, _ []netip.Addr, _ string) (net.Conn, error) {
+		t.Fatal("dialPinned must not be called on bypass")
+		return nil, errors.New("unreachable")
+	}
+	proxyURL := newProxyForTest(t, p)
+
+	client := &http.Client{
+		Transport: &http.Transport{
+			Proxy: http.ProxyURL(mustParseURL(t, proxyURL)),
+		},
+		Timeout: 5 * time.Second,
+	}
+
+	resp, err := client.Get("http://internal.example/")
+	if err != nil {
+		t.Fatalf("GET via proxy: %v", err)
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode != http.StatusOK {
+		t.Fatalf("status = %d, want 200", resp.StatusCode)
+	}
+	if got := bypassCalls.Load(); got != 1 {
+		t.Fatalf("dialBypass called %d times, want 1", got)
+	}
+}
+
+func TestPinningProxy_Forward_StripsHopByHopHeaders(t *testing.T) {
+	var upstreamSawProxyAuth bool
+	upstream := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if r.Header.Get("Proxy-Authorization") != "" {
+			upstreamSawProxyAuth = true
+		}
+		w.Header().Set("Connection", "close")
+		w.Header().Set("Proxy-Connection", "close")
+		w.Header().Set("X-Downstream", "ok")
+		w.WriteHeader(http.StatusOK)
+	}))
+	t.Cleanup(upstream.Close)
+	upstreamURL := mustParseURL(t, upstream.URL)
+
+	p := newPinningProxy(nil, nil, false)
+	p.decide = func(_ context.Context, _ string, _, _ []*regexp2.Regexp, _ time.Time) (gotenberg.OutboundDecision, error) {
+		return gotenberg.OutboundDecision{Pinned: []netip.Addr{netip.MustParseAddr("127.0.0.1")}}, nil
+	}
+	p.dialPinned = func(ctx context.Context, network string, _ []netip.Addr, _ string) (net.Conn, error) {
+		return net.Dial(network, upstreamURL.Host)
+	}
+	proxyURL := newProxyForTest(t, p)
+
+	req, err := http.NewRequest(http.MethodGet, "http://example.com/", nil)
+	if err != nil {
+		t.Fatalf("new request: %v", err)
+	}
+	req.Header.Set("Proxy-Authorization", "Basic Zm9vOmJhcg==")
+
+	client := &http.Client{
+		Transport: &http.Transport{
+			Proxy: http.ProxyURL(mustParseURL(t, proxyURL)),
+		},
+		Timeout: 5 * time.Second,
+	}
+
+	resp, err := client.Do(req)
+	if err != nil {
+		t.Fatalf("GET via proxy: %v", err)
+	}
+	defer resp.Body.Close()
+
+	if upstreamSawProxyAuth {
+		t.Fatalf("upstream received Proxy-Authorization, proxy did not strip it")
+	}
+	if resp.Header.Get("Proxy-Connection") != "" {
+		t.Fatalf("response retained Proxy-Connection, proxy did not strip it")
+	}
+	if resp.Header.Get("X-Downstream") != "ok" {
+		t.Fatalf("response missing X-Downstream header")
+	}
+}
+
+func TestPinningProxy_Forward_RejectsNonAbsoluteURL(t *testing.T) {
+	p := newPinningProxy(nil, nil, false)
+	p.decide = func(_ context.Context, _ string, _, _ []*regexp2.Regexp, _ time.Time) (gotenberg.OutboundDecision, error) {
+		t.Fatal("decide must not be called for malformed proxy request")
+		return gotenberg.OutboundDecision{}, nil
+	}
+	proxyURL := newProxyForTest(t, p)
+
+	conn, err := net.Dial("tcp", strings.TrimPrefix(proxyURL, "http://"))
+	if err != nil {
+		t.Fatalf("dial proxy: %v", err)
+	}
+	defer conn.Close()
+
+	// Send a request with a path-only target, not an absolute URI, which
+	// the proxy should reject with 400.
+	_, err = fmt.Fprint(conn, "GET /path HTTP/1.1\r\nHost: example.com\r\n\r\n")
+	if err != nil {
+		t.Fatalf("write request: %v", err)
+	}
+	resp, err := http.ReadResponse(bufio.NewReader(conn), nil)
+	if err != nil {
+		t.Fatalf("read response: %v", err)
+	}
+	defer resp.Body.Close()
+	if resp.StatusCode != http.StatusBadRequest {
+		t.Fatalf("status = %d, want 400", resp.StatusCode)
+	}
+}
+
+func TestPinningProxy_CONNECT_Pinned_Success(t *testing.T) {
+	upstreamAddr, stop := newRawTCPServer(t, func(c net.Conn) {
+		defer c.Close()
+		_, _ = c.Write([]byte("HI"))
+		buf := make([]byte, 4)
+		n, _ := io.ReadFull(c, buf)
+		_, _ = c.Write(buf[:n])
+	})
+	t.Cleanup(stop)
+
+	var decideCalls atomic.Int32
+	p := newPinningProxy(nil, nil, false)
+	p.decide = func(_ context.Context, _ string, _, _ []*regexp2.Regexp, _ time.Time) (gotenberg.OutboundDecision, error) {
+		decideCalls.Add(1)
+		return gotenberg.OutboundDecision{Pinned: []netip.Addr{netip.MustParseAddr("127.0.0.1")}}, nil
+	}
+	p.dialPinned = func(_ context.Context, network string, _ []netip.Addr, _ string) (net.Conn, error) {
+		return net.Dial(network, upstreamAddr)
+	}
+	proxyURL := newProxyForTest(t, p)
+
+	// Connect to the proxy, send CONNECT, splice raw bytes.
+	conn, err := net.Dial("tcp", strings.TrimPrefix(proxyURL, "http://"))
+	if err != nil {
+		t.Fatalf("dial proxy: %v", err)
+	}
+	defer conn.Close()
+
+	_ = conn.SetDeadline(time.Now().Add(5 * time.Second))
+
+	_, err = fmt.Fprintf(conn, "CONNECT example.com:443 HTTP/1.1\r\nHost: example.com:443\r\n\r\n")
+	if err != nil {
+		t.Fatalf("write CONNECT: %v", err)
+	}
+
+	br := bufio.NewReader(conn)
+	statusLine, err := br.ReadString('\n')
+	if err != nil {
+		t.Fatalf("read status: %v", err)
+	}
+	if !strings.Contains(statusLine, " 200 ") {
+		t.Fatalf("CONNECT status = %q, want 200", statusLine)
+	}
+	// Consume the blank line after headers.
+	for {
+		line, err := br.ReadString('\n')
+		if err != nil {
+			t.Fatalf("read headers: %v", err)
+		}
+		if line == "\r\n" || line == "\n" {
+			break
+		}
+	}
+
+	hi := make([]byte, 2)
+	_, err = io.ReadFull(br, hi)
+	if err != nil {
+		t.Fatalf("read greeting: %v", err)
+	}
+	if string(hi) != "HI" {
+		t.Fatalf("greeting = %q, want HI", hi)
+	}
+
+	_, err = conn.Write([]byte("PONG"))
+	if err != nil {
+		t.Fatalf("write PONG: %v", err)
+	}
+	echo := make([]byte, 4)
+	_, err = io.ReadFull(br, echo)
+	if err != nil {
+		t.Fatalf("read echo: %v", err)
+	}
+	if string(echo) != "PONG" {
+		t.Fatalf("echo = %q, want PONG", echo)
+	}
+	if got := decideCalls.Load(); got != 1 {
+		t.Fatalf("decide called %d times, want 1", got)
+	}
+}
+
+func TestPinningProxy_CONNECT_BlockedByDecide(t *testing.T) {
+	p := newPinningProxy(nil, nil, false)
+	p.decide = func(_ context.Context, _ string, _, _ []*regexp2.Regexp, _ time.Time) (gotenberg.OutboundDecision, error) {
+		return gotenberg.OutboundDecision{}, fmt.Errorf("nope: %w", gotenberg.ErrFiltered)
+	}
+	p.dialPinned = func(_ context.Context, _ string, _ []netip.Addr, _ string) (net.Conn, error) {
+		t.Fatal("dialPinned must not be called when decide returns an error")
+		return nil, errors.New("unreachable")
+	}
+	proxyURL := newProxyForTest(t, p)
+
+	conn, err := net.Dial("tcp", strings.TrimPrefix(proxyURL, "http://"))
+	if err != nil {
+		t.Fatalf("dial proxy: %v", err)
+	}
+	defer conn.Close()
+	_ = conn.SetDeadline(time.Now().Add(5 * time.Second))
+
+	_, err = fmt.Fprintf(conn, "CONNECT rebind.example:443 HTTP/1.1\r\nHost: rebind.example:443\r\n\r\n")
+	if err != nil {
+		t.Fatalf("write CONNECT: %v", err)
+	}
+
+	resp, err := http.ReadResponse(bufio.NewReader(conn), nil)
+	if err != nil {
+		t.Fatalf("read response: %v", err)
+	}
+	defer resp.Body.Close()
+	if resp.StatusCode != http.StatusForbidden {
+		t.Fatalf("CONNECT status = %d, want 403", resp.StatusCode)
+	}
+}
+
+// TestPinningProxy_DNSRebind_SingleResolution is the regression test for
+// the DNS rebinding window. It simulates a DNS authority that returns a
+// public IP on the first lookup and a loopback IP on subsequent lookups.
+// The proxy must resolve the host exactly once per request and dial the
+// IP validated at that moment, so that a second resolution by any later
+// layer cannot pivot the connection to an internal target.
+func TestPinningProxy_DNSRebind_SingleResolution(t *testing.T) {
+	upstream := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		_, _ = fmt.Fprint(w, "public-upstream")
+	}))
+	t.Cleanup(upstream.Close)
+	upstreamURL := mustParseURL(t, upstream.URL)
+
+	var lookupCount atomic.Int32
+	stubDecide := func(_ context.Context, _ string, _, _ []*regexp2.Regexp, _ time.Time) (gotenberg.OutboundDecision, error) {
+		n := lookupCount.Add(1)
+		if n == 1 {
+			// First lookup: returns a public IP, validation passes, the
+			// proxy pins it for the dial.
+			return gotenberg.OutboundDecision{Pinned: []netip.Addr{netip.MustParseAddr("93.184.216.34")}}, nil
+		}
+		// Any subsequent lookup for the same host would return a
+		// loopback IP. This return value must not influence the dial
+		// because the proxy must not call decide again for this request.
+		return gotenberg.OutboundDecision{}, fmt.Errorf("rebind lookup: %w", gotenberg.ErrFiltered)
+	}
+
+	p := newPinningProxy(nil, nil, false)
+	p.decide = stubDecide
+	p.dialPinned = func(_ context.Context, network string, addrs []netip.Addr, _ string) (net.Conn, error) {
+		if len(addrs) != 1 || addrs[0].String() != "93.184.216.34" {
+			t.Errorf("dialPinned got addrs %v, want [93.184.216.34]", addrs)
+		}
+		return net.Dial(network, upstreamURL.Host)
+	}
+	proxyURL := newProxyForTest(t, p)
+
+	client := &http.Client{
+		Transport: &http.Transport{
+			Proxy: http.ProxyURL(mustParseURL(t, proxyURL)),
+		},
+		Timeout: 5 * time.Second,
+	}
+
+	resp, err := client.Get("http://rebind.example/")
+	if err != nil {
+		t.Fatalf("GET via proxy: %v", err)
+	}
+	defer resp.Body.Close()
+	body, _ := io.ReadAll(resp.Body)
+
+	if resp.StatusCode != http.StatusOK {
+		t.Fatalf("status = %d, want 200", resp.StatusCode)
+	}
+	if string(body) != "public-upstream" {
+		t.Fatalf("body = %q, want %q", body, "public-upstream")
+	}
+	if got := lookupCount.Load(); got != 1 {
+		t.Fatalf("decide called %d times, want exactly 1 (rebind protection)", got)
+	}
+}
+
+func TestPinningProxy_StartTwice(t *testing.T) {
+	p := newPinningProxy(nil, nil, false)
+	err := p.Start(testLogger())
+	if err != nil {
+		t.Fatalf("first Start: %v", err)
+	}
+	t.Cleanup(func() { _ = p.Stop(testLogger()) })
+
+	err = p.Start(testLogger())
+	if err == nil {
+		t.Fatal("second Start: expected error, got nil")
+	}
+}
+
+func TestPinningProxy_StopIdempotent(t *testing.T) {
+	p := newPinningProxy(nil, nil, false)
+	// Stop on a never-started proxy is a no-op.
+	if err := p.Stop(testLogger()); err != nil {
+		t.Fatalf("Stop on never-started proxy: %v", err)
+	}
+
+	if err := p.Start(testLogger()); err != nil {
+		t.Fatalf("Start: %v", err)
+	}
+	if err := p.Stop(testLogger()); err != nil {
+		t.Fatalf("first Stop: %v", err)
+	}
+	if err := p.Stop(testLogger()); err != nil {
+		t.Fatalf("second Stop on stopped proxy: %v", err)
+	}
+}

pkg/modules/chromium/routes.go

@@ -7,6 +7,7 @@ import (
 	"fmt"
 	"html/template"
 	"net/http"
+	"net/url"
 	"os"
 	"path/filepath"
 	"strconv"
@@ -402,6 +403,34 @@ func FormDataChromiumScreenshotOptions(ctx *api.Context) (*api.FormData, Screens
 	return form, screenshotOptions
 }
 
+// rejectFileScheme returns an HTTP 400 [api] error when rawURL uses the
+// file:// scheme. /forms/chromium/convert/url and
+// /forms/chromium/screenshot/url accept user-supplied URLs and are
+// intended for navigating to remote HTTP(S) resources; allowing file://
+// lets a caller reach Chromium's working directory through the default
+// deny-list's /tmp/ allowance, which exists only to serve main-page
+// HTML/Markdown that the other routes generate. Filter the scheme at the
+// route layer where no request-scoped allowedFilePrefixes exists.
+func rejectFileScheme(rawURL string) error {
+	parsed, err := url.Parse(rawURL)
+	if err != nil {
+		return api.WrapError(
+			fmt.Errorf("parse URL: %w", err),
+			api.NewSentinelHttpError(http.StatusBadRequest, fmt.Sprintf("Invalid URL: %s", err)),
+		)
+	}
+	if strings.EqualFold(parsed.Scheme, "file") {
+		return api.WrapError(
+			fmt.Errorf("file:// scheme not allowed on URL route"),
+			api.NewSentinelHttpError(
+				http.StatusBadRequest,
+				"file:// URLs are not accepted on this route. Use the /convert/html or /convert/markdown routes to render local HTML",
+			),
+		)
+	}
+	return nil
+}
+
 // convertUrlRoute returns an [api.Route] which can convert a URL to PDF.
 func convertUrlRoute(chromium Api, engine gotenberg.PdfEngine) api.Route {
 	return api.Route{
@@ -431,6 +460,11 @@ func convertUrlRoute(chromium Api, engine gotenberg.PdfEngine) api.Route {
 				return fmt.Errorf("validate form data: %w", err)
 			}
 
+			err = rejectFileScheme(url)
+			if err != nil {
+				return fmt.Errorf("reject URL scheme: %w", err)
+			}
+
 			if (watermark.Source == gotenberg.StampSourceImage || watermark.Source == gotenberg.StampSourcePDF) && watermarkFile != "" {
 				watermark.Expression = watermarkFile
 			}
@@ -467,6 +501,11 @@ func screenshotUrlRoute(chromium Api) api.Route {
 				return fmt.Errorf("validate form data: %w", err)
 			}
 
+			err = rejectFileScheme(url)
+			if err != nil {
+				return fmt.Errorf("reject URL scheme: %w", err)
+			}
+
 			err = screenshotUrl(ctx, chromium, url, options)
 			if err != nil {
 				return fmt.Errorf("URL screenshot: %w", err)

test/integration/features/chromium_convert_url.feature

@@ -478,6 +478,41 @@ Feature: /forms/chromium/convert/url
       # Modern browsers block file URIs from being loaded into iframes when the parent page is served over HTTP/HTTPS.
       | 'file:///etc/passwd' does not match any expression from the allowed list |
 
+  Scenario: POST /forms/chromium/convert/url (file:// scheme rejected at route layer)
+    Given I have a default Gotenberg container
+    When I make a "POST" request to Gotenberg at the "/forms/chromium/convert/url" endpoint with the following form data and header(s):
+      | url | file:///tmp/foo/index.html | field |
+    Then the response status code should be 400
+    Then the response header "Content-Type" should be "text/plain; charset=UTF-8"
+    Then the response body should match string:
+      """
+      file:// URLs are not accepted on this route. Use the /convert/html or /convert/markdown routes to render local HTML
+      """
+
+  Scenario: POST /forms/chromium/convert/url (Main URL resolves to a non-public IP, allow-private-ips off)
+    Given I have a Gotenberg container with the following environment variable(s):
+      | CHROMIUM_ALLOW_LIST |  |
+    Given I have a static server
+    When I make a "POST" request to Gotenberg at the "/forms/chromium/convert/url" endpoint with the following form data and header(s):
+      | url | http://host.docker.internal:%d/html/testdata/page-1-html/index.html | field |
+    Then the response status code should be 403
+    Then the response header "Content-Type" should be "text/plain; charset=UTF-8"
+    Then the response body should match string:
+      """
+      Forbidden
+      """
+
+  Scenario: POST /forms/chromium/convert/url (Main URL resolves to a non-public IP, allow-private-ips on)
+    Given I have a Gotenberg container with the following environment variable(s):
+      | CHROMIUM_ALLOW_LIST        |      |
+      | CHROMIUM_ALLOW_PRIVATE_IPS | true |
+    Given I have a static server
+    When I make a "POST" request to Gotenberg at the "/forms/chromium/convert/url" endpoint with the following form data and header(s):
+      | url | http://host.docker.internal:%d/html/testdata/page-1-html/index.html | field |
+    Then the response status code should be 200
+    Then the response header "Content-Type" should be "application/pdf"
+    Then there should be 1 PDF(s) in the response
+
   Scenario: POST /forms/chromium/convert/url (JavaScript Enabled)
     Given I have a default Gotenberg container
     Given I have a static server

test/integration/features/chromium_screenshot_url.feature

@@ -44,6 +44,17 @@ Feature: /forms/chromium/screenshot/url
     Then the response status code should be 400
     Then the response header "Content-Type" should be "text/plain; charset=UTF-8"
 
+  Scenario: POST /forms/chromium/screenshot/url (file:// scheme rejected at route layer)
+    Given I have a default Gotenberg container
+    When I make a "POST" request to Gotenberg at the "/forms/chromium/screenshot/url" endpoint with the following form data and header(s):
+      | url | file:///tmp/foo/index.html | field |
+    Then the response status code should be 400
+    Then the response header "Content-Type" should be "text/plain; charset=UTF-8"
+    Then the response body should match string:
+      """
+      file:// URLs are not accepted on this route. Use the /convert/html or /convert/markdown routes to render local HTML
+      """
+
   @webhook
   Scenario: POST /forms/chromium/screenshot/url (Webhook)
     Given I have a default Gotenberg container

test/integration/features/debug.feature

@@ -78,6 +78,7 @@ Feature: /debug
           "chromium-allow-file-access-from-files": "false",
           "chromium-allow-insecure-localhost": "false",
           "chromium-allow-list": "[.+]",
+          "chromium-allow-private-ips": "false",
           "chromium-auto-start": "false",
           "chromium-clear-cache": "false",
           "chromium-clear-cookies": "false",
@@ -210,6 +211,7 @@ Feature: /debug
           "chromium-allow-file-access-from-files": "false",
           "chromium-allow-insecure-localhost": "false",
           "chromium-allow-list": "[.+]",
+          "chromium-allow-private-ips": "false",
           "chromium-auto-start": "false",
           "chromium-clear-cache": "false",
           "chromium-clear-cookies": "false",