fix(api): bound downloadFrom decoding by the entry limit

pkg/modules/api/api.go

@@ -215,7 +215,7 @@ func (a *Api) Descriptor() gotenberg.ModuleDescriptor {
 			fs.Bool("api-download-from-enable-environment-proxy", false, "Route downloadFrom fetches through the proxy defined by the standard HTTP_PROXY, HTTPS_PROXY, and NO_PROXY variables, including credentials")
 			fs.Int("api-download-from-max-retry", 4, "Set the maximum number of retries for the download from feature")
 			fs.Int("api-download-from-max-concurrency", 10, "Set the maximum number of downloadFrom entries fetched concurrently per request - bounds the outbound fan-out. Set to 0 to disable this feature")
-			fs.Int("api-download-from-max-entries", 0, "Set the maximum number of downloadFrom entries allowed per request. Set to 0 to disable this feature")
+			fs.Int("api-download-from-max-entries", 1000, "Set the maximum number of downloadFrom entries allowed per request. Set to 0 to disable this limit, which lets a single request expand into an arbitrarily large array")
 			fs.Bool("api-disable-download-from", false, "Disable the download from feature")
 			fs.Bool("api-disable-health-check-route-telemetry", true, "Disable telemetry for health check route")
 			fs.Bool("api-disable-root-route-telemetry", true, "Disable telemetry for the root route")

pkg/modules/api/context.go

@@ -80,6 +80,48 @@ func (t *trackingReader) Read(p []byte) (int, error) {
 	return n, nil
 }
 
+// errTooManyDownloadFromEntries is returned by [decodeDownloadFrom] when the
+// array holds more entries than the configured maximum.
+var errTooManyDownloadFromEntries = errors.New("too many downloadFrom entries")
+
+// decodeDownloadFrom decodes the downloadFrom form field, refusing to
+// accumulate more than maxEntries. A maxEntries of 0 means no limit.
+//
+// It decodes element by element rather than calling [json.Unmarshal] on the
+// whole value. A compact array such as "[{},{},{}]" costs three bytes per
+// entry on the wire and expands to roughly seventy times that once
+// unmarshalled, so counting the entries afterwards is too late to bound the
+// allocation. Streaming keeps the cost proportional to maxEntries no matter
+// how long the array is.
+func decodeDownloadFrom(raw string, maxEntries int) ([]downloadFrom, error) {
+	dec := json.NewDecoder(strings.NewReader(raw))
+
+	token, err := dec.Token()
+	if err != nil {
+		return nil, err
+	}
+	if delim, ok := token.(json.Delim); !ok || delim != '[' {
+		return nil, fmt.Errorf("expected a JSON array, got '%v'", token)
+	}
+
+	var dls []downloadFrom
+	for dec.More() {
+		if maxEntries > 0 && len(dls) >= maxEntries {
+			return nil, errTooManyDownloadFromEntries
+		}
+
+		var dl downloadFrom
+		err = dec.Decode(&dl)
+		if err != nil {
+			return nil, err
+		}
+
+		dls = append(dls, dl)
+	}
+
+	return dls, nil
+}
+
 type downloadFrom struct {
 	// Url is the URL to download a file from.
 	Url string `json:"url"`
@@ -213,8 +255,13 @@ func newContext(echoCtx echo.Context, logger *slog.Logger, fs *gotenberg.FileSys
 	// any.
 	raw, ok := ctx.values["downloadFrom"]
 	if !downloadFromCfg.disable && ok {
-		var dls []downloadFrom
-		err = json.Unmarshal([]byte(raw[0]), &dls)
+		dls, err := decodeDownloadFrom(raw[0], downloadFromCfg.maxEntries)
+		if errors.Is(err, errTooManyDownloadFromEntries) {
+			return nil, cancel, WrapError(
+				fmt.Errorf("decode downloadFrom: %w", err),
+				NewSentinelHttpError(http.StatusBadRequest, fmt.Sprintf("Invalid 'downloadFrom' form field value: too many entries, the maximum is %d", downloadFromCfg.maxEntries)),
+			)
+		}
 		if err != nil {
 			return nil, cancel, WrapError(
 				fmt.Errorf("unmarshal json: %w", err),
@@ -222,16 +269,6 @@ func newContext(echoCtx echo.Context, logger *slog.Logger, fs *gotenberg.FileSys
 			)
 		}
 
-		// Reject oversized arrays at the trust boundary, before allocating
-		// the results slice or spawning any goroutine, so a compact request
-		// cannot inflate into an unbounded fan-out.
-		if downloadFromCfg.maxEntries > 0 && len(dls) > downloadFromCfg.maxEntries {
-			return nil, cancel, WrapError(
-				fmt.Errorf("too many downloadFrom entries: got %d, max %d", len(dls), downloadFromCfg.maxEntries),
-				NewSentinelHttpError(http.StatusBadRequest, fmt.Sprintf("Invalid 'downloadFrom' form field value: too many entries, the maximum is %d", downloadFromCfg.maxEntries)),
-			)
-		}
-
 		// Each goroutine writes to its own results slot. The main
 		// goroutine merges into ctx.files, ctx.diskToOriginal, and
 		// ctx.filesByField after eg.Wait() to avoid concurrent map

pkg/modules/api/context_test.go

@@ -11,6 +11,8 @@ import (
 	"net/http"
 	"net/http/httptest"
 	"os"
+	"runtime"
+	"strings"
 	"sync"
 	"sync/atomic"
 	"testing"
@@ -528,3 +530,75 @@ func TestNewContext_DownloadFromExpiredBudgetFailsClosed(t *testing.T) {
 		t.Fatal("newContext never returned: an entry starting past the deadline built an unbounded client")
 	}
 }
+
+func TestDecodeDownloadFrom(t *testing.T) {
+	for _, tc := range []struct {
+		scenario   string
+		raw        string
+		maxEntries int
+		expectErr  error
+		expectLen  int
+	}{
+		{"empty array", `[]`, 10, nil, 0},
+		{"under the limit", `[{"url":"http://a"},{"url":"http://b"}]`, 10, nil, 2},
+		{"exactly the limit", `[{"url":"http://a"},{"url":"http://b"}]`, 2, nil, 2},
+		{"over the limit", `[{"url":"http://a"},{"url":"http://b"}]`, 1, errTooManyDownloadFromEntries, 0},
+		{"no limit", `[{"url":"http://a"},{"url":"http://b"}]`, 0, nil, 2},
+		{"not an array", `{"url":"http://a"}`, 10, nil, 0},
+		{"malformed", `[{"url":`, 10, nil, 0},
+		{"not json", `nope`, 10, nil, 0},
+	} {
+		t.Run(tc.scenario, func(t *testing.T) {
+			dls, err := decodeDownloadFrom(tc.raw, tc.maxEntries)
+
+			if tc.expectErr != nil {
+				if !errors.Is(err, tc.expectErr) {
+					t.Fatalf("error = %v, want %v", err, tc.expectErr)
+				}
+				return
+			}
+			if tc.scenario == "not an array" || tc.scenario == "malformed" || tc.scenario == "not json" {
+				if err == nil {
+					t.Fatalf("expected an error for %q", tc.raw)
+				}
+				return
+			}
+			if err != nil {
+				t.Fatalf("unexpected error: %v", err)
+			}
+			if len(dls) != tc.expectLen {
+				t.Fatalf("decoded %d entries, want %d", len(dls), tc.expectLen)
+			}
+		})
+	}
+}
+
+// A compact array costs three bytes per entry on the wire and expands by
+// roughly seventy times once unmarshalled. Decoding must stop at the limit
+// rather than materialize the whole array and count afterwards.
+func TestDecodeDownloadFrom_StopsBeforeMaterializingTheArray(t *testing.T) {
+	const entries = 2_000_000
+
+	raw := "[" + strings.Repeat("{},", entries) + "{}]"
+
+	var before, after runtime.MemStats
+	runtime.GC()
+	runtime.ReadMemStats(&before)
+
+	_, err := decodeDownloadFrom(raw, 1000)
+
+	runtime.ReadMemStats(&after)
+
+	if !errors.Is(err, errTooManyDownloadFromEntries) {
+		t.Fatalf("error = %v, want errTooManyDownloadFromEntries", err)
+	}
+
+	// json.Unmarshal on the same input allocates hundreds of MiB. Bounded
+	// decoding should stay in the low single-digit MiB, so this threshold is
+	// deliberately loose and still fails loudly on a regression.
+	allocated := after.TotalAlloc - before.TotalAlloc
+	if allocated > 32<<20 {
+		t.Fatalf("decoding allocated %d MiB for a %d-entry array, want the limit to bound it", allocated>>20, entries)
+	}
+	t.Logf("allocated %d KiB decoding a %d-entry array with a limit of 1000", allocated>>10, entries)
+}