Traefik's errors middleware forwards Authorization and Cookie headers to separate error page service

Summary

There is a medium severity information disclosure vulnerability in Traefik's errors (custom error pages) middleware. When the backend returns a response matching the configured status range, the middleware forwards the original request's complete header set, including Authorization, Cookie, and other authentication material, to the separate error page service rather than only the minimal context needed to render the error page. This behavior is undocumented: the documentation states only that Host is forwarded by default, so operators are not warned that sensitive credentials are shared across service boundaries. Deployments using the errors middleware with a distinct error page service may inadvertently expose end-user credentials to infrastructure that was not intended to receive them.

Patches

• https://github.com/traefik/traefik/releases/tag/v2.11.44
• https://github.com/traefik/traefik/releases/tag/v3.6.15
• https://github.com/traefik/traefik/releases/tag/v3.7.0-rc.3

For more information

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

<details> <summary>Original Description</summary>

Description

Traefik v3.6.13's supported HTTP errors middleware discloses sensitive request headers to the configured error page service when the original backend response matches the configured status range and the middleware takes its default header-forwarding path. In the reproduced configuration, the business router audit-customerrors@docker pointed to backend service audit-backend, attached middleware audit-leak@docker, and the middleware was configured with errors.status=500-599, errors.service=audit-error, and errors.query=/collect. A request to the business route caused the backend to return 500, after which Traefik created a secondary request to the error service and copied the original Authorization and Cookie headers into that cross-service request.

This is a normal feature path on an ordinary HTTP route. It does not depend on api.insecure, the dashboard, pprof, or a debug-only mode. The confidentiality boundary that breaks here is the service boundary between the original backend chain and the separate error page service: credentials that were only meant for the original backend are automatically delivered to another service.

The root cause is in pkg/middlewares/customerrors/custom_errors.go:151-160:

if len(c.forwardNginxHeaders) > 0 {
    utils.CopyHeaders(pageReq.Header, c.forwardNginxHeaders)
    pageReq.Header.Set("X-Code", strconv.Itoa(code))
    pageReq.Header.Set("X-Format", req.Header.Get("Accept"))
    pageReq.Header.Set("X-Original-Uri", req.URL.RequestURI())
} else {
    utils.CopyHeaders(pageReq.Header, req.Header)
}

Unless the NginxHeaders branch is explicitly used, the middleware copies the entire original request header map into the error page request. The documentation at docs/content/reference/routing-configuration/http/middlewares/errorpages.md:103-107 only states that Host is forwarded by default, so operators are not warned that Authorization, Cookie, and other authentication material are forwarded as well.

Steps To Reproduce

1. Deploy Traefik v3.6.13 with a normal business route that uses the supported errors middleware and points errors.service to a distinct service. The attached PoC uses BASE_URL = "http://127.0.0.1:28080", API_BASE_URL = "http://127.0.0.1:28180", ROUTER_PATH = "/audit-customerrors", AUTHORIZATION = "Bearer audit-secret-token", and COOKIE = "sessionid=audit-cookie; theme=dark".

2. Start the two attached helper services customerrors_backend.py and customerrors_error.py. The backend listens on port 8000 and always returns 500. The error service listens on port 8000 and returns the request method, path, and received headers as JSON. The PoC starts them with the router and middleware labels below so that the business request is handled by the backend, while the error page is fetched from the separate error service:

traefik.http.routers.audit-customerrors.rule=PathPrefix(`/audit-customerrors`)
traefik.http.routers.audit-customerrors.entrypoints=web
traefik.http.routers.audit-customerrors.priority=100
traefik.http.routers.audit-customerrors.service=audit-backend
traefik.http.routers.audit-customerrors.middlewares=audit-leak
traefik.http.services.audit-backend.loadbalancer.server.port=8000
traefik.http.middlewares.audit-leak.errors.status=500-599
traefik.http.middlewares.audit-leak.errors.service=audit-error
traefik.http.middlewares.audit-leak.errors.query=/collect

3. Confirm that Traefik has loaded the route and middleware. The attached customerrors_router.json shows that audit-customerrors@docker uses middleware audit-leak@docker, and the attached customerrors_middleware.json shows that the middleware is enabled with status 500-599, service audit-error, and query /collect.

4. Send a request containing sensitive credentials through the business route. The manual reproduction used the following request, and the automated PoC sends the same header values:

curl -i \
  -H 'Authorization: Bearer audit-secret-token' \
  -H 'Cookie: sessionid=audit-cookie; theme=dark' \
  http://127.0.0.1:28080/audit-customerrors

5. Observe that the backend returns 500, Traefik internally requests /collect from the error service, and the error service receives the original Authorization and Cookie headers. The attached manual_curl_customerrors.txt response shows the leaked headers directly, and the attached poc_customerrors_header_leak.output.txt execution log shows the same result from the automated PoC.

Recommendations

The default behavior should forward only the minimal context needed to render an error page instead of copying the full original header set with utils.CopyHeaders(pageReq.Header, req.Header). At minimum, Traefik should strip Authorization, Proxy-Authorization, Cookie, Set-Cookie, and common custom authentication headers such as X-Api-Key before issuing the error page request. If operators truly need additional headers, that behavior should be opt-in through an explicit allowlist rather than the default. The documentation should also describe the current behavior and warn that routing an error page to a separate service can otherwise disclose end-user credentials across service boundaries.

PoC

The main PoC attachment is poc_customerrors_header_leak.py.

import json
import os
import subprocess
import sys
import time
import urllib.error
import urllib.request
from pathlib import Path


TARGET = "traefik customErrors sensitive header leak"
BASE_URL = "http://127.0.0.1:28080"
API_BASE_URL = "http://127.0.0.1:28180"
TRAEFIK_CONTAINER = "traefik-openclaw"
NETWORK = ""
DOCKER_IMAGE = "python:3.12-alpine"
BACKEND_CONTAINER = "traefik-audit-backend"
ERROR_CONTAINER = "traefik-audit-error"
ROUTER_NAME = "audit-customerrors"
ROUTER_PATH = "/audit-customerrors"
AUTHORIZATION = "Bearer audit-secret-token"
COOKIE = "sessionid=audit-cookie; theme=dark"
TIMEOUT_SECONDS = 10
ROUTER_WAIT_SECONDS = 20

EVIDENCE_DIR = Path(__file__).resolve().parent
BACKEND_SCRIPT = EVIDENCE_DIR / "customerrors_backend.py"
ERROR_SCRIPT = EVIDENCE_DIR / "customerrors_error.py"


def run_command(command):
    print(f"$ {' '.join(command)}")
    completed = subprocess.run(command, capture_output=True, text=True, check=True)
    stdout = completed.stdout.strip()
    stderr = completed.stderr.strip()
    if stdout:
        print(stdout)
    if stderr:
        print(stderr)
    return stdout


def remove_container(name):
    subprocess.run(["docker", "rm", "-f", name], capture_output=True, text=True)


def detect_network():
    if NETWORK:
        return NETWORK

    output = run_command(
        ["docker", "inspect", TRAEFIK_CONTAINER, "--format", "{{json .NetworkSettings.Networks}}"]
    )
    networks = json.loads(output)
    network_names = sorted(networks.keys())
    if not network_names:
        raise RuntimeError("No docker network found for Traefik container")
    return network_names[0]


def ensure_image():
    run_command(["docker", "pull", DOCKER_IMAGE])


def start_error_container(network_name):
    run_command(
        [
            "docker", "run", "-d", "--name", ERROR_CONTAINER,
            "--network", network_name,
            "-v", f"{ERROR_SCRIPT}:/srv/error.py:ro",
            "-l", "traefik.enable=true",
            "-l", f"traefik.docker.network={network_name}",
            "-l", "traefik.http.services.audit-error.loadbalancer.server.port=8000",
            DOCKER_IMAGE, "python", "/srv/error.py",
        ]
    )


def start_backend_container(network_name):
    run_command(
        [
            "docker", "run", "-d", "--name", BACKEND_CONTAINER,
            "--network", network_name,
            "-v", f"{BACKEND_SCRIPT}:/srv/backend.py:ro",
            "-l", "traefik.enable=true",
            "-l", f"traefik.docker.network={network_name}",
            "-l", f"traefik.http.routers.{ROUTER_NAME}.rule=PathPrefix(`{ROUTER_PATH}`)",
            "-l", f"traefik.http.routers.{ROUTER_NAME}.entrypoints=web",
            "-l", f"traefik.http.routers.{ROUTER_NAME}.priority=100",
            "-l", f"traefik.http.routers.{ROUTER_NAME}.service=audit-backend",
            "-l", f"traefik.http.routers.{ROUTER_NAME}.middlewares=audit-leak",
            "-l", "traefik.http.services.audit-backend.loadbalancer.server.port=8000",
            "-l", "traefik.http.middlewares.audit-leak.errors.status=500-599",
            "-l", "traefik.http.middlewares.audit-leak.errors.service=audit-error",
            "-l", "traefik.http.middlewares.audit-leak.errors.query=/collect",
            DOCKER_IMAGE, "python", "/srv/backend.py",
        ]
    )


def fetch_json(url, headers=None):
    request = urllib.request.Request(url, headers=headers or {}, method="GET")
    try:
        response = urllib.request.urlopen(request, timeout=TIMEOUT_SECONDS)
    except urllib.error.HTTPError as exc:
        response = exc

    with response:
        return json.loads(response.read().decode())


def wait_for_router():
    deadline = time.time() + ROUTER_WAIT_SECONDS
    while time.time() < deadline:
        try:
            data = fetch_json(f"{API_BASE_URL}/api/rawdata")
            if f"{ROUTER_NAME}@docker" in data.get("routers", {}):
                return data
        except Exception:
            pass
        time.sleep(1)
    raise RuntimeError("Timed out waiting for router")


def trigger_request():
    headers = {
        "Authorization": AUTHORIZATION,
        "Cookie": COOKIE,
    }
    return fetch_json(f"{BASE_URL}{ROUTER_PATH}", headers=headers)


def validate(response_json):
    leaked_headers = response_json.get("headers", {})
    leaked_auth = leaked_headers.get("Authorization")
    leaked_cookie = leaked_headers.get("Cookie")

    print("Response JSON:")
    print(json.dumps(response_json, indent=2, sort_keys=True))

    if leaked_auth != AUTHORIZATION:
        raise RuntimeError(f"Authorization not leaked as expected, got: {leaked_auth!r}")
    if leaked_cookie != COOKIE:
        raise RuntimeError(f"Cookie not leaked as expected, got: {leaked_cookie!r}")

    print("Validation result: error page service received the original Authorization and Cookie.")


def main():
    print(f"TARGET={TARGET}")
    network_name = detect_network()
    print(f"Using docker network: {network_name}")

    remove_container(BACKEND_CONTAINER)
    remove_container(ERROR_CONTAINER)

    try:
        ensure_image()
        start_error_container(network_name)
        start_backend_container(network_name)
        wait_for_router()
        response_json = trigger_request()
        validate(response_json)
    finally:
        remove_container(BACKEND_CONTAINER)
        remove_container(ERROR_CONTAINER)
        print("Cleaned up temporary containers.")


if __name__ == "__main__":
    try:
        main()
    except subprocess.CalledProcessError as exc:
        if exc.stdout:
            print(exc.stdout)
        if exc.stderr:
            print(exc.stderr, file=sys.stderr)
        raise

Supporting backend helper used by the PoC, from customerrors_backend.py:

from http.server import BaseHTTPRequestHandler, HTTPServer


class Handler(BaseHTTPRequestHandler):
    def do_GET(self):
        self.send_response(500)
        self.send_header("Content-Type", "text/plain; charset=utf-8")
        self.end_headers()
        self.wfile.write(b"backend forced 500\n")

    def log_message(self, format, *args):
        return


def main():
    HTTPServer(("0.0.0.0", 8000), Handler).serve_forever()


if __name__ == "__main__":
    main()

Supporting error service helper used by the PoC, from customerrors_error.py:

import json
from http.server import BaseHTTPRequestHandler, HTTPServer


class Handler(BaseHTTPRequestHandler):
    def do_GET(self):
        body = json.dumps(
            {
                "method": self.command,
                "path": self.path,
                "headers": {key: value for key, value in self.headers.items()},
            },
            indent=2,
            sort_keys=True,
        ).encode()
        self.send_response(200)
        self.send_header("Content-Type", "application/json; charset=utf-8")
        self.send_header("Content-Length", str(len(body)))
        self.end_headers()
        self.wfile.write(body)

    def log_message(self, format, *args):
        return


def main():
    HTTPServer(("0.0.0.0", 8000), Handler).serve_forever()


if __name__ == "__main__":
    main()

Evidence Files

customerrors_middleware.json proves that the active middleware is the supported errors middleware and that it was configured with status 500-599, service audit-error, and query /collect.

{
    "errors": {
        "status": [
            "500-599"
        ],
        "service": "audit-error",
        "query": "/collect"
    },
    "status": "enabled",
    "usedBy": [
        "audit-customerrors@docker"
    ],
    "name": "audit-leak@docker",
    "provider": "docker",
    "type": "errors"
}

customerrors_router.json proves that the business router audit-customerrors@docker was enabled on the web entrypoint, routed to audit-backend, and used middleware audit-leak@docker.

{
    "entryPoints": [
        "web"
    ],
    "middlewares": [
        "audit-leak@docker"
    ],
    "service": "audit-backend",
    "rule": "PathPrefix(`/audit-customerrors`)",
    "priority": 100,
    "observability": {
        "accessLogs": true,
        "metrics": true,
        "tracing": true,
        "traceVerbosity": "minimal"
    },
    "status": "enabled",
    "using": [
        "web"
    ],
    "name": "audit-customerrors@docker",
    "provider": "docker",
    "priorityStr": "100"
}

manual_curl_customerrors.txt proves that a direct request through Traefik caused the separate error service to receive the original Authorization and Cookie values.

HTTP/1.1 500 Internal Server Error
Content-Length: 461
Content-Type: application/json; charset=utf-8
Date: Mon, 13 Apr 2026 13:09:58 GMT
Server: BaseHTTP/0.6 Python/3.12.13

{
  "headers": {
    "Accept": "*/*",
    "Accept-Encoding": "gzip",
    "Authorization": "Bearer audit-secret-token",
    "Cookie": "sessionid=audit-cookie; theme=dark",
    "Host": "127.0.0.1:28080",
    "User-Agent": "curl/8.7.1",
    "X-Forwarded-Host": "127.0.0.1:28080",
    "X-Forwarded-Port": "28080",
    "X-Forwarded-Proto": "http",
    "X-Forwarded-Server": "c231be677a1b",
    "X-Real-Ip": "172.19.0.1"
  },
  "method": "GET",
  "path": "/collect"
}

poc_customerrors_header_leak.output.txt is the automated execution log for the Python PoC. The source material provided the following excerpt from that output, which shows the same credential disclosure and the PoC's validation result.

Response JSON:
{
  "headers": {
    "Accept-Encoding": "identity",
    "Authorization": "Bearer audit-secret-token",
    "Cookie": "sessionid=audit-cookie; theme=dark",
    "Host": "127.0.0.1:28080",
    "User-Agent": "Python-urllib/3.14",
    "X-Forwarded-Host": "127.0.0.1:28080",
    "X-Forwarded-Port": "28080",
    "X-Forwarded-Proto": "http",
    "X-Forwarded-Server": "c231be677a1b",
    "X-Real-Ip": "172.19.0.1"
  },
  "method": "GET",
  "path": "/collect"
}
Validation result: error page service received the original Authorization and Cookie.

Impact

Any deployment that uses the supported errors middleware with a separate error page service can silently copy end-user credentials to that second service whenever the configured error status range is triggered. In practice, this means bearer tokens, session cookies, and other custom authentication headers can be disclosed to infrastructure that was never meant to receive them. If the error service is maintained by a different team, shared across tenants, hosted by a third party, or simply logged more broadly than the primary application service, this expands the exposure of valid credentials and can enable unauthorized API access or account compromise depending on what the leaked tokens authorize.

</details>