Technical information leak In github.com/traefik/traefik/v2
Description
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.
Original Description
## Description Traefik v3.6.13's supported HTTPerrors 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: go 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: text 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: bash 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. ```python 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()Mitigation
Update Impact
Minimal update. May introduce new vulnerabilities or breaking changes.
Ecosystem | Component | Affected version | Patched versions |
|---|---|---|---|
 go | 3.6.15, 3.7.0-rc.3 | ||
go | 2.11.44 |
Aliases
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References
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