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](https://github.com/traefik/traefik/issues).
<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:
```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_middlewa
