CVE-2026-88004: Traefik entrypoint header-name sanitization bypassed via request trailers
Traefik is an open source HTTP reverse proxy and load balancer. From 3.2.0 until 3.7.13, Traefik entrypoint defenses aliasHeadersStrategy, underscoreHeadersStrategy, and forwardedHeaders inspect req.Header but not req.Trailer, allowing an unauthenticated client to submit an aliasing or trusted header name in an HTTP/1.1 chunked trailer or an HTTP/2 trailer. When the retry or buffering middleware reads the body before the reverse proxy clones the request, the attacker-controlled trailer value reaches a backend that merges trailers into the header namespace, bypassing the documented delete or reject behavior and potentially spoofing identity or forwarded routing data. This issue is fixed in 3.7.13.
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
Traefikto a version that resolves this vulnerability.Fixed in 3.7.13 - Compensating control
If upgrading to Traefik 3.7.13 is not immediately possible, mitigate by preventing clients from sending HTTP trailers (e.g., block/drop requests containing HTTP/1.1 or HTTP/2 trailers at the edge/WAF or by configuring an upstream reverse proxy to reject requests with trailer headers).
Event History
Frequently Asked Questions
Which deployments are exposed to this bypass?
Traefik versions from 3.2.0 through 3.7.12 are affected when entrypoint header defenses are relied upon and a request can reach a backend that merges trailers into the header namespace. Exploitation also requires the retry or buffering middleware to read the request body before the reverse proxy clones the request.
What does an attacker need to send?
An unauthenticated client must send a crafted HTTP/1.1 chunked trailer or HTTP/2 trailer containing an aliasing or trusted header name. The trailer value can then reach the backend as a header value, potentially spoofing identity or forwarded routing data.
Are the documented header delete or reject behaviors sufficient?
No. The affected entrypoint defenses inspect request headers but not request trailers, so the documented delete or reject behavior can be bypassed through trailers under the described middleware and backend conditions.
What is the available remediation?
Upgrade Traefik to 3.7.13, which fixes the issue.