# CVE-2026-88004: Traefik entrypoint header sanitization bypass via request trailers

Source: https://junglewise.ai/threats/cve-2026-88004-traefik-entrypoint-header-sanitization-bypass-via-request  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-09-10. Updated: 2026-09-25  
License: CC BY 4.0 with a link back

CVE-2026-88004 · Severity: high · CVSS 7.4 · Published 2026-09-10

Traefik is a reverse proxy that provides built-in protections to prevent clients from spoofing trusted security headers used for authentication and access control. A flaw in Traefik v3 allows attackers to bypass these protections by smuggling forged headers through a secondary HTTP channel (request trailers). When certain middleware components buffer request bodies, attackers can inject forged identity headers with attacker-chosen values that reach backend services, potentially enabling account takeover, privilege escalation, or IP-based access bypass in downstream systems that rely on these headers.

Technologies: [github.com/traefik/traefik (Go)](https://junglewise.ai/threats/technologies/github-com-traefik-traefik), [github.com/traefik/traefik/v2 (Go)](https://junglewise.ai/threats/technologies/github-com-traefik-traefik-v2), [github.com/traefik/traefik/v3 (Go)](https://junglewise.ai/threats/technologies/github-com-traefik-traefik-v3), [Traefik](https://junglewise.ai/threats/technologies/traefik). Vendors: [Go](https://junglewise.ai/threats/vendors/go), [Traefik](https://junglewise.ai/threats/vendors/traefik).

## Quick answers

**Is CVE-2026-88004 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-88004 being exploited in the wild.

**How severe is CVE-2026-88004?** High severity, with a CVSS score of 7.4, published 10 September 2026.

**Which products and versions are affected?** github.com/traefik/traefik (Go) SEMVER:introduced=0; github.com/traefik/traefik/v2 (Go) SEMVER:introduced=0; github.com/traefik/traefik/v3 (Go) >= 3.2.0, < 3.7.13, SEMVER:introduced=3.2.0,fixed=3.7.13.

**Is there a fix for CVE-2026-88004?** Yes. github.com/traefik/traefik/v3 (Go): upgrade to 3.7.13 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-88004 attacked?** It is attacked over the network, in a web application.

**Am I affected by CVE-2026-88004?** Check whether you run github.com/traefik/traefik (Go), github.com/traefik/traefik/v2 (Go), github.com/traefik/traefik/v3 (Go) or one of the others listed at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## How to fix

- github.com/traefik/traefik/v3 (Go): upgrade to 3.7.13 or later

  ```sh
  go get github.com/traefik/traefik/v3@v3.7.13
  ```


## Technical details

The vulnerability is a header-name sanitization bypass in Traefik v3's custom reverse proxy (`pkg/proxy/httputil`). All entrypoint sanitization handlers—`removeAliasingHeaders`, `rejectAliasingHeaders`, `removeHeadersWithUnderscores`, `rejectHeadersWithUnderscores`, and `DeleteXForwardedHeaders`—iterate only `req.Header` and ignore `req.Trailer`, despite doc comments claiming coverage of both. Unauthenticated clients can declare HTTP trailers containing aliasing names (e.g., `X_Auth_User`) or trusted names (e.g., `X-Forwarded-Prefix`). Go's HTTP server pre-fills trailer names in `req.Trailer` before the handler runs, making them invisible to entrypoint checks. On bare proxy paths, trailer values are dropped (limiting impact to name-level); however, when body-buffering middleware (retry with `status` codes or the `buffering` middleware) reads the body first, trailer values are populated before the proxy clone, allowing attacker-controlled values to reach the backend. Backends that merge trailers into header namespaces (e.g., pre-fix libevent per CVE-2026-63379) then trust and act on the smuggled headers for authorization decisions. Traefik v2 is unaffected because it uses Go's standard-library `httputil.ReverseProxy`, which does not forward request trailer values. The fix stops forwarding request trailer values to backends while preserving trailer names as permitted by RFC 9110 §6.6.2.

## Affected products

- Traefik Traefik v3.2.0 through v3.7.12

## Identifiers

- CVE-2026-88004
- GHSA-v67p-phpq-fc8x
- GO-2026-6467

## Timeline

- 2026-09-07: disclosed
- 2026-09-10: advisory
- 2026-09-10: patched: Traefik v3.7.13 released with fix; v3.2–v3.6 end-of-life, users must upgrade to v3.7.13

## References

- [https://api.github.com/users/bipol4r](https://api.github.com/users/bipol4r)
- [https://github.com/bipol4r](https://github.com/bipol4r)
- [https://api.github.com/users/bipol4r/gists%7B/gist_id%7D](https://api.github.com/users/bipol4r/gists%7B/gist_id%7D)
- [https://api.github.com/users/bipol4r/repos](https://api.github.com/users/bipol4r/repos)
- [https://avatars.githubusercontent.com/u/109329336?v=4](https://avatars.githubusercontent.com/u/109329336?v=4)
- [https://api.github.com/users/bipol4r/events%7B/privacy%7D](https://api.github.com/users/bipol4r/events%7B/privacy%7D)

## Related threats

- [Traefik ForwardAuth connection pool identity leak via NTLM/Negotiate](https://junglewise.ai/threats/cve-2026-121966-traefik-2-x-before-2-11-60-and-3-x-before-3-7-16-sends)
- [Traefik FastProxy data race in HTTP/1 upgrades](https://junglewise.ai/threats/cve-2026-119574-traefik-from-3-2-0-before-3-7-15-contains-a-data-race-in-the)
- [Traefik authentication bypass in TLS options resolution](https://junglewise.ai/threats/cve-2026-114656-traefik-before-2-11-58-and-3-7-13-contains-an-authentication)
- [Traefik NTLM/Kerberos TLS configuration bypass](https://junglewise.ai/threats/cve-2026-114642-traefik-2-11-0-before-2-11-58-and-3-0-0-before-3-7-14-stores-its)
- [Traefik authentication bypass in backend connection pool](https://junglewise.ai/threats/cve-2026-114639-traefik-2-11-0-before-2-11-58-and-3-0-0-before-3-7-14-contains)