# CVE-2026-88877: Traefik authentication bypass in ingress-nginx provider

Source: https://junglewise.ai/threats/cve-2026-88877-traefik-authentication-bypass-in-ingress-nginx-provider  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-09-10. Updated: 2026-09-26  
License: CC BY 4.0 with a link back

CVE-2026-88877 · Severity: critical · CVSS 9.8 · Published 2026-09-10

Traefik is a widely-used HTTP reverse proxy and load balancer for Kubernetes. In affected versions, a specific annotation combination causes the system to create an extra route that bypasses all security middlewares, including authentication (BasicAuth, IP allowlisting) and forwards requests directly to protected backends. An attacker can exploit this by sending requests with malformed port numbers in the Host header, completely circumventing login requirements and access controls.

Technologies: [Traefik](https://junglewise.ai/threats/technologies/traefik). Vendors: [Traefik](https://junglewise.ai/threats/vendors/traefik).

## Quick answers

**Is CVE-2026-88877 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-88877 being exploited in the wild.

**How severe is CVE-2026-88877?** Critical severity, with a CVSS score of 9.8, published 10 September 2026.

**Which products and versions are affected?** Traefik >= v3.7.0, <= v3.7.11.

**Is there a fix for CVE-2026-88877?** A patch was published on 2026: Fixed in v3.7.12 Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-88877 attacked?** It is attacked over the network, in a hosted service.

**Am I affected by CVE-2026-88877?** Check whether you run Traefik 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.

## Technical details

The vulnerability is an authorization bypass in Traefik's Kubernetes ingress-nginx provider. When an Ingress resource carries both an authentication annotation (e.g., basic-auth) and the nginx.ingress.kubernetes.io/from-to-www-redirect annotation, the provider creates a "sibling" router that matches only on the host with a RedirectRegex middleware but points to the same protected backend service. RedirectRegex is not a terminal handler, so unmatched requests pass through to the backend. The redirect pattern only accepts numeric ports (regex: (https?)://[^/:]+(:[0-9]+)?/(.*)), but Traefik's host matcher canonicalizes authorities via net.SplitHostPort, which accepts non-numeric ports. A request with a malformed port (e.g., Host: www.example.com:x) bypasses the redirect pattern, selects the sibling router lacking all middlewares, and reaches the protected backend unauthenticated. This affects all annotation-derived middlewares: authentication, source-IP allowlisting, and others. Versions >= v3.7.0 and <= v3.7.11 are vulnerable; the fix is available in v3.7.12.

## Affected products

- Traefik Traefik >= v3.7.0, <= v3.7.11

## Timeline

- 2026-09-10: disclosed: CVE-2026-88877 published
- 2026: patched: Fixed in v3.7.12

## References

- [https://github.com/traefik/traefik/security/advisories/GHSA-cjr6-pf59-jq29](https://github.com/traefik/traefik/security/advisories/GHSA-cjr6-pf59-jq29)
- [https://www.vulncheck.com/advisories/traefik-3.7.0-authentication-bypass-via-from-to-www-redirect](https://www.vulncheck.com/advisories/traefik-3.7.0-authentication-bypass-via-from-to-www-redirect)

## 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)