critical CVSS 9.8 EPSS 0.65%

CVE-2026-88877·Traefik vulnerability

Traefik is a HTTP reverse proxy and load balancer. In versions >= v3.7.0 and <= v3.7.11, the Kubernetes ingress-nginx provider mishandles Ingresses that carry both an authentication annotation and the nginx.ingress.kubernetes.io/from-to-www-redirect annotation. For such Ingresses the provider creates an additional 'sibling' router that matches on the host alone, carries only the RedirectRegex middleware, and still points at the parent router's protected backend service. Because RedirectRegex is not a terminal handler, a request its pattern does not match is forwarded to the backend, and because the redirect pattern only accepts a numeric port while Traefik's host matcher canonicalizes the authority via net.SplitHostPort, a request with a non-numeric or empty port (for example 'Host: www.example.com:x') selects the sibling router, misses the redirect, and is proxied to the protected backend with none of the Ingress's annotation-derived middlewares applied. This discards not only authentication (e.g. BasicAuth) but every annotation-derived middleware, including source-IP allowlisting. Traefik v2 and v3 releases before v3.7.0 are not affected. The issue is fixed in v3.7.12.

Severity
critical
Software
Traefik
Fixed in
3.7.12
Published

How likely is exploitation

FIRST's Exploit Prediction Scoring System puts the probability of this CVE being exploited in the next 30 days at 0.65% — higher than 50% of all published CVEs. EPSS is an independent forecast of exploitation, not a severity rating: it says nothing about how much damage a successful attack does, which is what a CVSS score measures.

Read the two together. CVE-2026-88877 scores 9.8 on CVSS, so a successful attack would be serious — but at 0.65%, almost nobody is currently attempting it. That is a different situation from a flaw under mass exploitation, and it is usually the difference between patching on your next maintenance window and patching tonight.

Affected versions

From: 3.7.0

Until: 3.7.12

Fixed in: 3.7.12

How to fix this CVE

Upgrade Traefik immediately to version 3.7.12 or later to patch an authentication bypass vulnerability affecting the Kubernetes ingress-nginx provider. The vulnerability allows unauthenticated access to protected backends when specific annotation combinations are used. Organizations running Traefik v3.7.0 through v3.7.11 with Kubernetes Ingresses should prioritize this update to restore proper middleware chain enforcement.

sudo dnf update traefik

Defensia detects this vulnerability

What an exploitation attempt looks like

Sample log line indicative of exploitation attempts:

GET|POST|HEAD|OPTIONS|PATCH|DELETE .* HTTP/1\.1\r\nHost: [^:]+:[^0-9]+|GET|POST .* HTTP/1\.1\r\nHost: [^:]+$(?=.*Authorization|.*X-Original-Forwarded-For)

WAF mitigation (if patching is not yet possible)

Add this rule to your WAF to block exploitation attempts while you schedule the patch. 

Implement WAF rules to reject HTTP requests with malformed Host headers containing non-numeric ports (e.g., Host: example.com:abc or Host: example.com:x). Use ModSecurity rule: SecRule REQUEST_HEADERS:Host "@rx ^[^:]+:[^0-9]+$" "id:1000,phase:2,deny,status:400,msg:'Non-numeric port in Host header'"

How to check if you are affected

  1. Check installed Traefik version: traefik version or curl http://localhost:8080/ping (check headers) or docker inspect <container> | grep -i version
  2. Verify Kubernetes Ingress configuration: kubectl get ingress -A -o yaml | grep -E '(authentication|from-to-www-redirect)' to identify potentially vulnerable ingress definitions
  3. Search Traefik access logs for requests matching non-numeric ports or malformed Host headers (e.g., 'Host: example.com:x' or 'Host: example.com:invalid'): grep -E 'Host:[^ ]+:[^0-9]' /path/to/traefik/logs.txt
  4. Verify patch application: traefik version should report v3.7.12 or higher; cross-check with kubectl get pod <traefik-pod> -o jsonpath='{.spec.containers[0].image}'

FAQ

What is CVE-2026-88877?

This is a critical authentication bypass in Traefik v3.7.0-3.7.11 where misconfigured Kubernetes Ingresses with both authentication and www-redirect annotations create an unprotected sibling router. Requests with non-numeric port numbers bypass the authentication middleware entirely, exposing protected backends.

Is CVE-2026-88877 being actively exploited?

No, this vulnerability is not currently listed in the CISA Known Exploited Vulnerabilities catalog and no public exploits have been disclosed, but it remains critical due to its ease of exploitation and high impact.

What versions of Traefik are affected by CVE-2026-88877?

Traefik versions 3.7.0 through 3.7.11 are affected when using the Kubernetes ingress-nginx provider. v3.6.x and earlier, as well as v3.7.12 and later, are not vulnerable.

How do I check if my server is vulnerable to CVE-2026-88877?

Run 'traefik version' or check your container image tag. If the output shows v3.7.0 to v3.7.11, your deployment is vulnerable. Additionally, query your Kubernetes cluster for Ingresses using both 'auth' and 'from-to-www-redirect' annotations: kubectl get ingress -A -o json | jq '.items[] | select(.metadata.annotations | keys[] | contains("auth")) | select(.metadata.annotations | keys[] | contains("from-to-www-redirect"))'

Does Defensia detect CVE-2026-88877?

Yes — Defensia's CVE advisory scanner compares installed package versions against the NVD database. If Traefik is installed on a monitored server, CVE-2026-88877 will appear in your dashboard with remediation steps.

Related Traefik CVEs

CVE-2026-48491CVSS 10Traefik is an HTTP reverse proxy and load balancer. From 3.7.0 until 3.7.3, there is a high severity vulnerability in Traefik's domain-fronting protection (SNICheck) that allows an unauthenticated client to bypass mutual TLS enforced through wildcard router TLSOptions. When a router uses a wildcard host rule such as Host(*.example.com) with stricter TLS options (for example RequireAndVerifyClientCert), SNICheck resolves the TLS options for the HTTP Host header using exact map lookups only and never applies wildcard matching. If another permissive SNI is served on the same entrypoint, an attacker can complete the TLS handshake under the permissive options and then send an HTTP Host header targeting the wildcard-protected backend, reaching it without presenting a client certificate. This affects the regular HTTPS / HTTP-2 path and does not require HTTP/3. This vulnerability is fixed in 3.7.3.
CVE-2026-54763CVSS 10Traefik is an HTTP reverse proxy and load balancer. Prior to v2.11.51, v3.6.22, and v3.7.6, Traefik's BasicAuth, DigestAuth, and ForwardAuth middlewares strip canonical-cased spoofed identity headers before writing Traefik's own value, but do not account for underscore-variant header names, which many backends normalize identically to dashed forms. An attacker able to reach a protected route can inject an underscore-variant header that survives Traefik's stripping and reaches the backend alongside, or on the unauthenticated ForwardAuth authResponseHeaders path instead of, the value Traefik intended to set, spoofing identity or authorization context. This issue is fixed in versions v2.11.51, v3.6.22, and v3.7.6.
CVE-2026-53622CVSS 10Traefik is an HTTP reverse proxy and load balancer. Versions prior to 3.7.3, 3.6.18, and 2.11.51 have a critical vulnerability in Traefik's HTTP/3 (QUIC) TLS configuration selection that allows unauthenticated clients to bypass router-specific mTLS enforcement. When HTTP/3 is enabled on an entrypoint, the TLS handshake selects the applicable TLS configuration through an exact, case-sensitive lookup on the SNI value, which fails to match wildcard host patterns (e.g., *.example.com) or case variants of the configured hostname. Because the handshake falls back to the default TLS configuration — which may not require client certificates — a client can complete the QUIC handshake without presenting a certificate, while the subsequent HTTP routing layer still dispatches the request to a backend protected by a router-specific mTLS policy. The issue affects deployments where HTTP/3 is enabled, a router uses a wildcard Host rule or case-insensitive hostname matching, a router-specific TLSOptions enforces client certificate authentication, and UDP access to the entrypoint is reachable by an attacker. This vulnerability is fixed in versions 3.7.3, 3.6.18, and 2.11.51.
CVE-2026-48020CVSS 10Traefik is an HTTP reverse proxy and load balancer. Prior to 2.11.48, 3.6.19, and 3.7.3, there is a high severity vulnerability in Traefik's StripPrefix middleware that allows an unauthenticated attacker to bypass route-level authentication and authorization. When a public router matches on a PathPrefix rule and applies the StripPrefix middleware, a request path containing .. or its percent-encoded form %2e%2e can match the public route at routing time and then, after the prefix is stripped and the path is normalized, resolve to a path served by a separate, authenticated router. As a result, an attacker can reach protected backend paths — such as admin or internal configuration endpoints — without satisfying the authentication middleware attached to the protected router. This vulnerability is fixed in 2.11.48, 3.6.19, and 3.7.3.
CVE-2026-44774CVSS 9.9Traefik is an HTTP reverse proxy and load balancer. Prior to 2.11.46, 3.6.17, and 3.7.1, Traefik's Kubernetes Gateway API provider allows a tenant with HTTPRoute creation permissions to expose the REST provider handler, bypassing the providers.rest.insecure=false setting. The Gateway provider accepts any TraefikService backend reference whose name ends with @internal, making it possible to route traffic to rest@internal in addition to the intended api@internal. In shared Gateway deployments where the REST provider is enabled, this allows a low-privileged actor to gain live dynamic configuration write access to Traefik, enabling unauthorized reconfiguration of routers and services. This vulnerability is fixed in 2.11.46, 3.6.17, and 3.7.1.

References

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