critical CVSS 10 EPSS 0.36%

CVE-2026-48491·Traefik vulnerability

Traefik 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.

Severity
critical
Software
Traefik
Fixed in
3.7.3
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 0.36% — higher than 27% 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-2026-48491 scores 10 on CVSS, so a successful attack would be serious — but at 0.36%, 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: 3.7.3

Fixed in: 3.7.3

How to fix this CVE

Upgrade Traefik to version 3.7.3 or later immediately to patch the SNICheck bypass vulnerability that allows attackers to circumvent mutual TLS authentication on wildcard-protected routes. This vulnerability permits unauthenticated clients to reach backend services requiring client certificates by exploiting inconsistent TLS option resolution between exact and wildcard domain matches. Restart Traefik after the upgrade to apply the security fix.

sudo dnf update traefik

Defensia detects this vulnerability

What an exploitation attempt looks like

Sample log line indicative of exploitation attempts:

POST|GET.*Host:.*\.example\.com.*\s+ followed by TLS handshake completion without client certificate verification errors; look for access logs showing successful requests to wildcard-protected backends that should have been rejected due to missing mandatory client certificates

How to check if you are affected

  1. Run 'traefik version' or check the binary version with 'traefik --version' to confirm the installed version is below 3.7.3
  2. Review your Traefik configuration (traefik.yml or dynamic configs) to identify any wildcard router rules (Host(*.domain.com)) combined with TLSOptions requiring client certificates
  3. Examine Traefik access logs for requests to wildcard-protected backends that lack expected client certificate validation errors or TLS handshake failures
  4. After upgrading to 3.7.3, verify the fix with 'traefik version' and restart Traefik with 'sudo systemctl restart traefik' to confirm the patched version is running

FAQ

What is CVE-2026-48491?

This vulnerability affects Traefik's SNICheck mechanism, which fails to apply wildcard domain matching when resolving strict TLS requirements. An attacker can bypass mutual TLS authentication by completing a TLS handshake under permissive options and then sending an HTTP Host header targeting a wildcard-protected backend that requires client certificates.

Is CVE-2026-48491 being actively exploited?

No, CVE-2026-48491 is not listed in the CISA KEV (Known Exploited Vulnerabilities) catalog and no public exploits are currently available, but the critical CVSS 10.0 score indicates organizations should prioritize patching immediately.

What versions of Traefik are affected by CVE-2026-48491?

Traefik versions 3.7.0 through 3.7.2 are vulnerable; version 3.7.3 and later contain the fix.

How do I check if my server is vulnerable to CVE-2026-48491?

Run 'traefik version' and check if the output shows a version between 3.7.0 and 3.7.2. If you're running one of these versions with wildcard Host rules and TLSOptions requiring client certificates (RequireAndVerifyClientCert), your server is vulnerable.

Does Defensia detect CVE-2026-48491?

Yes — Defensia's CVE advisory scanner compares installed package versions against the NVD database. If Traefik is installed on a monitored server, CVE-2026-48491 will appear in your dashboard with remediation steps.

Related Traefik CVEs

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-2026-54763CVSS 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-2026-48020CVSS 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.
CVE-2026-85595CVSS 9.8Traefik versions before v2.11.55 and versions v3.0.0 through v3.7.10 contain an authentication bypass vulnerability in the digestAuth middleware where unknown usernames receive an empty secret instead of rejection. Attackers can compute a valid digest response using the empty secret and arbitrary credentials to bypass authentication on any digestAuth-protected route without a valid username or password.

References

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