CVE-2026-114640 in Traefik
Summary
by MITRE • 10/23/2026
Traefik from v3.7.0 before v3.7.14 contains an authentication bypass vulnerability in its Kubernetes Ingress-NGINX provider that forwards client-supplied Ssl-Client-* headers on the plaintext HTTP router. When ssl-redirect is disabled alongside auth-tls-pass-certificate-to-upstream, unauthenticated attackers can send Ssl-Client-Verify: SUCCESS with a chosen certificate subject and issuer to impersonate mTLS identities.
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Analysis
by VulDB Data Team • 10/23/2026
The vulnerability identified in Traefik versions prior to 3.7.14 represents a critical authentication bypass within the Kubernetes Ingress-NGINX provider, specifically affecting configurations where SSL redirection is disabled while simultaneously enabling the forwarding of TLS client certificates to upstream services. This flaw stems from an improper validation mechanism regarding headers that are typically associated with mutual Transport Layer Security (mTLS) handshakes. Specifically, Traefik forwards client-supplied Ssl-Client-* headers on plaintext HTTP routers without sufficient verification against actual cryptographic evidence or trusted certificate authorities. In a properly secured mTLS environment, the identity of a client is established through a rigorous handshake process where the server validates the client's digital signature using its public key and verifies the certificate chain against known trust anchors. However, in this vulnerable configuration, Traefik treats incoming HTTP headers as authoritative sources for client identity without ensuring they correspond to an actual successful TLS negotiation or valid cryptographic proof.
The operational impact of this flaw is severe, allowing unauthenticated attackers to impersonate any mTLS-protected service identity by simply crafting malicious requests with forged Ssl-Client-* headers. An attacker can inject a header such as Ssl-Client-Verify: SUCCESS along with arbitrary values for the certificate subject and issuer fields. Because Traefik forwards these headers directly to the backend application, the upstream service may erroneously trust the request as coming from an authorized client based solely on the presence of these headers. This effectively nullifies the security guarantees provided by mTLS, which is often used in microservices architectures to ensure strict access control and data integrity between services. Attackers can exploit this to gain unauthorized access to sensitive APIs, exfiltrate confidential data, or perform actions with elevated privileges associated with the impersonated identity.
From a classification perspective, this vulnerability aligns with CWE-287, which describes Improper Authentication, as the system fails to correctly verify the identity of a user before granting access. It also relates to CWE-346, Origin Validation Error, because the application trusts headers that can be easily spoofed by any client capable of sending HTTP requests, rather than deriving trust from secure channel properties or server-side validation. In terms of offensive security frameworks, this technique is consistent with ATT&CK T1078, Valid Accounts, where an adversary uses legitimate credentials or identities to bypass authentication mechanisms. The specific method of forging headers maps closely to techniques involving header injection and manipulation, allowing the attacker to masquerade as a trusted entity within the internal network topology protected by Traefik.
Mitigation strategies must prioritize upgrading to version 3.7.14 or later, where this logic has been corrected to prevent the forwarding of unverified client headers on plaintext routers when SSL redirection is disabled. For environments that cannot immediately upgrade, administrators should ensure that ssl-redirect remains enabled for all routes requiring mTLS authentication, thereby forcing clients through a TLS handshake that provides cryptographic proof of identity before any application-level processing occurs. Additionally, backend services should not rely solely on forwarded Ssl-Client-* headers for critical authorization decisions; instead, they should implement their own validation logic or require explicit token-based authentication mechanisms that are independent of proxy-injected headers. Regular security audits and penetration testing focusing on header manipulation can help identify similar misconfigurations in other reverse proxies or load balancers within the infrastructure.