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CVE-2026-45063 – Authentication Bypass – Symfony X509Authenticator (prior to 5.4.52 / 6.4.40 / 7….

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Summary

CVE-2026-45063 is a critical authentication bypass (identity spoofing) vulnerability in the X509Authenticator component of the Symfony PHP framework, rated CVSS 4.0: 9.1 (Critical). An attacker who can obtain a certificate signed by a CA trusted by the target application can craft the certificate’s CN field to embed a victim’s email address, causing Symfony to authenticate the attacker as the victim. All Symfony applications using mTLS client-certificate authentication via X509Authenticator and running versions prior to 5.4.52, 6.4.40, 7.4.12, or 8.0.12 are affected.

Technical details

  • Root cause: X509Authenticator extracts the user identifier from $_SERVER['SSL_CLIENT_S_DN'] (the client certificate’s Subject Distinguished Name) using a regex that matches emailAddress= at any position in the DN string. Because the regex is unanchored, it matches inside the value of other RDN fields — not just at proper RDN boundaries (CWE-290: Authentication Bypass by Spoofing).
  • Trigger conditions: The application must be configured to use Symfony’s X509Authenticator for mutual TLS (mTLS) client-certificate authentication. This is a non-default, explicitly configured security component.
  • Attack vector: A network-accessible attacker obtains a certificate issued by any CA trusted by the target application with a crafted CN value, e.g., CN=Attacker [email protected]. When this certificate is presented during the TLS handshake, the unanchored regex extracts [email protected] from within the CN value and authenticates the attacker as that victim user rather than matching the legitimate emailAddress= RDN field.
  • Impact: Complete authentication bypass enabling full impersonation of arbitrary users registered on the application, leading to unauthorized access to confidential data (VC:H) and unauthorized modification of application state (VI:H).

Affected software

  • symfony/security-http and symfony/symfony:
    • All versions prior to 5.4.52 (5.4.x branch)
    • Versions 6.0.0 through 6.4.39 (6.4.x branch)
    • Versions 7.0.0 through 7.4.11 (7.4.x branch)
    • Versions 8.0.0 through 8.0.11 (8.0.x branch)

Severity

CVSS 4.0 Base Score: 9.1 (Critical)
Vector: CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N

Note: A CVSS v3.1 score has not been published for this CVE. The official CVSS 4.0 score is 9.1. The AT:P (Attack Requirements: Present) qualifier reflects the prerequisite of holding a CA-trusted certificate; this does not lower the critical severity designation.

Mitigation and recommended actions

  • Immediate action — upgrade to a patched version:

    • Symfony 5.4.52 (5.4.x branch)
    • Symfony 6.4.40 (6.4.x branch)
    • Symfony 7.4.12 (7.4.x branch)
    • Symfony 8.0.12 (8.0.x branch)

    The fix anchors the emailAddress= extraction regex to RDN boundaries (start of string or following a , / / separator), ensuring the pattern only matches a legitimate emailAddress RDN and cannot be satisfied by content embedded in another RDN’s value.

  • If immediate patching is not possible: No official workarounds have been documented by the Symfony security team. Organizations should prioritize patching and consider restricting access to mTLS-protected endpoints at the network perimeter until the upgrade can be applied.

IONIX Status

The IONIX research team is tracking ongoing exploitation attempts and recommends immediate patching. Potentially affected assets are outlined in this post.

References

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