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CVE-2026-15341 – Unauthenticated Authentication Bypass / Account Takeover – User Session Synchronize

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Summary

CVE-2026-15341 is a critical unauthenticated authentication bypass vulnerability in the User Session Synchronizer WordPress plugin, allowing full account takeover including of administrator accounts. The plugin’s synchronize_session() function fails to validate any nonce, capability, or shared-secret against attacker-supplied parameters, letting an unauthenticated attacker impersonate any user. The flaw carries a CVSS v3.1 base score of 9.8 (Critical).

Technical details

  • Root cause: synchronize_session(), which runs on every request via the init hook, does not validate a nonce, user capability, or shared secret against the ussync-key, ussync-token, and ussync-ref request parameters supplied by the client.
  • Trigger conditions: when a request references an unregistered synchronization slot, the plugin’s encryption key degrades to a predictable value and its referrer allowlist check becomes ineffective, removing the remaining protections against forged requests.
  • Attack vector: network, unauthenticated, no user interaction required.
  • Impact: an attacker can craft a request that authenticates the session as any WordPress user, including administrators, resulting in full account takeover and site compromise.

Affected software

  • User Session Synchronizer (WordPress plugin) by rafasashi, versions up to and including 1.4.0

Severity

  • CVSS v3.1 Base Score: 9.8 (Critical)
  • Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

Mitigation and recommended actions

  • Immediate: No patched version has been released by the plugin author at the time of disclosure. The plugin is closed on the WordPress.org plugin repository.
  • Recommended action: Deactivate and remove the User Session Synchronizer plugin from all WordPress installations immediately, since no fixed version currently exists.
  • If the plugin cannot be removed immediately: block or filter requests containing the ussync-key, ussync-token, and ussync-ref parameters at the web application firewall / reverse proxy layer, and monitor authentication logs for unexpected administrator logins.

References

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