CVE-2026-15341 in User Session Synchronizer Plugin
Summary
by MITRE • 08/15/2026
The User Session Synchronizer plugin for WordPress is vulnerable to Authentication Bypass leading to Account Takeover in all versions up to, and including, 1.4.0. The `synchronize_session()` function, hooked on `init` and therefore executed on every request, performs no nonce, capability, or shared-secret validation against the attacker-supplied `ussync-key`, `ussync-token`, and `ussync-ref` parameters; when `ussync-key` references an unregistered slot, `get_option()` returns `false` for both the secret key and the domain list, causing the AES-256-CBC encryption key to degrade to the fully predictable `md5('')` and the referer allowlist to collapse to an empty-string match, while the AES IV is unconditionally hard-coded as `md5('another-secret')`. This makes it possible for unauthenticated attackers to supply a crafted request encrypting any known or guessable user email address in the `ussync-ref` parameter, causing the handler to call `wp_set_auth_cookie()` for the matched user and granting full authentication as that user — including administrators — with no prior knowledge of site secrets.
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Analysis
by VulDB Data Team • 08/15/2026
The User Session Synchronizer plugin for WordPress presents a critical authentication bypass vulnerability that enables account takeover across all versions up to and including 140. This flaw resides in the synchronize_session() function which executes on every request due to its hook on the init action. The vulnerability stems from the complete absence of security validation mechanisms against attacker-supplied parameters including ussync-key, ussync-token, and ussync-ref. When an attacker provides an unregistered slot reference through ussync-key, the get_option() function returns false for both the secret key and domain list, creating a cascade of predictable security failures that fundamentally undermine the plugin's cryptographic protection.
The technical implementation of this vulnerability exploits predictable cryptographic degradation patterns that render the AES-256-CBC encryption ineffective. When the system fails to retrieve valid configuration data for an unregistered slot, the encryption key automatically reverts to md5(''), a fully predictable value that eliminates any cryptographic security. Additionally, the referer allowlist collapses to an empty-string match due to the false return values, while the AES initialization vector remains hard-coded as md5('another-secret'). These predictable elements work together to create a scenario where attackers can craft malicious requests without needing knowledge of legitimate site secrets or configuration parameters.
The operational impact of this vulnerability extends to complete administrative compromise across affected WordPress installations. An unauthenticated attacker can supply a crafted request containing any known or guessable user email address within the ussync-ref parameter, allowing the system to execute wp_set_auth_cookie() for the matched user account. This authentication bypass grants attackers full access privileges including administrator rights without requiring prior knowledge of site secrets, configuration parameters, or legitimate credentials. The vulnerability affects all user roles from standard subscribers to privileged administrators, making it particularly dangerous for sites where administrative accounts are targeted.
This vulnerability aligns with CWE-287 Authentication Bypass and maps directly to ATT&CK technique T1078 Valid Accounts, representing a critical weakness in WordPress plugin security architecture. The flaw demonstrates poor input validation and insufficient access control mechanisms that should be implemented at multiple layers of the authentication system. Organizations using this plugin face immediate risk of unauthorized access, data theft, privilege escalation, and potential full system compromise. The vulnerability's exploitation requires no specialized knowledge beyond basic understanding of the WordPress plugin architecture and standard web application attack patterns.
Recommended mitigations include immediate removal or deactivation of the affected plugin versions, implementation of network-level protections such as firewall rules blocking malicious request patterns, and enforcement of strong authentication measures including multi-factor authentication. Additionally, organizations should conduct comprehensive security audits of all installed plugins to identify similar vulnerabilities in other third-party components, implement proper input validation and parameter sanitization, and establish monitoring protocols for suspicious authentication attempts. The vulnerability highlights the critical importance of proper cryptographic implementation and access control verification in web application security, particularly within content management systems where plugin vulnerabilities can have cascading effects on entire site security postures.