External risk intelligence

WordPress User Session Synchronizer Plugin Authentication Bypass

CVE advisorySeverity: CRITICAL (CVSS 9.8)

CVE-2026-15341

The vulnerability exists in a WordPress plugin that executes code on every request to the site. Because WordPress is most commonly deployed as a public-facing web application, and the vulnerable function is part of the standard request lifecycle, the attack surface is exposed to the internet by design.

Authentication Bypass

Halo Surface Signal: 4 out of 5 — likely to be public-facing.

External exposure likelihood

Horizon Alert

Summary of the vulnerability and why it matters

A critical vulnerability has been identified in the User Session Synchronizer plugin for WordPress, affecting all versions up to 1.4.0. This flaw allows unauthenticated attackers to bypass authentication and take over any user account, including administrative ones, by exploiting a flaw in how session data is handled. The primary concern is confirming relevance and exposure given the plugin's function and the nature of the vulnerability.

Unauthenticated users can take over accounts.
High impact vulnerability affecting WordPress.
Confirm relevance and assess potential exposure.

Attack Path

How an attacker could exploit the issue

An unauthenticated attacker can bypass security checks by sending a specially crafted request to the User Session Synchronizer plugin. This request manipulates session synchronization parameters, leading to predictable encryption keys and an empty referer allowlist. The vulnerability allows an attacker to forge authentication cookies for any user, including administrators, granting them full control of the site.

No prior access or authentication needed.
Crafted request exploits session sync.
Complete account takeover risk.

Live Threat

Current exploitation, exposure, and threat context

This vulnerability could allow an unauthenticated attacker to bypass authentication and take over any user account, including administrator accounts, on a WordPress site using the User Session Synchronizer plugin. This is possible because the plugin does not properly validate the parameters used in session synchronization requests, allowing an attacker to craft a request that grants them authenticated access.

User account access could be compromised.
Attackers can send crafted requests.
Full administrative control may be gained.

Operational Fix

Recommended remediation, mitigation, and detection steps

System owners and application teams are responsible for addressing this critical vulnerability in the User Session Synchronizer plugin for WordPress. The first step is to identify all WordPress instances utilizing this plugin, determine their exposure to the internet, and confirm which business-critical applications rely on them. This will help prioritize remediation efforts and engage the correct accountable owners.

Identify plugin deployment and owner.
Verify external reachability and business impact.
Plan vendor coordination and patching.

Supplementary metadata

CVSS vector

Validate whether this threat affects your internet-facing exposure.

Halo Threat Intelligence helps prioritize remediation with Halo Surface Signal and H/A/L/O context. Start exposure validation with a free external attack surface trial.

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Frequently asked questions

What is the WordPress User Session Synchronizer plugin?

This plugin is a component designed to manage and sync user sessions across WordPress installations. It handles the verification and transmission of session data to ensure users stay logged in as they navigate. Because it interacts with the standard WordPress request lifecycle, it processes incoming data on every page load to maintain these session states.

What does CWE-287 mean for CVE-2026-15341?

CWE-287 refers to Improper Authentication. In this specific case, the plugin fails to verify the identity of the person requesting a session sync. Because the plugin does not check for required security tokens or permissions, an attacker can manipulate specific data inputs to trick the software into granting them a valid session, effectively skipping the login process entirely.

How can an attacker trigger this vulnerability?

An attacker sends a specially crafted web request to the server containing specific parameters. The bug is triggered when the plugin attempts to process these parameters against an unregistered or missing configuration. It is important to note that this flaw does not require the attacker to have existing login credentials or any prior access to the site to successfully forge the authentication cookies.

Is my site at risk according to Halo Surface Signal?

Halo Surface Signal indicates the risk is high because this plugin executes code on every incoming request. Since WordPress sites are typically exposed to the internet by design, the vulnerability is reachable by anyone online. If your site is accessible via the public web, this plugin's presence creates an entry point for potential unauthorized access.

What should I do if I run this plugin?

First, audit your environment to identify all WordPress instances where the User Session Synchronizer plugin is currently active. Determine which of these sites are reachable from the internet and prioritize those handling sensitive or administrative data. Once identified, coordinate with your team to remove or disable the plugin until a secure version is available.

References

Written by Nicholas Merritt

Nicholas Merritt is VP of Security Solutions at Halo Security. With more than 25 years across application security, vulnerability prioritization, and offensive security, he helps teams translate threat data into practical exposure validation and remediation focus. He authors Halo Threat Intelligence CVE advisories using Halo Surface Signal and H/A/L/O context to prioritize internet-facing risk.