CVE-2026-16264: Newsletters WordPress Plugin Auth Bypass

CVE-2026-16264 Overview

CVE-2026-16264 is a broken access control vulnerability in the Newsletters WordPress plugin before version 4.18.1. The plugin fails to perform ownership checks on several subscriber management actions and issues a management session to unauthenticated visitors on request. Attackers can read any subscriber's personal data and overwrite any subscriber's record, including the email address. The flaw is classified as [CWE-639] (Authorization Bypass Through User-Controlled Key) and is network-exploitable without authentication or user interaction.

Critical Impact
Unauthenticated attackers can enumerate subscriber personal data and hijack subscriber records by overwriting email addresses on any target site running the vulnerable plugin.

Affected Products

Newsletters WordPress plugin versions prior to 4.18.1
WordPress sites with the Newsletters plugin installed and active
Any deployment exposing the plugin's subscriber management endpoints to the internet

Discovery Timeline

2026-09-23 - CVE-2026-16264 published to the National Vulnerability Database (NVD)
2026-09-23 - Last updated in NVD database

Technical Details for CVE-2026-16264

Vulnerability Analysis

The vulnerability originates in the Newsletters plugin's subscriber management workflow. The plugin exposes actions that operate on subscriber records identified by user-controlled parameters. It does not verify that the requester owns the subscriber record being accessed or modified.

The plugin also issues a management session to unauthenticated visitors on demand. This means an attacker does not need valid credentials or a legitimate subscriber token to reach management functionality. Once a session is granted, the attacker can pivot to any subscriber ID and invoke read or write operations.

The combined effect is full horizontal privilege escalation across the subscriber base. Attackers can enumerate personal data such as names and email addresses, and can overwrite the email address on any record. Overwriting an email redirects future newsletter content and can be chained with password-reset or account-recovery flows on integrated services.

Root Cause

The root cause is missing authorization on subscriber-scoped endpoints, aligned with [CWE-639]. The plugin trusts client-supplied identifiers to select the target subscriber and does not bind the active session to a specific record. The additional design choice of issuing management sessions to anonymous callers removes the only remaining barrier to abuse.

Attack Vector

The attack vector is network-based and requires no authentication or user interaction. An attacker requests a management session from the plugin's public endpoint, then issues subscriber management actions while substituting arbitrary subscriber identifiers. Each substituted identifier returns or modifies the corresponding record. See the WPScan Vulnerability Report for endpoint-level technical details.

Detection Methods for CVE-2026-16264

Indicators of Compromise

Unauthenticated HTTP requests to the Newsletters plugin's subscriber management endpoints from a single source iterating sequential subscriber IDs
Unexpected changes to subscriber email addresses in the plugin's database tables without a corresponding administrator action
Newsletter delivery bounces or complaints from subscribers whose email addresses were silently rewritten
Session cookies issued to clients that never completed a subscriber login or double opt-in

Detection Strategies

Review web server access logs for high-volume GET or POST requests targeting Newsletters plugin action parameters from unauthenticated sources
Audit the plugin's subscriber table for recent UPDATE operations on the email column and correlate with administrative activity
Alert on anonymous requests that receive a management session cookie response from the plugin

Monitoring Recommendations

Enable WordPress audit logging to capture subscriber record modifications with timestamps and source IP addresses
Monitor outbound email volume and bounce rates for anomalies that suggest hijacked subscriber addresses
Retain web application firewall (WAF) logs for the plugin's endpoints for at least 90 days to support retrospective investigation

How to Mitigate CVE-2026-16264

Immediate Actions Required

Upgrade the Newsletters plugin to version 4.18.1 or later on all WordPress installations
Inventory subscriber records and identify email address changes made during the exposure window
Notify affected subscribers if personal data may have been accessed or modified
Rotate any downstream credentials or integrations that rely on subscriber email addresses for verification

Patch Information

The vendor addressed the vulnerability in Newsletters plugin version 4.18.1 by enforcing ownership checks on subscriber management actions and restricting management session issuance. Refer to the WPScan Vulnerability Report for advisory details and fixed version confirmation.

Workarounds

Restrict access to the plugin's subscriber management endpoints using a WAF rule until the patch is applied
Temporarily deactivate the Newsletters plugin on sites that cannot be updated immediately
Block unauthenticated requests to plugin action handlers at the reverse proxy or edge layer

bash

# Example: block anonymous access to Newsletters plugin endpoints via nginx
location ~* /wp-content/plugins/newsletters/ {
    deny all;
    return 403;
}