CVE-2026-16267: Newsletters WordPress Plugin RCE Flaw

CVE-2026-16267 Overview

CVE-2026-16267 is a PHP object injection vulnerability in the Newsletters WordPress plugin versions before 4.16. The plugin fails to restrict allowed classes when calling unserialize() on values taken from a public form submission. Unauthenticated remote attackers can inject arbitrary PHP objects into the application context. When combined with a suitable gadget chain present in WordPress core, another plugin, or the theme, this can lead to remote code execution, arbitrary file operations, or data exfiltration. The issue is tracked under CWE-502: Deserialization of Untrusted Data.

Critical Impact
Unauthenticated attackers can inject arbitrary PHP objects through a public form, enabling potential remote code execution when a gadget chain is available.

Affected Products

Newsletters WordPress plugin versions prior to 4.16
WordPress sites exposing the plugin's public subscription form
Any hosting environment where the plugin processes untrusted form input

Discovery Timeline

2026-08-08 - CVE-2026-16267 published to NVD
2026-08-11 - Last updated in NVD database

Technical Details for CVE-2026-16267

Vulnerability Analysis

The Newsletters plugin accepts input from a public-facing form and passes at least one field through PHP's unserialize() function without enforcing an allowed_classes allowlist. PHP object injection occurs when attacker-controlled serialized data instantiates arbitrary classes available in the runtime. Instantiation triggers magic methods such as __wakeup(), __destruct(), and __toString() on those objects. If any loaded class implements a usable POP (Property-Oriented Programming) gadget, the attacker can pivot from object instantiation to file writes, SQL execution, or command execution.

Because the entry point is an unauthenticated public form, no credentials or user interaction beyond a crafted HTTP request are required. Attack complexity remains elevated because successful exploitation depends on the presence of a reachable gadget chain in the target's PHP class graph.

Root Cause

The root cause is the use of unserialize() on untrusted input without the ['allowed_classes' => false] option introduced in PHP 7. This coding pattern is the canonical example of [CWE-502]. The plugin trusts serialized data submitted through a form field and reconstructs PHP objects from it, giving attackers indirect control over object lifecycle callbacks.

Attack Vector

The attack vector is a network-based HTTP request to the plugin's public form endpoint. The attacker submits a form payload containing a serialized PHP object graph in the vulnerable field. On processing, the plugin calls unserialize() and instantiates the attacker's objects. When those objects go out of scope or interact with the request handler, their magic methods fire and execute the gadget chain. Refer to the WPScan Vulnerability Report for technical detail on the affected parameter.

Detection Methods for CVE-2026-16267

Indicators of Compromise

POST requests to Newsletters plugin form endpoints containing serialized PHP markers such as O:, a:, or s: followed by class or property counts
Unexpected PHP error log entries referencing __wakeup, __destruct, or unserialize around form submission timestamps
New or modified PHP files under wp-content/ immediately after subscription form activity
Outbound network connections from the web server process to unfamiliar hosts following form submissions

Detection Strategies

Inspect web server access logs for form submissions where request bodies contain O: followed by a quoted class name, a strong indicator of a serialized object payload
Enable and review WordPress plugin debug logging for unserialize() warnings triggered by malformed class references
Use a web application firewall rule to flag serialized PHP object patterns in POST bodies destined for /wp-admin/admin-ajax.php or plugin form handlers

Monitoring Recommendations

Alert on file integrity changes in wp-content/plugins/, wp-content/uploads/, and theme directories
Monitor the wp_options table for unexpected serialized values written after form submissions
Track outbound egress from PHP-FPM or Apache worker processes and alert on connections to non-corporate destinations

How to Mitigate CVE-2026-16267

Immediate Actions Required

Update the Newsletters WordPress plugin to version 4.16 or later on every affected site
Audit web server and application logs for evidence of serialized payloads submitted to the plugin's form endpoints
If update is not immediately possible, disable the Newsletters plugin until patching completes

Patch Information

Upgrade to Newsletters plugin version 4.16 or newer. The fix restricts the classes accepted during deserialization of form input. See the WPScan Vulnerability Report for the fixed version reference.

Workarounds

Restrict access to the plugin's public form endpoints using a web application firewall rule that blocks request bodies containing serialized PHP object signatures
Deploy a virtual patch that strips or rejects fields matching /^O:\d+:"/ in submissions to Newsletters endpoints
Remove the plugin's public subscription form from the site until the update is applied

bash

# Example WAF rule pattern (ModSecurity) blocking PHP object injection on subscribe endpoints
SecRule REQUEST_URI "@contains newsletters" \
    "chain,phase:2,deny,status:403,id:1602616267,msg:'CVE-2026-16267 PHP object injection attempt'"
    SecRule REQUEST_BODY "@rx O:\d+:\"[A-Za-z_\\\\]+\"" "t:none,t:urlDecodeUni"