CVE-2026-78265: The Events Calendar Auth Bypass Vulnerability

CVE-2026-78265 Overview

CVE-2026-78265 is an unauthenticated PHP Object Injection vulnerability affecting The Events Calendar plugin for WordPress in versions up to and including 6.17.2. The flaw is classified under CWE-502: Deserialization of Untrusted Data. Remote attackers can trigger deserialization of attacker-controlled data without authentication, potentially leading to arbitrary code execution when a suitable POP (Property-Oriented Programming) gadget chain is available in the WordPress environment.

Critical Impact
Unauthenticated attackers can send crafted serialized payloads over the network to compromise site confidentiality, integrity, and availability.

Affected Products

The Events Calendar plugin for WordPress, versions <= 6.17.2
WordPress sites running the vulnerable plugin without a Web Application Firewall filtering serialized payloads
Multisite WordPress deployments where the plugin is network-activated

Discovery Timeline

2026-08-24 - CVE-2026-78265 published to the National Vulnerability Database
2026-08-26 - Last updated in NVD database

Technical Details for CVE-2026-78265

Vulnerability Analysis

The vulnerability stems from insecure deserialization of untrusted input inside The Events Calendar plugin. PHP Object Injection occurs when user-supplied data reaches a unserialize() call without validation. An attacker crafts a serialized payload that instantiates existing PHP classes whose magic methods (__wakeup, __destruct, __toString) perform sensitive operations.

Exploitation does not require authentication, and the network attack vector allows delivery through standard HTTP requests. Because The Events Calendar runs inside the WordPress process, a successful gadget chain execution can lead to arbitrary file writes, SQL execution, or remote code execution depending on the loaded plugin and theme code base.

Refer to the Patchstack Vulnerability Report for advisory-level details.

Root Cause

The root cause is deserialization of attacker-controlled input inside plugin request handlers. PHP's unserialize() function reconstructs arbitrary objects from serialized strings, triggering class-defined magic methods during hydration. Passing untrusted data to this function without a strict allowed_classes allow-list violates CWE-502 and enables object injection.

Attack Vector

A remote attacker submits a crafted serialized payload to a plugin endpoint that accepts request parameters, cookies, or form fields containing serialized data. When the plugin unserializes the input, controlled objects instantiate and their magic methods execute using attacker-supplied properties. Chained through gadgets present in WordPress core, other plugins, or Composer dependencies, the injection can escalate to arbitrary code execution.

See the Patchstack advisory for reproduction context. No verified public exploit code is available at time of publication.

Detection Methods for CVE-2026-78265

Indicators of Compromise

HTTP requests to WordPress endpoints containing serialized PHP patterns such as O: (object), a: (array), or s: (string) inside parameter values
Unexpected PHP files written to wp-content/uploads/, wp-content/plugins/, or theme directories following requests to Events Calendar endpoints
Outbound network connections from the PHP-FPM or Apache worker process to unfamiliar hosts
New administrator accounts or modified wp_optionsactive_plugins entries without a corresponding admin session

Detection Strategies

Inspect web server access logs for POST or GET requests to Events Calendar routes containing serialized object markers in parameter values
Deploy WAF signatures that flag PHP serialization syntax in untrusted request fields
Monitor filesystem integrity on the WordPress installation and alert on writes outside expected update windows
Correlate PHP error logs for __wakeup, __destruct, or class-not-found exceptions coinciding with plugin request paths

Monitoring Recommendations

Enable verbose access logging for /wp-admin/admin-ajax.php and /wp-json/tribe/* routes
Forward WordPress and web server logs to a centralized analytics platform for pattern detection
Alert on process creation from PHP interpreter processes spawning shells such as sh, bash, or python
Track plugin version inventory across the estate to identify hosts still running The Events Calendar <= 6.17.2

How to Mitigate CVE-2026-78265

Immediate Actions Required

Identify every WordPress site running The Events Calendar and confirm the installed plugin version
Update The Events Calendar to a patched release above 6.17.2 as published by the vendor
Temporarily disable the plugin on sites that cannot be patched immediately
Rotate WordPress administrator credentials and API keys on any host that shows suspicious activity

Patch Information

Apply the patched version of The Events Calendar as referenced in the Patchstack Vulnerability Report. After upgrading, review site content, uploads, user accounts, and scheduled tasks (wp_cron) for artifacts introduced during any exposure window.

Workarounds

Deploy a Web Application Firewall rule that blocks serialized PHP object markers (O:, C:) in inbound request parameters
Restrict access to Events Calendar endpoints by IP allow-list where administrative usage is limited
Enforce least-privilege file permissions on wp-content/ to reduce impact of arbitrary file writes
Use a virtual patching service such as Patchstack until the official fix can be installed

bash

# Example WAF rule (ModSecurity) blocking PHP object serialization patterns in request bodies
SecRule ARGS "@rx (?:^|[^a-zA-Z0-9_])[OC]:\d+:\"[a-zA-Z_\\\\]+\":\d+:" \
    "id:1026078265,phase:2,deny,status:403,log,\
     msg:'Possible PHP Object Injection payload (CVE-2026-78265)'"