CVE-2026-42687: EventPrime Auth Bypass Vulnerability

CVE-2026-42687 Overview

CVE-2026-42687 is an unauthenticated PHP Object Injection vulnerability in the EventPrime Event Calendar Management plugin for WordPress, affecting versions up to and including 4.3.2.1. The flaw is categorized as Insecure Deserialization under [CWE-502]. An unauthenticated attacker can submit crafted serialized PHP data that the plugin deserializes without validation. Depending on PHP classes available in the WordPress installation or other active plugins, this can lead to property-oriented programming (POP) chain exploitation. Successful exploitation can result in arbitrary file operations, SQL injection, or remote code execution.

Critical Impact
Unauthenticated attackers can trigger PHP object injection on vulnerable WordPress sites running EventPrime, potentially leading to remote code execution when a suitable gadget chain is present.

Affected Products

EventPrime Event Calendar Management plugin for WordPress, versions <= 4.3.2.1
WordPress installations with the vulnerable plugin enabled
Sites running additional plugins or themes that expose usable PHP POP gadget chains

Discovery Timeline

2026-06-15 - CVE-2026-42687 published to NVD
2026-06-17 - Last updated in NVD database

Technical Details for CVE-2026-42687

Vulnerability Analysis

The vulnerability stems from unsafe deserialization of attacker-controlled input inside the EventPrime plugin. PHP's unserialize() function reconstructs objects from string input and invokes magic methods such as __wakeup(), __destruct(), and __toString(). When user input reaches unserialize() without validation, attackers can instantiate arbitrary classes loaded in the runtime.

Object injection alone does not always produce code execution. Exploitation requires a usable gadget chain — a sequence of class methods invoked during object lifecycle events that performs sensitive actions. WordPress environments commonly expose such chains through core, plugins, or themes. This makes EventPrime sites broadly exploitable in practice.

The attack does not require authentication, which removes the most significant barrier to mass exploitation. The high attack complexity reflects the need to identify and target a viable gadget chain in the specific installation. Once a chain is identified, impact extends to file write, file read, SQL injection, and arbitrary code execution under the WordPress process account.

Root Cause

The root cause is the use of unserialize() on untrusted input received through plugin request handlers. EventPrime accepts data that is passed into deserialization routines without integrity checks or allow-listing of expected classes. PHP's deserialization process automatically triggers magic methods, giving attacker-supplied objects an execution context.

Attack Vector

The attack vector is network-based. An attacker sends an HTTP request to a vulnerable EventPrime endpoint containing a serialized PHP payload. No credentials or user interaction are required. The plugin parses the payload and invokes unserialize(), instantiating the attacker's objects. Magic methods on the instantiated classes then execute gadget logic that performs the attacker's intended action.

Technical details and the affected request path are described in the Patchstack WordPress Vulnerability Analysis.

Detection Methods for CVE-2026-42687

Indicators of Compromise

HTTP requests to EventPrime endpoints containing serialized PHP markers such as O:, a:, s:, or C: followed by class names and lengths.
Unexpected PHP files written under wp-content/uploads/ or plugin directories after EventPrime requests.
Outbound network connections from the web server process to unfamiliar hosts shortly after suspicious POST requests.
New or modified WordPress admin users created without an audit trail of legitimate authentication events.

Detection Strategies

Inspect web server access logs for POST and GET parameters containing PHP serialization syntax targeting EventPrime routes.
Apply WAF rules that flag serialized object patterns in request bodies and query strings sent to WordPress plugin endpoints.
Correlate web request events with file system write events and child process spawns from php-fpm or the web server worker.
Monitor WordPress database for unauthorized changes to the wp_users and wp_options tables following plugin requests.

Monitoring Recommendations

Enable verbose logging on the WordPress site and forward logs to a centralized analytics platform for query and retention.
Alert on PHP error log entries referencing __wakeup, __destruct, or unserialize failures involving EventPrime classes.
Baseline normal EventPrime request patterns and alert on payload size or content anomalies.
Track plugin version inventory across the WordPress fleet to identify hosts running 4.3.2.1 or earlier.

How to Mitigate CVE-2026-42687

Immediate Actions Required

Update the EventPrime Event Calendar Management plugin to a version later than 4.3.2.1 as soon as a fixed release is available.
If a patched version is not yet deployed, deactivate and remove the EventPrime plugin from production WordPress sites.
Audit web server and WordPress logs for prior exploitation attempts targeting EventPrime endpoints.
Rotate WordPress administrator credentials and API keys on any site suspected of compromise.

Patch Information

Consult the Patchstack WordPress Vulnerability Analysis for the current fixed version and upgrade guidance. Apply the vendor-provided update through the WordPress plugin manager or by replacing plugin files manually after backing up the site.

Workarounds

Deploy a Web Application Firewall rule blocking requests to EventPrime endpoints that contain serialized PHP patterns such as O:\d+: or a:\d+:{.
Restrict access to the WordPress site at the network edge while testing the patch.
Run the WordPress process under a least-privileged user account with no write access to plugin and core directories.
Disable PHP functions such as exec, system, passthru, and shell_exec via disable_functions in php.ini to reduce RCE impact from gadget chains.

bash

# Example ModSecurity rule to block PHP serialized payloads to EventPrime endpoints
SecRule REQUEST_URI "@contains /wp-admin/admin-ajax.php" \
    "chain,id:1004287,phase:2,deny,status:403,log,msg:'Possible PHP Object Injection targeting EventPrime (CVE-2026-42687)'"
    SecRule ARGS|REQUEST_BODY "@rx (?:^|[^a-zA-Z0-9])(O|C):[0-9]+:\"[A-Za-z_\\\\]+\":[0-9]+:\{" \
        "t:none,t:urlDecodeUni"