CVE-2026-78292: Hash Form PHP Object Injection Vulnerability

CVE-2026-78292 Overview

CVE-2026-78292 is an unauthenticated PHP Object Injection vulnerability affecting the Hash Form WordPress plugin in versions 1.4.1 and earlier. The flaw allows remote attackers to submit crafted serialized data that the plugin deserializes without validation. Successful exploitation can lead to arbitrary code execution, data tampering, or full site compromise when a suitable PHP gadget chain is present. The vulnerability requires no authentication and no user interaction, making WordPress sites running the affected plugin accessible to opportunistic attacks over the network. The issue is tracked under CWE-502: Deserialization of Untrusted Data.

Critical Impact
Unauthenticated attackers can trigger PHP object deserialization to achieve remote code execution or destructive actions on WordPress sites running Hash Form <= 1.4.1.

Affected Products

Hash Form WordPress plugin versions <= 1.4.1
WordPress installations with the Hash Form plugin installed and activated
Sites exposing plugin form endpoints to unauthenticated visitors

Discovery Timeline

2026-08-27 - CVE-2026-78292 published to NVD
2026-08-27 - Last updated in NVD database

Technical Details for CVE-2026-78292

Vulnerability Analysis

The Hash Form plugin processes attacker-controlled input through PHP's unserialize() function without validating or restricting the resulting object types. When PHP deserializes untrusted input, it instantiates arbitrary classes and invokes magic methods such as __wakeup(), __destruct(), or __toString(). Attackers use these entry points to chain existing application or library classes into a gadget chain that performs unintended operations.

In a WordPress context, gadget chains commonly leverage classes from WordPress core, other installed plugins, or Composer dependencies. Depending on the available gadgets, exploitation can lead to arbitrary file write, SQL injection, or remote code execution. The attack requires no credentials, so any exposed form submission handler can serve as the entry point.

Root Cause

The root cause is unsafe deserialization of untrusted input [CWE-502]. The plugin accepts serialized data from an unauthenticated request path and passes it directly to unserialize(). Without a class allowlist (allowed_classes) or a switch to safer formats such as JSON, PHP instantiates any class the attacker names.

Attack Vector

An attacker sends an HTTP request containing a serialized PHP payload to a Hash Form endpoint that processes form data. The plugin deserializes the payload, triggering magic methods on attacker-chosen classes. Chaining gadgets from the WordPress environment allows the attacker to escalate object instantiation into concrete impact, including code execution under the web server user. No prior access, session, or user interaction is required.

For technical details, see the Patchstack Vulnerability Report.

Detection Methods for CVE-2026-78292

Indicators of Compromise

HTTP POST requests to Hash Form endpoints containing serialized PHP markers such as O:, a:, or s: followed by class or property names
Unexpected PHP files, webshells, or modified theme and plugin files with recent timestamps under wp-content/
New or modified WordPress administrator accounts and unexplained option changes in wp_options
Outbound network connections from the web server to unknown hosts shortly after suspicious form submissions

Detection Strategies

Inspect web server and WordPress request logs for form submissions containing serialized object patterns targeting Hash Form endpoints
Deploy a Web Application Firewall (WAF) rule that blocks request bodies matching PHP serialization signatures on plugin URLs
Perform file integrity monitoring across wp-content/plugins/, wp-content/themes/, and wp-content/uploads/ to detect unauthorized changes
Correlate PHP error logs for unexpected class instantiation or __wakeup/__destruct warnings

Monitoring Recommendations

Alert on process creation events where PHP-FPM or the web server spawns shells, curl, wget, or scripting interpreters
Track outbound connections from web server hosts to non-approved destinations
Monitor WordPress admin activity for new users, role changes, and plugin or theme installations

How to Mitigate CVE-2026-78292

Immediate Actions Required

Update the Hash Form plugin to a version later than 1.4.1 as soon as the vendor releases a fix
If no fixed version is available, deactivate and remove the Hash Form plugin from all affected WordPress sites
Review web server, WordPress, and database logs for signs of prior exploitation and rotate all administrator credentials and secret keys
Restore known-good backups if unauthorized file changes or accounts are identified

Patch Information

Refer to the Patchstack Vulnerability Report for the current patch status and vendor guidance. Apply the vendor-supplied update through the WordPress plugin management interface once available, and confirm the installed version is greater than 1.4.1.

Workarounds

Block or rate-limit unauthenticated requests to Hash Form endpoints at the WAF or reverse proxy layer
Add WAF signatures that reject request bodies matching PHP serialized object patterns such as O:\d+:"
Restrict PHP execution in wp-content/uploads/ using web server configuration to limit post-exploitation impact
Enforce least privilege on the web server process and database user to reduce blast radius

bash

# Example WAF rule (ModSecurity) to block PHP serialized payloads in requests to Hash Form
SecRule REQUEST_URI "@contains /wp-admin/admin-ajax.php" \
    "chain,phase:2,deny,status:403,id:1002026,\
    msg:'Possible PHP Object Injection targeting Hash Form (CVE-2026-78292)'"
    SecRule ARGS|REQUEST_BODY "@rx O:[0-9]+:\"[a-zA-Z_\\\\]+\"" "t:none"