CVE-2026-42473: MixPHP Deserialization Vulnerability

CVE-2026-42473 Overview

CVE-2026-42473 is an unsafe deserialization vulnerability affecting MixPHP Framework versions 2.x through 2.2.17. The framework's session and cache handlers invoke PHP's unserialize() function on data read from the filesystem inside the FileHandler object. An attacker who can write or influence the contents of session or cache files can trigger PHP object instantiation through gadget chains, leading to arbitrary code execution. The flaw is classified under CWE-502: Deserialization of Untrusted Data.

Critical Impact
Successful exploitation can yield remote code execution in the context of the PHP application, compromising confidentiality, integrity, and availability of the host process.

Affected Products

MixPHP Framework 2.x
MixPHP Framework versions up to and including 2.2.17
Applications using MixPHP FileHandler for session or cache storage

Discovery Timeline

2026-05-01 - CVE-2026-42473 published to NVD
2026-05-05 - Last updated in NVD database

Technical Details for CVE-2026-42473

Vulnerability Analysis

MixPHP Framework provides session and cache subsystems backed by a FileHandler class that persists serialized PHP data to disk. When the framework reads a session or cache entry, the handler calls unserialize() directly on the file contents without integrity verification. PHP's unserialize() reconstructs arbitrary objects defined in the autoloadable namespace, invoking magic methods such as __wakeup(), __destruct(), and __toString() during the process. An attacker who can drop a crafted file into the session or cache directory, or who can influence the path resolution to point at attacker-controlled content, can introduce malicious serialized payloads. These payloads chain existing classes (gadgets) loaded by the application or its dependencies to achieve code execution, file write, or command execution.

Root Cause

The root cause is unsafe deserialization of filesystem-sourced data inside FileHandler. The handler treats stored bytes as trusted and passes them straight to unserialize(). No HMAC, signature, or schema validation guards the deserialization path. Refer to the MixPHP repository and the Server.php source for v2.2.17 for the affected code.

Attack Vector

The attack vector is network-based when an attacker can influence session or cache file content through application functionality, shared hosting environments, or chained file-write primitives. The attacker crafts a serialized PHP object referencing gadget classes available in the application's autoload graph. When the framework loads the session or cache entry, unserialize() materializes the object and triggers gadget magic methods, resulting in arbitrary code execution. A proof-of-concept payload structure is documented in this GitHub Gist.

Detection Methods for CVE-2026-42473

Indicators of Compromise

Session or cache files containing serialized PHP objects with class names not used by the application (for example, O: markers referencing unexpected namespaces).
Unexpected child processes spawned by the PHP-FPM or CLI worker that loads MixPHP, such as shells or network utilities.
New or modified files in the session storage directory with timestamps that do not align with legitimate user activity.

Detection Strategies

Inspect session and cache directories for entries that begin with serialized object signatures (O: or C:) referencing classes outside the application's session schema.
Enable PHP audit logging and alert on calls to unserialize() originating from FileHandler paths.
Monitor process trees for the PHP runtime invoking sh, bash, python, curl, or wget shortly after session or cache reads.

Monitoring Recommendations

Capture filesystem write events on session and cache directories and forward them to a centralized log platform for correlation.
Track web request patterns that precede anomalous PHP behavior, focusing on requests that touch session-issuing endpoints.
Baseline outbound network connections from PHP workers to identify command-and-control or download activity following exploitation.

How to Mitigate CVE-2026-42473

Immediate Actions Required

Upgrade MixPHP Framework to a version later than 2.2.17 once a fixed release is published by the maintainers.
Restrict filesystem permissions on session and cache directories so only the PHP process owner can read or write entries.
Audit existing session and cache storage and purge entries that contain unexpected serialized objects.

Patch Information

No vendor patch reference is included in the NVD entry at the time of publication. Track the mix-php/mix GitHub repository for fixed releases and security advisories.

Workarounds

Replace FileHandler with a session or cache backend that does not rely on unserialize(), such as a JSON-based custom handler.
Wrap stored session and cache values with an HMAC signature and verify the signature before invoking unserialize().
Move session and cache storage to an isolated directory outside any web-writable path to prevent attacker-controlled file injection.

bash

# Configuration example: harden session storage directory
chown -R www-data:www-data /var/lib/mixphp/sessions
chmod 700 /var/lib/mixphp/sessions
find /var/lib/mixphp/sessions -type f -exec chmod 600 {} \;