CVE-2026-25923: my little forum RCE Vulnerability

CVE-2026-25923 Overview

CVE-2026-25923 is a high-severity insecure deserialization vulnerability affecting my little forum, a PHP and MySQL-based internet forum application. The vulnerability stems from insufficient URL validation that fails to filter the phar:// protocol, enabling attackers to exploit Phar deserialization through a multi-stage attack chain. By uploading a malicious Phar Polyglot file disguised as a JPEG image and triggering deserialization through BBCode [img] tag processing, attackers can leverage the Smarty 4.1.0 POP (Property-Oriented Programming) chain to achieve arbitrary file deletion on the target system.

Critical Impact

This vulnerability allows unauthenticated attackers to delete arbitrary files on the server through a combination of malicious image upload and Phar deserialization, potentially leading to complete system compromise or denial of service.

Affected Products

• my little forum versions prior to 20260208.1
• Installations using Smarty 4.1.0 template engine
• PHP environments with Phar stream wrapper enabled

Discovery Timeline

• 2026-02-09 - CVE CVE-2026-25923 published to NVD
• 2026-02-10 - Last updated in NVD database

Technical Details for CVE-2026-25923

Vulnerability Analysis

This vulnerability is classified as CWE-434 (Unrestricted Upload of File with Dangerous Type) combined with insecure deserialization. The attack exploits a gap in the application's URL validation logic, which does not properly sanitize or block the phar:// stream wrapper when processing image URLs embedded in BBCode tags.

The exploitation chain involves several interconnected components. First, the application's image upload feature accepts files based on file extension and basic content validation, which can be bypassed using Phar Polyglot techniques. Phar Polyglots are specially crafted files that are simultaneously valid in multiple formats—in this case, appearing as a legitimate JPEG image while containing a malicious Phar archive payload.

When a user references an uploaded file using the BBCode [img] tag with a phar:// wrapper (e.g., [img]phar://path/to/uploaded/image.jpg[/img]), the PHP engine automatically deserializes the Phar's metadata section. This triggers the instantiation of serialized objects stored within the Phar file.

The attack leverages the Smarty 4.1.0 POP chain, which provides a sequence of class methods that, when triggered during deserialization, ultimately lead to arbitrary file deletion. This type of gadget chain exploitation is a well-documented technique for converting limited deserialization primitives into more severe impacts like code execution or file system manipulation.

Root Cause

The root cause is twofold: first, the URL validation mechanism fails to implement a whitelist of allowed protocols, permitting the phar:// stream wrapper to pass validation checks. Second, the application processes user-controlled URLs in contexts that trigger PHP's stream handling, causing automatic Phar deserialization when the phar:// protocol is used.

Attack Vector

The attack is network-based and can be executed by unauthenticated remote attackers. The attack flow consists of three primary steps:

1. Malicious File Upload: The attacker uploads a Phar Polyglot file through the forum's image upload functionality. This file appears as a valid JPEG image but contains a Phar archive with serialized Smarty objects configured to exploit the POP chain.

2. Deserialization Trigger: The attacker creates or edits a forum post containing a BBCode [img] tag that references the uploaded file using the phar:// wrapper protocol.

3. Payload Execution: When the forum processes the BBCode to render the post, it attempts to access the image via the phar:// URL, triggering PHP's automatic Phar metadata deserialization and executing the Smarty POP chain payload.

The technical details of this vulnerability are documented in the GitHub Security Advisory GHSA-wr9p-3c3g-78fw.

Detection Methods for CVE-2026-25923

Indicators of Compromise

• Uploaded files containing Phar archive signatures (__HALT_COMPILER()) embedded within image data
• Forum posts or database entries containing BBCode [img] tags with phar:// protocol references
• Unexpected file deletions or missing system files on the web server
• Web server logs showing requests with phar:// in URL parameters or POST data

Detection Strategies

• Implement web application firewall (WAF) rules to block requests containing phar:// strings in user-supplied input
• Deploy file integrity monitoring (FIM) to detect unauthorized file deletions or modifications
• Analyze uploaded files for Phar signatures regardless of file extension using deep content inspection
• Review PHP application logs for deserialization errors or Smarty template processing anomalies

Monitoring Recommendations

• Monitor web server access logs for patterns indicative of Phar deserialization attempts, including URL-encoded variations of phar://
• Configure SIEM alerts for file deletion events in web application directories
• Implement real-time scanning of uploaded content for polyglot file characteristics
• Track changes to critical configuration files that could be targeted for deletion

How to Mitigate CVE-2026-25923

Immediate Actions Required

• Upgrade my little forum to version 20260208.1 or later immediately
• Audit existing uploaded files for potential Phar Polyglot content
• Review forum posts for suspicious BBCode [img] tags containing protocol wrappers
• Implement server-side filtering to block phar:// protocol in user inputs

Patch Information

The vulnerability has been addressed in my little forum version 20260208.1. The fix implements proper URL protocol validation to prevent the phar:// stream wrapper from being processed in user-supplied URLs. Organizations running affected versions should upgrade immediately by downloading the patched release from the official release page.

Workarounds

• Disable the Phar stream wrapper at the PHP configuration level by adding phar to the disable_functions directive if application functionality permits
• Implement application-level input filtering to reject any user input containing phar:// or URL-encoded equivalents
• Use a web application firewall to block requests containing Phar protocol references
• Restrict file upload functionality to authenticated users only and implement strict file type validation using magic byte verification

# PHP configuration to disable phar stream wrapper
# Add to php.ini or .htaccess
php_admin_value[disable_functions] = "phar://,phar_"
# Or use stream_wrapper_unregister in application bootstrap
# stream_wrapper_unregister('phar');