CVE-2026-25728: ClipBucket Race Condition Vulnerability

CVE-2026-25728 Overview

CVE-2026-25728 is a Time-of-Check to Time-of-Use (TOCTOU) race condition vulnerability in ClipBucket v5, an open source video sharing platform. The vulnerability exists in the avatar and background image upload functionality where the application moves uploaded files to a web-accessible location before validating them. This creates a critical window during which an attacker can execute arbitrary PHP code before the file is deleted by the validation process.

The vulnerable code path involves move_uploaded_file() placing the uploaded file in a web-accessible path, followed by ValidateImage() performing validation, and then @unlink() deleting the file if validation fails. This sequence allows attackers to race the validation process and execute malicious PHP payloads during the brief window between file placement and deletion.

Critical Impact

Unauthenticated attackers can achieve remote code execution on ClipBucket v5 servers by exploiting the race condition in the file upload validation process.

Affected Products

• ClipBucket v5 prior to version 5.5.3 - #40

Discovery Timeline

• 2026-02-10 - CVE-2026-25728 published to NVD
• 2026-02-11 - Last updated in NVD database

Technical Details for CVE-2026-25728

Vulnerability Analysis

This vulnerability is classified as CWE-367 (Time-of-Check Time-of-Use Race Condition). The fundamental flaw lies in the order of operations during file upload processing. The application implements a "move first, validate later" approach which is inherently insecure for file upload scenarios.

When a user uploads an avatar or background image, the file is immediately placed in a publicly accessible web directory using move_uploaded_file(). Only after the file has been moved does the application call ValidateImage() to verify the uploaded content is a legitimate image. If validation fails, the application attempts to remove the malicious file using @unlink().

This design creates a race condition window - the time between when the file becomes accessible on the web server and when validation completes. An attacker can craft a request to upload a malicious PHP file disguised with an image extension, then rapidly request that file before the validation and deletion occurs. If the timing is successful, the PHP code executes on the server with the web server's privileges.

The network-accessible attack vector with no authentication requirement makes this vulnerability particularly dangerous for internet-facing ClipBucket installations.

Root Cause

The root cause is an improper sequencing of security operations during file upload handling. Secure file upload implementations should validate files in a temporary, non-executable location before moving them to web-accessible directories. ClipBucket's implementation inverts this order, validating only after the file has been placed in a dangerous location.

The use of the error suppression operator (@) before unlink() further compounds the issue by potentially masking errors during the cleanup process, though this is secondary to the core TOCTOU vulnerability.

Attack Vector

The attack exploits the race condition through rapid, timed requests:

1. Attacker crafts a PHP webshell or malicious payload with an image-like filename
2. Attacker submits the file through the avatar or background image upload endpoint
3. The server moves the file to a web-accessible path immediately
4. Attacker rapidly sends HTTP requests to access the uploaded PHP file
5. If the request arrives before ValidateImage() completes and @unlink() removes the file, the PHP code executes
6. Attacker achieves remote code execution on the server

The attack can be automated with tools that send multiple concurrent requests to maximize the chance of winning the race condition.

The following patch was applied to address this vulnerability:

 {
   "version":"5.5.3",
-  "revision":"39",
+  "revision":"40",
   "status":"stable",
   "detail":[
     {

Source: GitHub Commit Changes

Detection Methods for CVE-2026-25728

Indicators of Compromise

• Unusual burst of HTTP requests targeting avatar or background upload endpoints followed by rapid requests to the uploaded file path
• PHP files appearing in image upload directories that contain suspicious code patterns
• Web server logs showing rapid sequential requests: POST to upload endpoint followed by GET to the same filename within milliseconds
• Unexpected PHP execution errors or webshell-related processes spawned by the web server user

Detection Strategies

• Monitor web server access logs for rapid POST/GET request pairs targeting upload functionality and uploaded file paths
• Implement file integrity monitoring on upload directories to detect PHP files or files with executable content
• Deploy web application firewall rules to detect and block attempts to access recently uploaded files with suspicious extensions or content types
• Analyze server process trees for unusual child processes spawned by the web server that may indicate successful code execution

Monitoring Recommendations

• Enable detailed logging for all file upload operations including timestamps, source IPs, and file metadata
• Set up alerts for any file with executable content appearing in designated image upload directories
• Monitor for PHP error logs indicating attempts to execute non-image files through the upload path
• Implement network traffic analysis to detect rapid request patterns characteristic of race condition exploitation attempts

How to Mitigate CVE-2026-25728

Immediate Actions Required

• Upgrade ClipBucket v5 to version 5.5.3 - #40 or later immediately
• If immediate upgrade is not possible, disable avatar and background image upload functionality temporarily
• Review server logs for evidence of exploitation attempts
• Audit upload directories for any suspicious PHP files or unexpected content

Patch Information

The vulnerability is fixed in ClipBucket v5 version 5.5.3 - #40. The security patch addresses the TOCTOU race condition by implementing proper validation sequencing. For detailed patch information, refer to the GitHub Security Advisory GHSA-xq7c-m5r2-9wqj and the commit 09536e6e2ca6d69a2ee83190b588c0b8116dd16d.

Workarounds

• Disable avatar and background image upload functionality by modifying application configuration or removing access to the affected endpoints
• Implement web server rules to prevent PHP execution in upload directories (e.g., php_flag engine off in .htaccess for Apache)
• Place upload directories outside the web root and serve images through a proxy script that validates content before serving
• Use a web application firewall to block requests attempting to access PHP files in upload directories

# Apache configuration to disable PHP execution in upload directories
<Directory "/path/to/clipbucket/uploads">
    php_flag engine off
    RemoveHandler .php .phtml .php3 .php4 .php5 .phps
    RemoveType .php .phtml .php3 .php4 .php5 .phps
</Directory>