CVE-2021-43970
📋 TL;DR
An authenticated attacker with low privileges can upload malicious files disguised as MP3s to execute arbitrary code on Quicklert for Digium servers. This vulnerability affects Quicklert for Digium version 10.0.0 (1043) and allows remote code execution with SYSTEM-level permissions.
💻 Affected Systems
- Quicklert for Digium
📦 What is this software?
Quicklert by Quicklert
⚠️ Risk & Real-World Impact
Worst Case
Full system compromise with attacker gaining SYSTEM privileges, allowing installation of persistent backdoors, data exfiltration, and lateral movement within the network.
Likely Case
Attacker uploads web shell to gain persistent access, executes commands to steal sensitive data, or uses server as pivot point for further attacks.
If Mitigated
With proper file upload validation and least privilege controls, impact limited to file upload denial or isolated application compromise.
🎯 Exploit Status
Exploitation requires authentication but uses simple file upload bypass technique. Weaponization likely due to RCE potential and SYSTEM privileges.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Unknown - check vendor for updated version
Vendor Advisory: https://quicklert.com
Restart Required: Yes
Instructions:
1. Contact Quicklert/Digium for patch information. 2. Apply vendor-provided update. 3. Restart Quicklert service. 4. Verify fix by testing file upload functionality.
🔧 Temporary Workarounds
Disable albumimages.jsp file upload
allRemove or restrict access to vulnerable JSP file upload endpoint
# Rename or move albumimages.jsp to disable
mv /path/to/albumimages.jsp /path/to/albumimages.jsp.disabled
Implement file upload validation
allAdd server-side validation to reject files with double extensions or JSP content
# Example: Add validation in web.xml or application config
# Reject files containing ';' in filename or .jsp extension
🧯 If You Can't Patch
- Restrict network access to Quicklert interface to trusted IPs only
- Implement WAF rules to block file uploads with double extensions or JSP content
🔍 How to Verify
Check if Vulnerable:
Check if Quicklert version is 10.0.0 (1043) and albumimages.jsp accepts files with .mp3;.jsp extension containing audio header.Check Version:
Check Quicklert web interface or configuration files for version informationVerify Fix Applied:
Test file upload with malicious .mp3;.jsp file - should be rejected. Verify version is updated.📡 Detection & Monitoring
Log Indicators:
- File upload requests to albumimages.jsp with double extensions
- JSP file execution in unexpected directories
- SYSTEM privilege processes spawned from web user context
Network Indicators:
- HTTP POST requests to albumimages.jsp with file uploads
- Outbound connections from Quicklert server to suspicious IPs
SIEM Query:
source="quicklert.log" AND (uri="*albumimages.jsp*" AND method="POST") OR (process_name="cmd.exe" AND parent_process="java.exe")