CVE-2021-46518
📋 TL;DR
CVE-2021-46518 is a heap buffer overflow vulnerability in Cesanta MJS JavaScript engine that allows attackers to execute arbitrary code or cause denial of service. This affects any application or device using MJS v2.20.0 for embedded JavaScript execution. The vulnerability is triggered through the mjs_disown function in the core component.
💻 Affected Systems
- Cesanta MJS
📦 What is this software?
Mjs by Cesanta
⚠️ Risk & Real-World Impact
Worst Case
Remote code execution leading to complete system compromise, data theft, or persistent backdoor installation.
Likely Case
Application crash causing denial of service, potentially leading to service disruption in embedded systems.
If Mitigated
Application crash contained within sandboxed environment with minimal impact.
🎯 Exploit Status
Exploitation requires ability to supply crafted JavaScript to the mjs_disown function. Public proof-of-concept demonstrates crash but not full RCE.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Version 2.20.1 and later
Vendor Advisory: https://github.com/cesanta/mjs/issues/195
Restart Required: Yes
Instructions:
1. Update MJS to version 2.20.1 or later. 2. Recompile any applications using MJS. 3. Restart affected services.
🔧 Temporary Workarounds
Disable vulnerable functionality
allRemove or disable calls to mjs_disown function if not required
# Review source code and remove/comment mjs_disown calls
Input validation
allImplement strict input validation for JavaScript passed to MJS
# Add input sanitization before passing data to MJS engine
🧯 If You Can't Patch
- Isolate affected systems in network segments with strict access controls
- Implement application-level firewalls to filter malicious JavaScript inputs
🔍 How to Verify
Check if Vulnerable:
Check if MJS version 2.20.0 is installed in your system or linked in applicationsCheck Version:
grep -r "MJS_VERSION" /path/to/mjs/source/ or check package managerVerify Fix Applied:
Verify MJS version is 2.20.1 or later and test mjs_disown functionality📡 Detection & Monitoring
Log Indicators:
- Segmentation faults in MJS processes
- Abnormal memory usage patterns
- Repeated crashes of MJS-based applications
Network Indicators:
- Unusual JavaScript payloads sent to MJS interfaces
- Traffic patterns indicating fuzzing attempts
SIEM Query:
process.name:"mjs" AND (event.action:"segmentation_fault" OR memory.usage > threshold)