CVE-2021-46524
📋 TL;DR
CVE-2021-46524 is a heap buffer overflow vulnerability in Cesanta MJS v2.20.0 that allows attackers to execute arbitrary code or cause denial of service. This affects any application or device using the vulnerable MJS JavaScript engine for embedded systems. Attackers can exploit this by providing specially crafted JSON input to trigger the overflow in the snquote function.
💻 Affected Systems
- Cesanta MJS JavaScript engine
📦 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
Denial of service causing application crashes or instability, potentially leading to service disruption
If Mitigated
Contained application crash without privilege escalation if proper sandboxing and memory protections are enabled
🎯 Exploit Status
Proof of concept available in GitHub issue #192; exploitation requires crafting specific JSON payloads
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Version 2.20.1 or later
Vendor Advisory: https://github.com/cesanta/mjs/issues/192
Restart Required: Yes
Instructions:
1. Update MJS to version 2.20.1 or later. 2. Recompile any applications using MJS. 3. Restart affected services. 4. Verify the fix by testing with known exploit payloads.
🔧 Temporary Workarounds
Input validation and sanitization
allImplement strict input validation for JSON data before passing to MJS parsing functions
Disable JSON parsing if unused
allRemove or disable MJS JSON parsing functionality if not required for application
🧯 If You Can't Patch
- Implement network segmentation to isolate systems using vulnerable MJS versions
- Deploy application firewalls to filter and inspect JSON payloads before reaching vulnerable systems
🔍 How to Verify
Check if Vulnerable:
Check MJS version in application dependencies or run 'strings' on binary to find MJS version stringsCheck Version:
Check build configuration or dependency files for 'mjs' version 2.20.0Verify Fix Applied:
Test with known exploit payloads from GitHub issue #192; successful parsing without crash indicates fix📡 Detection & Monitoring
Log Indicators:
- Application crashes with segmentation faults
- Memory corruption errors in system logs
- Unexpected process termination
Network Indicators:
- Unusual JSON payloads with crafted escape sequences
- Repeated connection attempts to JSON endpoints
SIEM Query:
source="application.log" AND ("segmentation fault" OR "heap corruption" OR "buffer overflow") AND process="*mjs*"