CVE-2021-46526
📋 TL;DR
CVE-2021-46526 is a global buffer overflow vulnerability in Cesanta MJS v2.20.0's JSON parsing functionality that allows attackers to execute arbitrary code or cause denial of service. This affects any application or device using the vulnerable MJS JavaScript engine. The vulnerability is triggered through specially crafted JSON input.
💻 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
Denial of service causing application crashes or instability, potentially leading to service disruption.
If Mitigated
Application crash with limited impact if proper sandboxing, memory protection, and privilege separation are implemented.
🎯 Exploit Status
The GitHub issue contains technical details and proof-of-concept. Buffer overflow vulnerabilities in parsing functions are commonly weaponized.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: v2.20.1 or later
Vendor Advisory: https://github.com/cesanta/mjs/issues/191
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 malicious JSON payloads.
🔧 Temporary Workarounds
Disable JSON parsing
allDisable MJS JSON parsing functionality if not required
Recompile MJS with JSON support disabled via build configuration
Input validation and sanitization
allImplement strict input validation for JSON data before passing to MJS
🧯 If You Can't Patch
- Implement network segmentation to isolate vulnerable systems
- Deploy web application firewall (WAF) with buffer overflow protection rules
🔍 How to Verify
Check if Vulnerable:
Check if application uses MJS v2.20.0. Review source code or dependencies for MJS inclusion.Check Version:
Check build configuration or dependency files for MJS version referenceVerify Fix Applied:
Verify MJS version is 2.20.1 or later. Test with proof-of-concept payload from GitHub issue.📡 Detection & Monitoring
Log Indicators:
- Application crashes with segmentation faults
- Memory corruption errors in logs
- Unusual JSON parsing errors
Network Indicators:
- Large or malformed JSON payloads to endpoints using MJS
- Repeated connection attempts with JSON data
SIEM Query:
source="application_logs" AND ("segmentation fault" OR "buffer overflow" OR "mjs")