CVE-2021-46520
📋 TL;DR
CVE-2021-46520 is a heap buffer overflow vulnerability in Cesanta MJS JavaScript engine 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 engine version. The vulnerability is triggered via the mjs_jprintf function in src/mjs_util.c.
💻 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
Application crash causing denial of service, potentially leading to service disruption.
If Mitigated
Contained crash within the application process with no privilege escalation if proper sandboxing exists.
🎯 Exploit Status
Proof of concept exists in GitHub issue #193. Exploitation requires crafting specific JavaScript input to trigger the buffer overflow.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: v2.20.1 or later
Vendor Advisory: https://github.com/cesanta/mjs/issues/193
Restart Required: Yes
Instructions:
1. Update MJS to version 2.20.1 or later. 2. Recompile any applications using MJS. 3. Restart affected services or devices.
🔧 Temporary Workarounds
Input validation and sanitization
allImplement strict input validation for JavaScript code processed by MJS to prevent triggering the vulnerable function.
Memory protection controls
linuxEnable ASLR, DEP, and other memory protection mechanisms to reduce exploit success probability.
echo 2 > /proc/sys/kernel/randomize_va_space
sysctl -w kernel.exec-shield=1
🧯 If You Can't Patch
- Network segmentation to isolate systems using vulnerable MJS version
- Implement strict input filtering and validation for all JavaScript input to MJS engine
🔍 How to Verify
Check if Vulnerable:
Check if MJS version is exactly 2.20.0. Review application dependencies and embedded libraries.Check Version:
Check build configuration or run application with --version flag if available. For embedded systems, check firmware version documentation.Verify Fix Applied:
Verify MJS version is 2.20.1 or later. Test with known proof-of-concept input to ensure no crash occurs.📡 Detection & Monitoring
Log Indicators:
- Application crashes with segmentation faults
- Memory corruption errors in system logs
- Unexpected process termination of MJS-based applications
Network Indicators:
- Unusual JavaScript payloads being sent to applications using MJS
- Traffic patterns indicating exploitation attempts
SIEM Query:
source="application_logs" AND ("segmentation fault" OR "heap corruption" OR "buffer overflow") AND process="*mjs*"