CVE-2021-46513
📋 TL;DR
CVE-2021-46513 is a 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 vulnerable versions of MJS for JavaScript execution. The vulnerability is triggered through specially crafted string inputs to the mjs_mk_string 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
Application crash causing denial of service, potentially disrupting dependent services.
If Mitigated
Contained crash within the MJS process with minimal impact if proper sandboxing and privilege separation are implemented.
🎯 Exploit Status
Proof of concept available in GitHub issue #189. Exploitation requires triggering the vulnerable function with crafted input.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: v2.20.1 or later
Vendor Advisory: https://github.com/cesanta/mjs/issues/189
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
Input validation and sanitization
allImplement strict input validation for strings passed to mjs_mk_string function
Memory protection controls
linuxEnable ASLR, DEP, and other memory protection mechanisms
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 versions
- Implement strict input validation and sanitization for all data processed by MJS
🔍 How to Verify
Check if Vulnerable:
Check if application uses MJS v2.20.0 by examining dependencies or running: strings binary_name | grep mjsCheck Version:
Check application documentation or build configuration for MJS versionVerify Fix Applied:
Verify MJS version is 2.20.1 or later and test with known exploit payloads📡 Detection & Monitoring
Log Indicators:
- Application crashes with segmentation faults
- Abnormal memory usage patterns
- Repeated failed string processing attempts
Network Indicators:
- Unusual network traffic to/from MJS-enabled applications
- Exploit kit traffic patterns
SIEM Query:
source="application_logs" AND ("segmentation fault" OR "buffer overflow" OR "mjs")