CVE-2021-46522
📋 TL;DR
CVE-2021-46522 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 library version. Attackers can exploit this by providing specially crafted input to trigger the overflow.
💻 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
Limited impact with proper memory protection mechanisms (ASLR, DEP) that may only cause crashes.
🎯 Exploit Status
The GitHub issue shows proof-of-concept details. Heap buffer overflows are commonly weaponized, especially in JavaScript engines that process untrusted code.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Versions after v2.20.0
Vendor Advisory: https://github.com/cesanta/mjs/issues/196
Restart Required: Yes
Instructions:
1. Check current MJS version. 2. Update to latest MJS version from official repository. 3. Recompile any applications using MJS. 4. Restart affected services.
🔧 Temporary Workarounds
Input Validation and Sanitization
allImplement strict input validation for all data processed by MJS to prevent triggering the overflow condition.
Memory Protection Hardening
linuxEnable ASLR, DEP, and other memory protection mechanisms to make exploitation more difficult.
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 data processed by MJS
🔍 How to Verify
Check if Vulnerable:
Check if MJS version 2.20.0 is installed: ldd /path/to/application | grep mjs or check build/package manifests.Check Version:
strings /path/to/binary | grep 'mjs\|MJS' or check package manager: dpkg -l | grep mjs or rpm -qa | grep mjsVerify Fix Applied:
Verify MJS version is updated beyond 2.20.0 and test with known safe inputs that previously triggered the issue.📡 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 network traffic to/from systems running MJS
- Exploit kit signatures targeting JavaScript engines
SIEM Query:
source="application.logs" AND ("segmentation fault" OR "heap corruption" OR "buffer overflow") AND process="*mjs*"