CVE-2021-31875
📋 TL;DR
CVE-2021-31875 is an off-by-one heap-based buffer overflow vulnerability in mjs_json_parse function of Cesanta MongooseOS mJS 1.26. A malicious JSON string can trigger this overflow, potentially allowing attackers to redirect control flow and execute arbitrary code. This affects systems using vulnerable versions of mJS for JSON parsing.
💻 Affected Systems
- Cesanta MongooseOS mJS
📦 What is this software?
⚠️ Risk & Real-World Impact
Worst Case
Remote code execution leading to complete system compromise, data theft, or ransomware deployment.
Likely Case
Application crash (denial of service) due to memory corruption; reliable code execution is difficult per reporter's assessment.
If Mitigated
Application crash with no data loss if proper sandboxing and memory protections are in place.
🎯 Exploit Status
Original reporter disputes exploit reliability; exploitation requires precise memory manipulation.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Fixed in versions after 1.26
Vendor Advisory: https://github.com/cesanta/mjs/releases/tag/1.26
Restart Required: Yes
Instructions:
1. Update mJS to version after 1.26. 2. Recompile applications using mJS. 3. Restart affected services.
🔧 Temporary Workarounds
Input Validation
allImplement strict JSON input validation and size limits before passing to mjs_json_parse.
Memory Protection
allEnable ASLR and DEP/stack protection on systems to reduce exploit success probability.
🧯 If You Can't Patch
- Isolate affected systems from untrusted networks
- Implement network filtering to block malicious JSON payloads
🔍 How to Verify
Check if Vulnerable:
Check mJS version: if using exactly version 1.26, system is vulnerable.Check Version:
Check application dependencies or build configuration for mjs versionVerify Fix Applied:
Verify mJS version is greater than 1.26 and applications have been recompiled.📡 Detection & Monitoring
Log Indicators:
- Application crashes with memory access violations
- Unexpected process termination
Network Indicators:
- Unusually large or malformed JSON payloads
SIEM Query:
Process termination events from mjs-based applications