CVE-2021-26530
📋 TL;DR
CVE-2021-26530 is a critical out-of-bounds write vulnerability in Cesanta Mongoose HTTPS server when compiled with OpenSSL support. Attackers can remotely execute arbitrary code or crash the server by sending connection requests after exhausting the memory pool. This affects any system running vulnerable versions of Mongoose server with TLS/HTTPS enabled.
💻 Affected Systems
- Cesanta Mongoose HTTPS Server
📦 What is this software?
Mongoose by Cesanta
⚠️ Risk & Real-World Impact
Worst Case
Remote code execution leading to complete system compromise, data theft, or ransomware deployment.
Likely Case
Server crash causing denial of service and potential data corruption.
If Mitigated
Limited to denial of service if memory protections or exploit mitigations are in place.
🎯 Exploit Status
Proof of concept available in GitHub issues. Exploitation requires sending connection requests after memory pool exhaustion.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Version 7.1 and later
Vendor Advisory: https://github.com/cesanta/mongoose/issues/1204
Restart Required: Yes
Instructions:
1. Update Mongoose to version 7.1 or later. 2. Recompile the application with the updated library. 3. Restart the server/service.
🔧 Temporary Workarounds
Disable TLS/HTTPS
allDisable TLS/HTTPS functionality if not required
Recompile Mongoose without OpenSSL support or disable HTTPS in configuration
Network Segmentation
linuxRestrict access to Mongoose server
iptables -A INPUT -p tcp --dport [PORT] -s [TRUSTED_IPS] -j ACCEPT
iptables -A INPUT -p tcp --dport [PORT] -j DROP
🧯 If You Can't Patch
- Implement strict network access controls to limit connections to trusted sources only
- Deploy web application firewall (WAF) rules to detect and block exploitation attempts
🔍 How to Verify
Check if Vulnerable:
Check if Mongoose version is 7.0 and compiled with OpenSSL support. Review build configuration and server logs for TLS initialization.Check Version:
Check application documentation or run the server with --version flag if availableVerify Fix Applied:
Verify Mongoose version is 7.1 or later and check that the mg_tls_init function has been patched.📡 Detection & Monitoring
Log Indicators:
- Multiple connection attempts followed by server crash
- Memory allocation errors in logs
- TLS handshake failures
Network Indicators:
- Multiple rapid connection requests to HTTPS port
- Unusual traffic patterns causing memory exhaustion
SIEM Query:
source="mongoose.log" AND ("crash" OR "memory" OR "tls_init")