CVE-2021-3810
📋 TL;DR
CVE-2021-3810 is a regular expression denial-of-service (ReDoS) vulnerability in code-server's URL path validation. Attackers can craft malicious URLs that cause excessive CPU consumption, potentially making the service unresponsive. This affects all code-server instances with the vulnerable code.
💻 Affected Systems
- code-server
📦 What is this software?
⚠️ Risk & Real-World Impact
Worst Case
Complete service unavailability due to CPU exhaustion, disrupting all users and potentially requiring service restart.
Likely Case
Degraded performance or temporary unavailability for users accessing the affected endpoint.
If Mitigated
Minimal impact with proper rate limiting and monitoring in place.
🎯 Exploit Status
Exploitation requires sending specially crafted HTTP requests to vulnerable endpoints.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 3.12.0 and later
Vendor Advisory: https://github.com/cdr/code-server/security/advisories/GHSA-7x4j-5xjq-8jvh
Restart Required: Yes
Instructions:
1. Stop code-server service. 2. Update to version 3.12.0 or later using your package manager or direct download. 3. Restart code-server service.
🔧 Temporary Workarounds
Implement rate limiting
allAdd rate limiting to HTTP requests to prevent abuse of the vulnerable endpoint.
# Use nginx or similar proxy with rate limiting
limit_req_zone $binary_remote_addr zone=code_server:10m rate=10r/s;
limit_req zone=code_server burst=20 nodelay;
Restrict network access
linuxLimit access to code-server to trusted networks only.
# Example using iptables
iptables -A INPUT -p tcp --dport 8080 -s trusted_network -j ACCEPT
iptables -A INPUT -p tcp --dport 8080 -j DROP
🧯 If You Can't Patch
- Implement Web Application Firewall (WAF) rules to block malicious URL patterns.
- Monitor CPU usage and set alerts for abnormal spikes to detect potential attacks.
🔍 How to Verify
Check if Vulnerable:
Check code-server version: if version is below 3.12.0, it is vulnerable.Check Version:
code-server --versionVerify Fix Applied:
Verify version is 3.12.0 or higher and test that crafted URLs no longer cause high CPU usage.📡 Detection & Monitoring
Log Indicators:
- Multiple requests to /static/ paths with unusual patterns
- High CPU usage alerts from monitoring systems
Network Indicators:
- Unusual volume of HTTP requests to code-server static endpoints
- Requests with long, repetitive patterns in URL paths
SIEM Query:
source="code-server" AND (url="/static/*" AND url_length>100) | stats count by src_ip