CVE-2021-26476
📋 TL;DR
CVE-2021-26476 is a critical OS command injection vulnerability in EPrints 3.4.2 that allows remote attackers to execute arbitrary commands on the server via specially crafted LaTeX input. This affects all EPrints 3.4.2 installations with the vulnerable calendar component exposed. Attackers can gain complete control of affected systems without authentication.
💻 Affected Systems
- EPrints
📦 What is this software?
Eprints by Eprints
⚠️ Risk & Real-World Impact
Worst Case
Full system compromise allowing attackers to execute arbitrary commands as the web server user, potentially leading to data theft, ransomware deployment, or complete server takeover.
Likely Case
Remote code execution leading to web shell installation, data exfiltration, or lateral movement within the network.
If Mitigated
Limited impact with proper input validation and command sanitization in place, though the vulnerability still presents significant risk.
🎯 Exploit Status
The exploit requires crafting malicious LaTeX input to the vulnerable calendar endpoint. Public proof-of-concept code exists and the attack vector is straightforward.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 3.4.3 and later
Vendor Advisory: https://files.eprints.org/2548/
Restart Required: No
Instructions:
1. Upgrade EPrints to version 3.4.3 or later. 2. Apply the security patch from the vendor advisory. 3. Verify the calendar component no longer accepts unsanitized LaTeX input.
🔧 Temporary Workarounds
Disable vulnerable calendar endpoint
allTemporarily disable or restrict access to the cgi/cal endpoint
# In Apache: RewriteRule ^/cgi/cal - [F]
# In Nginx: location ~ ^/cgi/cal { deny all; }
Input validation filter
linuxImplement WAF rules to block malicious LaTeX patterns
# Example ModSecurity rule: SecRule ARGS "@rx \\[^a-zA-Z0-9]" "id:1001,phase:2,deny"
🧯 If You Can't Patch
- Implement strict input validation and sanitization for all LaTeX input parameters
- Deploy a web application firewall (WAF) with rules specifically targeting OS command injection patterns
🔍 How to Verify
Check if Vulnerable:
Test if the /cgi/cal endpoint accepts and processes LaTeX input containing shell metacharacters like ;, |, or $()Check Version:
Check the EPrints version in the admin interface or review the software version filesVerify Fix Applied:
Verify that the calendar endpoint properly sanitizes LaTeX input and rejects commands containing shell metacharacters📡 Detection & Monitoring
Log Indicators:
- Unusual POST/GET requests to /cgi/cal with LaTeX parameters
- Commands containing shell metacharacters in web logs
- Unexpected process execution from web server user
Network Indicators:
- HTTP requests to calendar endpoint with encoded shell commands
- Outbound connections from web server to unexpected destinations
SIEM Query:
source="web_logs" AND (uri="/cgi/cal" AND (param="*;*" OR param="*|*" OR param="*$(*"))