CVE-2020-21526
📋 TL;DR
CVE-2020-21526 is a critical directory traversal vulnerability in Halo v1.1.3 that allows authenticated attackers to write arbitrary files to the server filesystem by bypassing path validation using the startsWith function. This affects all Halo installations running the vulnerable version, particularly those with administrative access enabled.
💻 Affected Systems
- Halo
📦 What is this software?
Halo by Halo
⚠️ Risk & Real-World Impact
Worst Case
Complete system compromise through remote code execution by writing malicious files to critical system locations, potentially leading to data theft, service disruption, or ransomware deployment.
Likely Case
Unauthorized file writes to web-accessible directories leading to defacement, data manipulation, or privilege escalation through configuration file modification.
If Mitigated
Limited impact if proper access controls, file system permissions, and input validation are in place, potentially only allowing writes to non-critical directories.
🎯 Exploit Status
Exploitation requires authenticated access but uses simple path traversal techniques with minimal technical complexity.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: v1.1.4 and later
Vendor Advisory: https://github.com/halo-dev/halo/issues/421
Restart Required: Yes
Instructions:
1. Backup your Halo installation and database. 2. Download Halo v1.1.4 or later from the official repository. 3. Replace the existing installation with the updated version. 4. Restart the Halo service.
🔧 Temporary Workarounds
Restrict Administrative Access
allLimit access to the Halo administrative interface to trusted IP addresses only
# Configure firewall rules to restrict access to Halo admin port
# Example: iptables -A INPUT -p tcp --dport 8090 -s TRUSTED_IP -j ACCEPT
# Example: iptables -A INPUT -p tcp --dport 8090 -j DROP
File System Permissions Hardening
linuxRestrict write permissions on Halo directories to minimize impact
# Make Halo directories read-only where possible
chmod -R 755 /path/to/halo
# Remove write permissions from critical system directories
chmod -R 555 /etc /usr /var
🧯 If You Can't Patch
- Implement strict network segmentation to isolate Halo instances from critical systems
- Enable comprehensive logging and monitoring for file write operations in Halo directories
🔍 How to Verify
Check if Vulnerable:
Check Halo version by accessing the admin interface and viewing the version in the footer or configuration files. If version is exactly 1.1.3, the system is vulnerable.Check Version:
grep -r "version" /path/to/halo/config/application.yml || cat /path/to/halo/META-INF/MANIFEST.MFVerify Fix Applied:
After updating, verify the version shows 1.1.4 or higher. Test the file upload functionality with traversal attempts to confirm proper path validation.📡 Detection & Monitoring
Log Indicators:
- Unusual file write operations in Halo logs
- Path traversal patterns in request logs (../, ..\, %2e%2e%2f)
- Multiple failed authentication attempts followed by successful login
Network Indicators:
- Unusual file upload patterns to Halo administrative endpoints
- POST requests containing path traversal sequences
SIEM Query:
source="halo.logs" AND (message="*../*" OR message="*..\\*" OR message="*%2e%2e%2f*")