CVE-2021-3943
📋 TL;DR
This vulnerability allows remote attackers to execute arbitrary code on Moodle servers by exploiting improper input validation during backup file restoration. It affects Moodle installations running vulnerable versions, potentially compromising the entire system. Any organization using affected Moodle versions for learning management is at risk.
💻 Affected Systems
- Moodle
📦 What is this software?
Moodle by Moodle
Moodle by Moodle
Moodle by Moodle
⚠️ Risk & Real-World Impact
Worst Case
Complete system compromise allowing attackers to execute arbitrary code, steal sensitive data, install malware, or pivot to other systems in the network.
Likely Case
Unauthorized access to the Moodle server, data exfiltration, and potential disruption of learning services.
If Mitigated
Limited impact if proper network segmentation, least privilege access, and input validation controls are implemented.
🎯 Exploit Status
Exploitation requires the ability to upload backup files, which typically requires authentication, but the vulnerability itself allows RCE once a malicious backup is processed.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Moodle 3.11.4, 3.10.8, 3.9.11
Vendor Advisory: https://moodle.org/mod/forum/discuss.php?d=429095
Restart Required: No
Instructions:
1. Backup your Moodle database and files. 2. Download the patched version from moodle.org. 3. Follow Moodle's standard upgrade procedure. 4. Verify the upgrade completed successfully.
🔧 Temporary Workarounds
Disable backup restoration
allTemporarily disable the ability to restore backup files to prevent exploitation.
Edit config.php and add: $CFG->enablebackuprestore = false;
🧯 If You Can't Patch
- Implement strict access controls to limit who can upload backup files.
- Deploy a web application firewall (WAF) with rules to detect and block malicious backup file uploads.
🔍 How to Verify
Check if Vulnerable:
Check your Moodle version via the Site Administration > Notifications page or by examining version.php in the Moodle root directory.Check Version:
grep "\$release" /path/to/moodle/version.phpVerify Fix Applied:
After patching, verify the version is 3.11.4, 3.10.8, or 3.9.11 or later, and test backup restoration functionality.📡 Detection & Monitoring
Log Indicators:
- Unusual backup file uploads, especially from unexpected IP addresses or users
- Errors in Moodle logs related to backup processing
Network Indicators:
- Large file uploads to backup restoration endpoints
- Suspicious outbound connections from the Moodle server post-upload
SIEM Query:
source="moodle.log" AND ("backup" AND "restore") AND (file_upload OR suspicious_activity)