CVE-2021-47857
📋 TL;DR
Moodle 3.10.3 contains a persistent cross-site scripting vulnerability in calendar event subtitles that allows attackers to inject malicious JavaScript. When users view a crafted calendar event, the malicious script executes in their browser, potentially stealing credentials or performing unauthorized actions. All Moodle instances running the affected version are vulnerable.
💻 Affected Systems
- Moodle
📦 What is this software?
Moodle by Moodle
⚠️ Risk & Real-World Impact
Worst Case
Attackers could steal administrator credentials, compromise user accounts, deface the platform, or redirect users to malicious sites, potentially leading to full system compromise.
Likely Case
Attackers with calendar event creation privileges could steal session cookies, perform actions as other users, or deploy phishing attacks against users viewing the malicious events.
If Mitigated
With proper input validation and output encoding, the malicious scripts would be rendered harmless as text rather than executable code.
🎯 Exploit Status
Exploitation requires attacker to have calendar event creation privileges. Public exploit code is available, making this easy to weaponize for attackers with the necessary access.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 3.10.4 and later
Vendor Advisory: https://moodle.org/
Restart Required: No
Instructions:
1. Backup your Moodle installation and database. 2. Download Moodle 3.10.4 or later from moodle.org. 3. Replace the affected files with the patched version. 4. Clear Moodle caches from Site administration > Development > Purge all caches.
🔧 Temporary Workarounds
Disable calendar event creation
allTemporarily restrict calendar event creation permissions to prevent exploitation while patching.
Navigate to Site administration > Users > Permissions > Define roles > Edit role capabilities > Search for 'moodle/calendar:manageentries' > Set to 'Prevent'
Content Security Policy
allImplement a strict Content Security Policy header to block inline script execution.
Add to web server config: Content-Security-Policy: script-src 'self' https://trusted.cdn.example;
🧯 If You Can't Patch
- Implement web application firewall rules to block XSS payloads in calendar event submissions
- Regularly audit calendar events for suspicious content and remove any containing script tags or JavaScript
🔍 How to Verify
Check if Vulnerable:
Check Moodle version in Site administration > Notifications page. If version is exactly 3.10.3, the system is vulnerable.Check Version:
Check Moodle config.php or visit Site administration > Notifications in web interfaceVerify Fix Applied:
After patching, verify version shows 3.10.4 or later. Test by creating a calendar event with <script>alert('test')</script> in subtitle - it should display as plain text, not execute.📡 Detection & Monitoring
Log Indicators:
- Unusual calendar event creation patterns
- Calendar events containing script tags or JavaScript in subtitles
- Multiple failed login attempts following calendar event views
Network Indicators:
- Outbound connections to suspicious domains following calendar event access
- Unusual POST requests to calendar event creation endpoints
SIEM Query:
source="moodle_logs" AND (event="calendar_event_created" AND subtitle CONTAINS "<script>") OR (event="user_login" AND src_ip!=user_normal_ip)