CVE-2021-43281
📋 TL;DR
This vulnerability allows authenticated MyBB administrators with 'Can manage settings?' permission to inject and execute arbitrary PHP code through the Admin Control Panel's Settings management module. The flaw enables remote code execution on affected MyBB installations, potentially compromising the entire web server. Only MyBB installations with vulnerable versions and administrators having specific permissions are affected.
💻 Affected Systems
- MyBB
📦 What is this software?
Mybb by Mybb
⚠️ Risk & Real-World Impact
Worst Case
Full server compromise allowing attacker to execute arbitrary code, steal data, install backdoors, pivot to other systems, and maintain persistent access.
Likely Case
Privileged administrator account compromise leading to website defacement, data theft, or installation of web shells for further attacks.
If Mitigated
Limited impact if proper access controls restrict admin permissions and regular security monitoring is in place.
🎯 Exploit Status
Exploitation requires admin credentials with specific permission. Public proof-of-concept exists in advisory.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 1.8.29
Vendor Advisory: https://github.com/mybb/mybb/security/advisories/GHSA-8gxx-vmr9-h39p
Restart Required: No
Instructions:
1. Backup your MyBB installation and database. 2. Download MyBB 1.8.29 or later from official site. 3. Replace all files except inc/config.php and uploads directory. 4. Run upgrade script if needed. 5. Verify functionality.
🔧 Temporary Workarounds
Remove settings management permission
allTemporarily revoke 'Can manage settings?' permission from all admin accounts until patching.
UPDATE mybb_adminoptions SET settings=0 WHERE uid IN (SELECT uid FROM mybb_users WHERE usergroup IN (SELECT gid FROM mybb_usergroups WHERE isadmin=1))
🧯 If You Can't Patch
- Implement strict access controls: Only grant 'Can manage settings?' permission to absolutely necessary administrators.
- Enable web application firewall rules to block PHP code injection patterns in admin panel requests.
🔍 How to Verify
Check if Vulnerable:
Check MyBB version in Admin CP dashboard or inc/version.php file. If version is less than 1.8.29, system is vulnerable.Check Version:
grep -i 'mybb_version' inc/version.phpVerify Fix Applied:
After upgrade, verify version shows 1.8.29 or higher in Admin CP. Test settings management functionality works without allowing PHP type settings.📡 Detection & Monitoring
Log Indicators:
- Admin CP log entries showing creation/modification of settings with 'php' type
- Web server logs showing POST requests to admin/modules/config/settings.php with suspicious parameters
Network Indicators:
- HTTP POST requests to admin control panel containing PHP code in parameters
SIEM Query:
source="web_logs" AND uri="*/admin/modules/config/settings.php*" AND (param="*php*" OR body="*eval(*" OR body="*system(*")