CVE-2025-27794
📋 TL;DR
This CVE describes a session hijacking vulnerability in Flarum forum software where an attacker controlling any subdomain under a parent domain can set cookies scoped to the parent domain, potentially replacing session tokens on sibling subdomains. This affects Flarum installations using subdomains under a common parent domain where the attacker controls any subdomain. The vulnerability requires specific domain configuration and browser security measures may limit practical exploitation.
💻 Affected Systems
- Flarum
📦 What is this software?
Flarum by Flarum
⚠️ Risk & Real-World Impact
Worst Case
Attacker gains unauthorized access to user accounts on Flarum forums by hijacking active sessions, potentially leading to account takeover, data theft, or privilege escalation.
Likely Case
Limited impact due to browser security restrictions and the requirement for attacker-controlled subdomains; most real-world exploitation would require specific domain misconfigurations.
If Mitigated
No impact if proper domain isolation, cookie security flags, or session token rotation are implemented.
🎯 Exploit Status
Exploitation requires attacker to control a subdomain under the same parent domain as the target Flarum installation; browser security measures may prevent practical exploitation as described.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 1.8.10
Vendor Advisory: https://github.com/flarum/framework/security/advisories/GHSA-hg9j-64wp-m9px
Restart Required: Yes
Instructions:
1. Backup your Flarum installation and database. 2. Update Flarum to version 1.8.10 or later using Composer: 'composer require flarum/core:^1.8.10'. 3. Clear cache and restart your web server. 4. Verify the update completed successfully.
🔧 Temporary Workarounds
Implement session token rotation
allAdd custom session management that rotates session tokens after authentication to prevent cookie replacement attacks.
Implement custom session handler in Flarum extensions
Configure cookie security flags
allSet HttpOnly, Secure, and SameSite=Strict flags on session cookies to limit cookie scope.
Configure in web server or application settings
🧯 If You Can't Patch
- Isolate Flarum installation to use dedicated domain rather than subdomain under shared parent domain
- Implement web application firewall rules to detect and block cookie manipulation attempts
🔍 How to Verify
Check if Vulnerable:
Check Flarum version: if version is below 1.8.10 and installation uses subdomains under a shared parent domain, it may be vulnerable.Check Version:
php flarum info | grep 'Flarum Core'Verify Fix Applied:
Verify Flarum version is 1.8.10 or higher and check that session cookies have proper security flags set.📡 Detection & Monitoring
Log Indicators:
- Multiple failed login attempts followed by successful login from different subdomain
- Unusual cookie manipulation in request headers
Network Indicators:
- Requests setting cookies with parent domain scope from attacker-controlled subdomains
SIEM Query:
source="web_logs" AND (cookie:*domain= OR set-cookie:*domain=) AND uri="*evil.*"