CVE-2021-25827
📋 TL;DR
CVE-2021-25827 is an authentication bypass vulnerability in Emby Server that allows attackers to bypass login requirements by setting the X-Forwarded-For HTTP header to a local IP address. This affects all Emby Server instances running versions below 4.7.12.0 that are exposed to network access.
💻 Affected Systems
- Emby Server
📦 What is this software?
Emby by Emby
⚠️ Risk & Real-World Impact
Worst Case
Complete compromise of the Emby Server instance, allowing unauthorized access to media content, configuration settings, and potential privilege escalation to underlying system.
Likely Case
Unauthorized access to media libraries and server configuration, potentially exposing sensitive media content and user data.
If Mitigated
Limited impact with proper network segmentation and access controls, though authentication bypass remains possible.
🎯 Exploit Status
Exploitation requires only HTTP header manipulation. Public proof-of-concept code exists demonstrating the bypass.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 4.7.12.0 and later
Vendor Advisory: https://github.com/EmbySupport/security/security/advisories/GHSA-fffj-6fr6-3fgf
Restart Required: Yes
Instructions:
1. Download Emby Server version 4.7.12.0 or later from emby.media. 2. Stop the Emby Server service. 3. Install the updated version. 4. Restart the Emby Server service.
🔧 Temporary Workarounds
Reverse Proxy Header Filtering
allConfigure reverse proxy (nginx, Apache, etc.) to strip or validate X-Forwarded-For headers before they reach Emby Server.
# nginx example: proxy_set_header X-Forwarded-For $remote_addr;
# Apache example: RequestHeader unset X-Forwarded-For
Network Access Control
allRestrict network access to Emby Server using firewall rules to only allow trusted IP addresses.
# Example iptables rule: iptables -A INPUT -p tcp --dport 8096 -s TRUSTED_IP -j ACCEPT
# Windows Firewall: New-InboundRule -DisplayName 'Emby Access' -LocalPort 8096 -RemoteAddress TRUSTED_IP -Protocol TCP -Action Allow
🧯 If You Can't Patch
- Implement strict network segmentation to isolate Emby Server from untrusted networks
- Deploy a web application firewall (WAF) configured to block requests with manipulated X-Forwarded-For headers
🔍 How to Verify
Check if Vulnerable:
Check Emby Server version via web interface Dashboard > Help > About, or via API endpoint /System/InfoCheck Version:
curl -s http://emby-server:8096/System/Info | grep -o '"Version":"[^"]*"'Verify Fix Applied:
Verify version is 4.7.12.0 or higher, then test authentication bypass by sending request with X-Forwarded-For: 127.0.0.1 header📡 Detection & Monitoring
Log Indicators:
- Failed login attempts followed by successful access from localhost IP
- Requests with X-Forwarded-For header containing local IP addresses (127.0.0.1, ::1, 192.168.*, 10.*, 172.16-31.*)
Network Indicators:
- HTTP requests to Emby Server with X-Forwarded-For header set to local IP addresses
- Unauthenticated access to protected endpoints
SIEM Query:
source="emby.log" AND ("X-Forwarded-For: 127.0.0.1" OR "X-Forwarded-For: ::1" OR "X-Forwarded-For: 192.168." OR "X-Forwarded-For: 10." OR "X-Forwarded-For: 172.16." OR "X-Forwarded-For: 172.17." OR "X-Forwarded-For: 172.18." OR "X-Forwarded-For: 172.19." OR "X-Forwarded-For: 172.20." OR "X-Forwarded-For: 172.21." OR "X-Forwarded-For: 172.22." OR "X-Forwarded-For: 172.23." OR "X-Forwarded-For: 172.24." OR "X-Forwarded-For: 172.25." OR "X-Forwarded-For: 172.26." OR "X-Forwarded-For: 172.27." OR "X-Forwarded-For: 172.28." OR "X-Forwarded-For: 172.29." OR "X-Forwarded-For: 172.30." OR "X-Forwarded-For: 172.31.")🔗 References
- https://emby.media/community/index.php?/topic/98191-emby-server-46-released/
- https://github.com/EmbySupport/security/security/advisories/GHSA-fffj-6fr6-3fgf
- https://github.com/MediaBrowser/Emby/issues/3784
- https://emby.media/community/index.php?/topic/98191-emby-server-46-released/
- https://github.com/EmbySupport/security/security/advisories/GHSA-fffj-6fr6-3fgf
- https://github.com/MediaBrowser/Emby/issues/3784
