CVE-2021-26088
📋 TL;DR
CVE-2021-26088 is an authentication bypass vulnerability in Fortinet's FSSO Collector that allows unauthenticated attackers to bypass firewall policies by sending specially crafted UDP packets. This affects organizations using FSSO Collector version 5.0.295 and below for network access control. Attackers can gain unauthorized access to protected network resources.
💻 Affected Systems
- Fortinet FSSO Collector
📦 What is this software?
⚠️ Risk & Real-World Impact
Worst Case
Complete network compromise where attackers bypass all FSSO-based firewall policies, gaining unrestricted access to internal resources and potentially moving laterally through the network.
Likely Case
Unauthorized access to specific protected resources behind the firewall, potentially leading to data exfiltration or further network exploitation.
If Mitigated
Limited impact with proper network segmentation and additional authentication layers preventing significant damage even if bypass occurs.
🎯 Exploit Status
Exploitation requires network access to the FSSO Collector UDP port (typically 8000). Crafted packets can be generated with basic networking tools.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 5.0.296 and above
Vendor Advisory: https://fortiguard.com/advisory/FG-IR-20-191
Restart Required: Yes
Instructions:
1. Download FSSO Collector version 5.0.296 or later from Fortinet support portal. 2. Backup current configuration. 3. Install the updated version. 4. Restart the FSSO Collector service. 5. Verify functionality.
🔧 Temporary Workarounds
Network Access Control
allRestrict access to FSSO Collector UDP port (default 8000) to only trusted authentication sources.
# Example firewall rule to restrict UDP port 8000
# iptables -A INPUT -p udp --dport 8000 -s <trusted_ip> -j ACCEPT
# iptables -A INPUT -p udp --dport 8000 -j DROP
Disable UDP Notifications
allConfigure FSSO to use TCP polling mode instead of UDP notifications if supported by your environment.
# In FSSO Collector configuration:
# Set PollingMode = TCP
# Disable UDPNotification = true
🧯 If You Can't Patch
- Implement strict network segmentation to limit potential damage from bypassed policies
- Deploy additional authentication mechanisms (MFA, NAC) for critical resources
🔍 How to Verify
Check if Vulnerable:
Check FSSO Collector version in administration interface or configuration files. Versions 5.0.295 and below are vulnerable.Check Version:
# On Windows: Check FSSO Collector About dialog
# On Linux: Check /opt/Fortinet/FSSO/version.txt or similarVerify Fix Applied:
Verify version is 5.0.296 or higher and test authentication flow with legitimate and crafted packets.📡 Detection & Monitoring
Log Indicators:
- Unexpected UDP packets to port 8000 from unauthorized sources
- Authentication events without corresponding legitimate user activity
- Firewall policy bypass events in FortiGate logs
Network Indicators:
- Unusual UDP traffic patterns to FSSO Collector port
- Authentication requests from unexpected IP addresses
- Network access from unauthorized sources
SIEM Query:
source_port=8000 AND protocol=UDP AND (src_ip NOT IN [trusted_auth_sources])