CVE-2021-34753
📋 TL;DR
This vulnerability allows unauthenticated remote attackers to bypass Cisco Firepower Threat Defense security rules for Ethernet Industrial Protocol traffic by sending specially crafted ENIP packets. It affects Cisco FTD software due to incomplete deep packet inspection processing. Organizations using Cisco FTD with ENIP traffic inspection enabled are vulnerable.
💻 Affected Systems
- Cisco Firepower Threat Defense (FTD) Software
📦 What is this software?
Firepower Threat Defense Software by Cisco
Firepower Threat Defense Software by Cisco
Firepower Threat Defense Software by Cisco
⚠️ Risk & Real-World Impact
Worst Case
Attackers could bypass all configured access control and intrusion policies for ENIP traffic, potentially allowing unauthorized access to industrial control systems or critical infrastructure.
Likely Case
Attackers bypass specific security rules for ENIP traffic, potentially allowing unauthorized communication with industrial devices that should be blocked.
If Mitigated
With proper network segmentation and defense-in-depth, impact is limited to ENIP traffic only, preventing broader network compromise.
🎯 Exploit Status
Exploitation requires sending crafted ENIP packets to vulnerable interfaces. No authentication required.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 6.6.5.2, 6.7.0.2, or 7.0.0 and later
Vendor Advisory: https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-ftd-enip-bypass-eFsxd8KP
Restart Required: Yes
Instructions:
1. Download appropriate patch from Cisco Software Center. 2. Upload to FTD management center. 3. Deploy update to affected devices. 4. Restart devices as required.
🔧 Temporary Workarounds
Disable ENIP Inspection
allTemporarily disable ENIP traffic inspection if not required
Configure via FMC: Devices > Device Management > Access Control > Edit Policy > Disable ENIP inspection rules
Network Segmentation
allIsolate ENIP traffic to separate VLANs with additional firewall controls
🧯 If You Can't Patch
- Implement strict network segmentation for ENIP traffic
- Deploy additional layer of firewall protection for industrial control network segments
🔍 How to Verify
Check if Vulnerable:
Check FTD version via CLI: 'show version' and compare to affected versions. Verify ENIP inspection is configured.Check Version:
show versionVerify Fix Applied:
After patching, verify version shows 6.6.5.2, 6.7.0.2, or 7.0.0+. Test ENIP traffic inspection functionality.📡 Detection & Monitoring
Log Indicators:
- Unexpected ENIP traffic bypassing configured rules
- ENIP packets from unauthorized sources
Network Indicators:
- Crafted ENIP packets with unusual structure
- ENIP traffic to/from unexpected IP addresses
SIEM Query:
source:ftd AND (event_type:bypass OR protocol:enip) AND action:allowed WHERE rule_action:block