CVE-2021-34781

Q: What is the severity of CVE-2021-34781?

CVE-2021-34781 has a CVSS score of 8.6 (HIGH). This vulnerability in Cisco Firepower Threat Defense (FTD) Software allows unauthenticated remote attackers to cause denial of service by flooding SSH connections. The attack exhausts system resources, requiring manual device reload for recovery. Organizations using multi-instance FTD deployments are affected.

8.6 HIGH

Denial of Service

📋 TL;DR

This vulnerability in Cisco Firepower Threat Defense (FTD) Software allows unauthenticated remote attackers to cause denial of service by flooding SSH connections. The attack exhausts system resources, requiring manual device reload for recovery. Organizations using multi-instance FTD deployments are affected.

💻 Affected Systems

Products:

Cisco Firepower Threat Defense (FTD) Software

Versions: Versions prior to 6.6.4.1 and 7.0.0

Operating Systems: FTD-specific OS

Default Config Vulnerable: ⚠️ Yes

Notes: Only affects multi-instance deployments; single-instance deployments are not vulnerable.

📦 What is this software?

⚠️ Risk & Real-World Impact

🔴

Worst Case

Complete device unavailability requiring manual reload, disrupting all network security functions and potentially causing extended downtime.

🟠

Likely Case

Service disruption affecting SSH management and potentially other functions due to resource exhaustion, requiring manual intervention.

🟢

If Mitigated

Limited impact with proper network segmentation and rate limiting, potentially avoiding complete DoS.

🌐 Internet-Facing: HIGH

🏢 Internal Only: MEDIUM

🎯 Exploit Status

Public PoC: ✅ No

Weaponized: LIKELY

Unauthenticated Exploit: ⚠️ Yes

Complexity: LOW

Attack requires sending high rate of crafted SSH connections; no authentication needed.

🛠️ Fix & Mitigation

✅ Official Fix

Patch Version: 6.6.4.1 and later, or 7.0.0 and later

Vendor Advisory: https://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-ftd-dos-rUDseW3r

Restart Required: Yes

Instructions:

1. Download appropriate FTD software version from Cisco. 2. Upload to device via management interface. 3. Install update following Cisco upgrade procedures. 4. Reload device to apply changes.

🔧 Temporary Workarounds

SSH Access Restriction

all

Limit SSH access to trusted management networks only

configure network management ssh-access [interface] [trusted-network]

Rate Limiting

all

Implement network-level rate limiting for SSH connections

🧯 If You Can't Patch

Restrict SSH access to management interfaces from trusted networks only
Implement network segmentation to isolate FTD management interfaces

🔍 How to Verify

Check if Vulnerable:

Check FTD version via CLI: 'show version' and verify if below 6.6.4.1 or 7.0.0

Check Version:

show version | include Version

Verify Fix Applied:

After update, verify version is 6.6.4.1+ or 7.0.0+ and test SSH connectivity

📡 Detection & Monitoring

Log Indicators:

High rate of failed SSH connection attempts
Resource exhaustion warnings
SSH service restart logs

Network Indicators:

Unusual high volume of SSH traffic to management interfaces
SSH connection floods from single sources

SIEM Query:

source="ftd_logs" AND ("SSH connection failed" OR "resource exhausted") | stats count by src_ip

📊 Metadata

CVE ID: CVE-2021-34781

CVSS v3 Score: 8.6 (HIGH)

CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:N/I:N/A:H

CWE: CWE-119

Published: October 27, 2021

Last Updated: November 21, 2024

🔗 References

https://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-ftd-dos-rUDseW3r Vendor Advisory
https://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-ftd-dos-rUDseW3r Vendor Advisory

📋 TL;DR

💻 Affected Systems

📦 What is this software?

Firepower Management Center Virtual Appliance by Cisco

Firepower Management Center Virtual Appliance by Cisco

Firepower Management Center Virtual Appliance by Cisco

Firepower Management Center Virtual Appliance by Cisco

Firepower Management Center Virtual Appliance by Cisco

Firepower Management Center Virtual Appliance by Cisco

Firepower Management Center Virtual Appliance by Cisco

Firepower Management Center Virtual Appliance by Cisco

Firepower Threat Defense by Cisco

Firepower Threat Defense by Cisco

Firepower Threat Defense by Cisco

Firepower Threat Defense by Cisco

Sourcefire Defense Center by Cisco

Sourcefire Defense Center by Cisco

Sourcefire Defense Center by Cisco

Sourcefire Defense Center by Cisco

Sourcefire Defense Center by Cisco

Sourcefire Defense Center by Cisco

Sourcefire Defense Center by Cisco

Sourcefire Defense Center by Cisco

⚠️ Risk & Real-World Impact

Worst Case

Likely Case

If Mitigated

🎯 Exploit Status

🛠️ Fix & Mitigation

✅ Official Fix

Instructions:

🔧 Temporary Workarounds

SSH Access Restriction

Rate Limiting

🧯 If You Can't Patch

🔍 How to Verify

Check if Vulnerable:

Check Version:

Verify Fix Applied:

📡 Detection & Monitoring

Log Indicators:

Network Indicators:

SIEM Query:

📊 Metadata

🔗 References

📤 Share & Export

🔗 Related Vulnerabilities