CVE-2024-20526
📋 TL;DR
An unauthenticated remote attacker can send crafted SSH messages to Cisco ASA devices to exhaust SSH resources, causing a denial of service for new SSH connections. Existing SSH sessions continue working, but the device requires manual reboot to restore SSH functionality. This affects Cisco ASA devices running vulnerable software versions.
💻 Affected Systems
- Cisco Adaptive Security Appliance (ASA) Software
📦 What is this software?
Adaptive Security Appliance Software by Cisco
View all CVEs affecting Adaptive Security Appliance Software →
Adaptive Security Appliance Software by Cisco
View all CVEs affecting Adaptive Security Appliance Software →
⚠️ Risk & Real-World Impact
Worst Case
Attackers completely block SSH administrative access to internet-facing ASA devices, requiring physical console access or ASDM for recovery.
Likely Case
Intermittent SSH connection failures for administrators, requiring manual device reboots to restore access.
If Mitigated
Minimal impact if SSH access is restricted to internal networks only with proper network segmentation.
🎯 Exploit Status
Crafted SSH messages required, but no authentication needed. Attack is straightforward once message format is known.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Refer to Cisco advisory for fixed versions
Vendor Advisory: https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-asa-ssh-dos-eEDWu5RM
Restart Required: Yes
Instructions:
1. Check Cisco advisory for fixed versions. 2. Download appropriate ASA software update. 3. Apply update following Cisco ASA upgrade procedures. 4. Reboot device after update.
🔧 Temporary Workarounds
Restrict SSH Access
allLimit SSH access to trusted management networks only using ACLs
access-list SSH-ACL extended permit tcp TRUSTED-NETWORK MASK ASA-IP eq 22
ssh TRUSTED-NETWORK MASK interface INSIDE
Disable SSH if Not Needed
allTemporarily disable SSH if alternative management methods (ASDM, console) are available
no ssh
🧯 If You Can't Patch
- Implement strict network ACLs to allow SSH only from trusted management IPs
- Monitor SSH connection failures and have console/ASDM access ready for emergency recovery
🔍 How to Verify
Check if Vulnerable:
Check ASA version with 'show version' and compare against Cisco advisory affected versions listCheck Version:
show version | include VersionVerify Fix Applied:
Verify running fixed version with 'show version' and test SSH connections remain stable under normal load📡 Detection & Monitoring
Log Indicators:
- Multiple SSH connection failures
- SSH resource exhaustion messages
- High rate of SSH connection attempts
Network Indicators:
- Unusual SSH traffic patterns to ASA devices
- SSH connections from unexpected sources
SIEM Query:
source="asa" AND ("SSH" AND ("failed" OR "denied" OR "resource"))