CVE-2024-20331

Q: What is the severity of CVE-2024-20331?

CVE-2024-20331 has a CVSS score of 6.8 (MEDIUM). This vulnerability in Cisco ASA and FTD software allows unauthenticated remote attackers to disrupt VPN authentication sessions by exploiting insufficient entropy. Attackers can terminate legitimate users' authentication attempts, preventing them from establishing remote access VPN connections. Organizations using affected Cisco VPN appliances are impacted.

6.8 MEDIUM

Denial of Service

📋 TL;DR

This vulnerability in Cisco ASA and FTD software allows unauthenticated remote attackers to disrupt VPN authentication sessions by exploiting insufficient entropy. Attackers can terminate legitimate users' authentication attempts, preventing them from establishing remote access VPN connections. Organizations using affected Cisco VPN appliances are impacted.

💻 Affected Systems

Products:

Cisco Adaptive Security Appliance (ASA) Software
Cisco Firepower Threat Defense (FTD) Software

Versions: Multiple versions - check Cisco advisory for specific affected releases

Operating Systems: Cisco ASA OS, Cisco FTD OS

Default Config Vulnerable: ⚠️ Yes

Notes: Only affects systems with Remote Access SSL VPN feature enabled. ASA in multi-context mode is vulnerable only in admin context.

📦 What is this software?

⚠️ Risk & Real-World Impact

🔴

Worst Case

Complete denial of remote access VPN services for all users, disrupting business operations for remote workers and branch offices.

🟠

Likely Case

Intermittent authentication failures for VPN users, requiring repeated authentication attempts and causing user frustration.

🟢

If Mitigated

Minimal impact with proper monitoring and quick response to authentication anomalies.

🌐 Internet-Facing: HIGH - VPN endpoints are typically internet-facing and directly accessible to attackers.

🏢 Internal Only: LOW - This primarily affects external VPN access, though internal VPN concentrators could be impacted if exposed.

🎯 Exploit Status

Public PoC: ✅ No

Weaponized: UNKNOWN

Unauthenticated Exploit: ⚠️ Yes

Complexity: MEDIUM

Requires attacker to determine user authentication handle, but no authentication needed to exploit.

🛠️ Fix & Mitigation

✅ Official Fix

Patch Version: Check Cisco advisory for specific fixed releases

Vendor Advisory: https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-asa-vpn-nyH3fhp

Restart Required: Yes

Instructions:

1. Review Cisco advisory for affected versions. 2. Download appropriate fixed software from Cisco. 3. Backup configuration. 4. Install update following Cisco upgrade procedures. 5. Verify VPN functionality post-update.

🔧 Temporary Workarounds

Disable Remote Access SSL VPN

all

Temporarily disable the vulnerable feature if not required

no webvpn
no enable outside

Implement Rate Limiting

all

Configure connection rate limiting to reduce attack effectiveness

rate-limit sessions maximum 10
rate-limit deny period 300

🧯 If You Can't Patch

Implement network segmentation to restrict access to VPN endpoints
Enable detailed logging and monitoring for authentication anomalies

🔍 How to Verify

Check if Vulnerable:

Check ASA/FTD version against Cisco advisory. Command: show version | include Version

Check Version:

show version | include Version

Verify Fix Applied:

Verify installed version matches fixed release from Cisco advisory. Command: show version | include Version

📡 Detection & Monitoring

Log Indicators:

Multiple authentication session terminations
Users reporting repeated authentication failures
Unexpected session handle values in logs

Network Indicators:

Unusual spike in authentication requests
Multiple TCP RST packets to VPN port

SIEM Query:

source="asa.log" AND ("authentication failed" OR "session terminated") | stats count by src_ip

📊 Metadata

CVE ID: CVE-2024-20331

CVSS v3 Score: 6.8 (MEDIUM)

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

CWE: CWE-330

Published: October 23, 2024

Last Updated: November 1, 2024

📋 TL;DR

💻 Affected Systems

📦 What is this software?

Adaptive Security Appliance Software by Cisco

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