CVE-2020-3259: Cisco Firepower Threat Defense Disclosure

CVE-2020-3259 Overview

A vulnerability in the web services interface of Cisco Adaptive Security Appliance (ASA) Software and Cisco Firepower Threat Defense (FTD) Software could allow an unauthenticated, remote attacker to retrieve memory contents on an affected device, which could lead to the disclosure of confidential information. The vulnerability is due to a buffer tracking issue when the software parses invalid URLs that are requested from the web services interface. An attacker could exploit this vulnerability by sending a crafted GET request to the web services interface. A successful exploit could allow the attacker to retrieve memory contents, which could lead to the disclosure of confidential information.

Critical Impact

This vulnerability is listed in CISA's Known Exploited Vulnerabilities (KEV) catalog, indicating active exploitation in the wild. Unauthenticated attackers can remotely extract sensitive memory contents including credentials, session tokens, and other confidential data from affected Cisco ASA and FTD devices.

Affected Products

• Cisco Adaptive Security Appliance (ASA) Software
• Cisco Firepower Threat Defense (FTD) Software
• Specific AnyConnect and WebVPN configurations

Discovery Timeline

• May 6, 2020 - CVE-2020-3259 published to NVD
• October 28, 2025 - Last updated in NVD database

Technical Details for CVE-2020-3259

Vulnerability Analysis

This vulnerability affects the web services interface of Cisco ASA and FTD software, specifically when AnyConnect or WebVPN features are enabled. The flaw originates from improper buffer tracking during URL parsing operations. When the software processes specially crafted invalid URLs, it fails to properly manage memory boundaries, allowing portions of device memory to be disclosed in HTTP responses.

The vulnerability is particularly dangerous because it requires no authentication, meaning any remote attacker with network access to the web services interface can attempt exploitation. The disclosed memory contents may contain sensitive information such as authentication credentials, session tokens, VPN configurations, private keys, or other confidential data stored in device memory.

Root Cause

The root cause is a buffer tracking issue (CWE-200: Information Exposure) in the URL parsing component of the web services interface. When invalid URLs are submitted, the software incorrectly tracks buffer boundaries, causing memory contents beyond the intended response data to be included in the HTTP response. This type of memory disclosure vulnerability occurs when input validation and memory management fail to properly constrain the data returned to users.

Attack Vector

An attacker can exploit this vulnerability remotely over the network without requiring any form of authentication or user interaction. The attack is executed by sending specially crafted HTTP GET requests containing malformed URLs to the web services interface of a vulnerable Cisco ASA or FTD device.

The attack workflow involves:

1. Identifying a target running vulnerable Cisco ASA or FTD software with web services (AnyConnect/WebVPN) enabled
2. Crafting malicious HTTP GET requests with invalid URL patterns designed to trigger the buffer tracking flaw
3. Sending multiple requests to harvest memory contents from the target device
4. Analyzing the disclosed memory data to extract credentials, tokens, or other sensitive information

The vulnerability has a network-based attack vector with low complexity, requiring no privileges or user interaction to exploit, making it highly attractive for threat actors seeking initial access to corporate networks.

Detection Methods for CVE-2020-3259

Indicators of Compromise

• Unusual or malformed HTTP GET requests to the web services interface (/+CSCOE+/, /+CSCOT+/, or similar WebVPN/AnyConnect paths)
• HTTP responses containing unexpected binary data or memory artifacts
• Repeated requests from the same source IP targeting web services endpoints
• Authentication events using credentials that may have been harvested through the vulnerability

Detection Strategies

• Monitor web server logs for requests containing unusual URL patterns or invalid characters targeting VPN endpoints
• Implement network intrusion detection rules to identify exploitation attempts against Cisco ASA/FTD web services
• Deploy web application firewall rules to block malformed requests to AnyConnect and WebVPN interfaces
• Analyze HTTP response sizes for anomalies that may indicate memory leakage

Monitoring Recommendations

• Enable detailed logging on Cisco ASA/FTD devices for all web services interface connections
• Configure SIEM correlation rules to detect patterns consistent with memory disclosure exploitation
• Review VPN authentication logs for unauthorized access attempts following potential credential theft
• Implement baseline monitoring of traffic to/from web services interfaces to identify anomalous activity

How to Mitigate CVE-2020-3259

Immediate Actions Required

• Apply the security patches provided by Cisco immediately to all affected ASA and FTD devices
• If patching is not immediately possible, evaluate disabling AnyConnect and WebVPN features temporarily
• Review logs for signs of prior exploitation and rotate credentials if compromise is suspected
• Restrict network access to web services interfaces to trusted IP ranges where feasible

Patch Information

Cisco has released software updates that address this vulnerability. Administrators should consult the Cisco Security Advisory for specific version information and upgrade guidance. Given the vulnerability's inclusion in the CISA Known Exploited Vulnerabilities catalog, organizations should prioritize remediation immediately.

Workarounds

• Limit access to the web services interface by configuring ACLs to allow connections only from trusted IP addresses
• Deploy a Web Application Firewall (WAF) in front of ASA/FTD devices to filter malicious requests
• Consider using an alternative VPN solution until patching can be completed if the risk is deemed critical
• Implement network segmentation to minimize the impact of potential credential disclosure

# Example: Restrict web services access to specific networks on Cisco ASA
access-list RESTRICT-WEBVPN extended permit tcp 10.0.0.0 255.255.255.0 host 192.168.1.1 eq 443
access-list RESTRICT-WEBVPN extended deny tcp any host 192.168.1.1 eq 443
access-group RESTRICT-WEBVPN in interface outside