Cisco ASA and FTD Web Services Interface Memory Information Disclosure Vulnerability (CVE-2020-3259) Analysis & Mitigation Guide
Executive & SecOps Summary
SECOPS ASSESSMENTCisco ASA Software and Cisco FTD Software with AnyConnect VPN or WebVPN configurations enabled
An unauthenticated, remote attacker can retrieve sensitive memory contents (such as user credentials and session tokens) by sending crafted HTTP GET requests
Apply the official firmware patches provided by Cisco immediately, or restrict access to the web services interface if patching is delayed
Affected Products & Version Scope
Remediation & Live Workaround Commands
PATCH & WORKAROUND1. Official Patch & Version Upgrade Commands
Download the appropriate patched software image for your hardware platform from the Cisco Software Central and perform the upgrade.
# Example: Copying and setting the new ASA image via CLI
copy tftp://10.1.1.50/asa9141-laddr-k8.spa disk0:/asa9141-laddr-k8.spa
# Update boot system configuration
no boot system disk0:/asa-old-image.bin
boot system disk0:/asa9141-laddr-k8.spa
# Save configuration and reload the device
write memory
reload2. Temporary Workaround & Hardening Configuration
If immediate patching is not feasible, restrict access to the WebVPN/AnyConnect interface using access control lists (ACLs) to allow only trusted source IPs.
# Example: Restricting HTTPS access to a trusted public IP range (e.g., 203.0.113.0/24)
access-list VPN_ACCESS_CONTROL extended permit tcp 203.0.113.0 255.255.255.0 any eq 443
access-list VPN_ACCESS_CONTROL extended deny tcp any any eq 443
# Apply the access list to the outside interface
access-group VPN_ACCESS_CONTROL in interface outside3. Log Audit & IoC Verification Commands
Verify if AnyConnect or WebVPN is enabled on your device and inspect system logs for anomalous web requests.
# Check if WebVPN/AnyConnect is enabled
show run webvpn
# Search system logs for abnormal web services interface activities
show logging | include WebVPN|HTTP|invalidStop Exposing VPN Ports to the Internet — Switch to Zero-Trust Access
Eliminate public-facing VPN concentrators and firewall management ports with identity-first Zero-Trust Network Access.
Technical Root Cause & Exploitation Mechanics
1. Vulnerability Overview & Root Cause
This vulnerability (CVE-2020-3259) is an information disclosure vulnerability in the web services interface of Cisco Adaptive Security Appliance (ASA) Software and Cisco Firepower Threat Defense (FTD) Software. The root cause is a buffer tracking issue that occurs when the software parses invalid URLs requested from the web services interface. Due to improper boundary validation, the system fails to isolate memory buffers, allowing internal memory contents to be leaked in response to specific requests.
2. Exploitation Mechanics & Attack Scenario
An attacker scans for target devices exposing the AnyConnect or WebVPN port (typically TCP 443) to the public internet. Without requiring any authentication, the attacker sends a specially crafted HTTP GET request to the web services interface. The vulnerable device processes this malformed request, triggers the buffer tracking flaw, and appends adjacent memory contents—which may contain active session tokens, usernames, and passwords—into the HTTP response body returned to the attacker. This vulnerability has been actively exploited in the wild (listed in CISA KEV).
3. Infrastructure & Business Impact
Although rated with a CVSS score of 7.5 (High), the real-world impact of this vulnerability is critical because it allows remote, unauthenticated attackers to harvest valid credentials. Since VPN gateways serve as the primary entry point to corporate networks, successful exploitation of this vulnerability often leads to unauthorized network entry, lateral movement, and subsequent ransomware deployment.
Official Vendor Advisories & References
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. Note: This vulnerability affects only specific AnyConnect and WebVPN configurations. For more information, see the Vulnerable Products section.
Frequently Asked SecOps Questions (CVE-2020-3259)
FAQQ1. How critical is CVE-2020-3259 and is it actively exploited in the wild?
CVE-2020-3259 is rated CVSS 7.5 (HIGH) with a 30-day EPSS exploitation probability of 71.5%. It is listed in the CISA Known Exploited Vulnerabilities (KEV) catalog, requiring immediate emergency remediation.
Q2. Which vendors and products are affected by CVE-2020-3259?
Primary impacted vendors include cisco across adaptive security appliance, firepower threat defense. Scope summary: All Cisco ASA Software and Cisco FTD Software releases with AnyConnect or WebVPN features enabled (refer to Cisco's advisory for specific patched versions)
Q3. What temporary mitigations or workarounds exist for CVE-2020-3259 before patching?
[Mitigation] Apply the official firmware patches provided by Cisco immediately, or restrict access to the web services interface if patching is delayed Follow the CLI remediation commands and interactive SecOps checklist on this page to restrict exposure prior to your maintenance window.
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