Critical Vulnerability Advisory: Defending Against Cisco ASA and FTD Memory Disclosure (CVE-2020-3259)
Cisco Adaptive Security Appliance (ASA) and Firepower Threat Defense (FTD) contain an information disclosure vulnerability. An attacker could retrieve memory contents on an affected device, which could lead to the disclosure of confidential information due to a buffer tracking issue when the software parses invalid URLs that are requested from the web services interface. This vulnerability affects only specific AnyConnect and WebVPN configurations.
FREQUENTLY ASKED
What is CVE-2020-3259 and why does it matter?
CVE-2020-3259 is an information disclosure vulnerability (CWE-200) within the web services interface of Cisco Adaptive Security Appliance (ASA) and Firepower Threat Defense (FTD). It allows an unauthenticated, remote attacker to retrieve sensitive memory contents. This matters because exposed memory can contain credentials, session tokens, or configuration data, providing a foothold for further attacks against the network perimeter.
Which versions of the product are affected?
This vulnerability affects multiple versions of Cisco software. Specifically, Cisco ASA Software versions 9.8, 9.9, 9.10, 9.12, and 9.13 are impacted. Additionally, Cisco Firepower Threat Defense (FTD) Software versions 6.2.3, 6.3.0, 6.4.0, and 6.5.0 are affected. Note that this risk is specifically tied to AnyConnect and WebVPN configurations on these devices.
Has a patch been released for CVE-2020-3259?
Yes, a patch has been released by Cisco. The vendor addresses this vulnerability through specific software updates for both ASA and FTD. Administrators should refer to the official Cisco Security Advisory (cisco-sa-asaftd-info-disclose-9eJtycMB) to identify the appropriate fixed release for their specific deployment. Applying these updates is the primary method to remediate the underlying buffer tracking issue.
What is the remediation deadline and what it means for compliance?
The remediation deadline for CVE-2020-3259 is March 7, 2024. For federal agencies and organizations following CISA BOD 22-01, this deadline is a mandatory requirement to mitigate known exploited vulnerabilities. Missing this deadline indicates a compliance failure and leaves the organization exposed to active threats, including potential ransomware deployment targeting these unpatched edge devices.
How do I check if an instance or deployment is affected?
To determine if an instance is affected, administrators should check if AnyConnect or WebVPN services are enabled on their Cisco ASA or FTD device. If these services are active, verify the running software version against the list of affected versions, such as ASA 9.12 or FTD 6.4.0. Running a version check via the command line or management console will confirm vulnerability status.
CVE-2020-3259 is a high-severity information disclosure vulnerability (CWE-200) affecting Cisco ASA and FTD software with a CVSS score of 7.5. This flaw allows unauthenticated attackers to remotely retrieve sensitive memory contents, and due to its known use by ransomware and active exploitation status, immediate remediation is required by the March 7, 2024 deadline.
Technical Deep Dive: CWE-200 and Memory Disclosure
The vulnerability identified as CVE-2020-3259 centers on CWE-200: Exposure of Sensitive Information to an Unauthorized Actor. In the context of Cisco ASA and FTD devices, this manifests as a critical failure in memory management within the web services interface. This interface is primarily responsible for handling client requests for VPN services, including Cisco AnyConnect and WebVPN.
The root cause is a buffer tracking issue that occurs during the parsing of malformed or invalid URLs. When the software encounters a specifically crafted GET request that deviates from expected URL structures, the internal pointer logic fails to validate the boundaries of the buffer being used for the request. This allows the system to inadvertently return data from adjacent memory locations instead of the requested resource.
Analyzing the Attack Chain
The attack chain for CVE-2020-3259 is notably simple, which contributes to its "LOW" attack complexity.
Reconnaissance: An attacker identifies a Cisco ASA or FTD device with the web services interface exposed to the internet, typically serving a VPN landing page.
Exploitation: The attacker sends a crafted HTTP GET request to the device. This request contains an invalid URL structure designed to trigger the buffer tracking logic failure.
Data Extraction: The device processes the request and, due to the vulnerability, responds with a chunk of its internal memory.
Information Gathering: The attacker repeats the process to leak various segments of memory. This disclosed information can include usernames, session tokens, passwords, and internal configuration details.
This type of memory leak is often referred to as a "heartbleed-style" vulnerability. Unlike an exploit that achieves Remote Code Execution (RCE), CVE-2020-3259 is a passive retrieval mechanism. However, its impact is devastating because it provides the attacker with the exact credentials needed to bypass authentication, making the "unauthenticated" nature of the attack particularly dangerous for the network's blast radius.
Who Is Affected
This vulnerability impacts organizations relying on Cisco ASA and FTD appliances for secure remote access. Specifically, the following software versions are confirmed vulnerable:
Cisco ASA Software: Versions 9.8, 9.9, 9.10, 9.12, and 9.13.
Cisco FTD Software: Versions 6.2.3, 6.3.0, 6.4.0, and 6.5.0.
The vulnerability is only exploitable when the web services interface is enabled. This typically includes deployments where AnyConnect or WebVPN services are active on the interface facing the public internet.
Compliance and CISA BOD 22-01
CISA has added CVE-2020-3259 to the Known Exploited Vulnerabilities (KEV) Catalog. For federal agencies and contractors, this carries a mandatory remediation requirement under Binding Operational Directive (BOD) 22-01. The Remediation Deadline of March 7, 2024, signifies that the threat is not theoretical; active exploitation has been observed in the wild, frequently as a precursor to ransomware deployment. Failure to patch by this date leaves the perimeter exposed to automated scanning and subsequent compromise.
Official Remediation Steps
To secure affected environments, administrators must move to fixed software releases as identified by Cisco. There is no viable workaround that does not involve disabling the affected VPN services.
Identify Vulnerable Instances: Execute the show version command on the ASA CLI or check the Firepower Management Center (FMC) to determine the current software version. Confirm if AnyConnect or WebVPN is enabled.
Download Fixed Software: Visit the Cisco Software Central and download the appropriate fixed release for your platform. Refer to the Cisco Security Advisory for the specific mapping of vulnerable versions to fixed releases.
Stage and Apply Patch: Follow standard organizational procedures for firmware updates. For ASA, this involves uploading the new image to flash and updating the boot variable. For FTD, the update should be pushed via the FMC or local manager.
Verify Remediation: After the reboot, verify that the device is running the target version. Test VPN connectivity to ensure services are operating correctly after the update.
Rotate Sensitive Credentials: Because this vulnerability allows memory disclosure, it is a security best practice to assume that any session tokens or administrative credentials active during the vulnerability window may have been compromised. Consider a mandatory password reset for VPN users and administrative accounts.
Security Best Practices
Implement Multi-Factor Authentication (MFA): Even if an attacker leaks a password through memory disclosure, MFA provides a secondary layer of defense that prevents unauthorized session establishment.
Egress Filtering and Segmentation: Ensure that the VPN termination point is segmented from the rest of the network. If a session is hijacked, segmentation limits the attacker's ability to move laterally.
Continuous Monitoring: Monitor web service logs for an unusual volume of malformed GET requests or 404/400 errors directed at the AnyConnect/WebVPN endpoints.
Geoblocking and IP Whitelisting: If your user base is geographically restricted, implement geoblocking on the VPN interface to reduce the attack surface available to global scanning bots.
Vulnerability Lifecycle Management: Use the EPSS score (currently 0.69725 for this CVE) to prioritize patching. High EPSS scores combined with "Active" SSVC status should always trigger emergency change windows.
Regular Configuration Audits: Periodically audit the ASA/FTD configuration to ensure that only necessary services are exposed to the internet. Disable the web services interface on any interface where it is not required.