CVE-2026-20105: 
Cisco Adaptive Security Appliance (ASA) vulnerability analysis and mitigation

CVE-2026-20105 is an authenticated memory exhaustion denial-of-service (DoS) vulnerability in the Remote Access SSL VPN functionality of Cisco Secure Firewall Adaptive Security Appliance (ASA) Software and Cisco Secure Firewall Threat Defense (FTD) Software. An authenticated, remote attacker with a valid VPN connection can send crafted packets to exhaust device memory, causing the device to reload and resulting in a DoS condition. The vulnerability was first published on March 4, 2026, and was discovered during internal security testing by Jason Crowder of the Cisco Advanced Security Initiatives Group (ASIG). It carries a CVSS v3.1 base score of 7.7 (High) (Cisco Advisory, Feedly).

Affected software versions include:

• ASA Software: 9.12.1–9.16.4.84, 9.17.1–9.18.4.65, 9.19.1–9.20.3.x, 9.22.1.1–9.22.2.3, 9.23.1–9.23.1.6
• FTD Software: 6.4.0–7.0.8.x, 7.1.0–7.2.10.x, 7.3.0–7.4.2.x, 7.6.0–7.6.3.x, 7.7.0–7.7.10.x

The vulnerability is classified as CWE-401 (Missing Release of Memory after Effective Lifetime), meaning the software fails to properly release allocated memory after it is no longer needed. The root cause is insufficient validation of user-supplied input within the Remote Access SSL VPN server component — the software trusts user input without adequate sanitization, allowing an attacker to trigger unbounded memory allocation. Exploitation requires the attacker to already hold a valid, authenticated VPN session, after which they send specially crafted packets to the SSL VPN server endpoint. The management and MUS interfaces are not affected by this specific CVE. The vulnerability is tracked under Cisco Bug ID CSCwo73891 (Cisco Advisory).

Successful exploitation causes complete memory exhaustion on the affected Cisco ASA or FTD device, forcing it to reload and resulting in a denial of service condition for all users and services dependent on that device. Because these are network security appliances, a reload can disrupt VPN connectivity, firewall enforcement, and network segmentation for all connected users and downstream systems. There is no confidentiality or integrity impact — the vulnerability is purely an availability concern — but the potential for disrupting critical network infrastructure makes it significant in enterprise environments (Cisco Advisory, Feedly).

As of the time of this report, there is no public proof-of-concept (PoC) exploit code and no evidence of active in-the-wild exploitation. Cisco PSIRT has confirmed it is not aware of any public announcements or malicious use of this vulnerability. The EPSS score is approximately 0.076%, indicating a low probability of exploitation in the near term. The vulnerability is not listed in the CISA Known Exploited Vulnerabilities (KEV) catalog. Exploitation requires prior authentication with valid VPN credentials, which meaningfully raises the bar for opportunistic attackers (Cisco Advisory, Feedly).

1. Obtain valid VPN credentials: The attacker must first acquire legitimate credentials for the target organization's Remote Access SSL VPN — through phishing, credential stuffing, or insider access — since exploitation requires an authenticated VPN session.
2. Establish a VPN connection: Using the obtained credentials, the attacker connects to the target Cisco ASA or FTD device's Remote Access SSL VPN endpoint.
3. Craft malicious packets: The attacker prepares specially crafted packets designed to trigger unbounded memory allocation within the SSL VPN server's input processing logic, exploiting the lack of input validation (CWE-401).
4. Send crafted packets to the VPN server: The attacker repeatedly transmits the crafted packets to the Remote Access SSL VPN server while maintaining the authenticated session.
5. Trigger memory exhaustion and device reload: As the device's memory is progressively exhausted, the device becomes unresponsive and eventually reloads, resulting in a DoS condition for all users and services on the affected appliance (Cisco Advisory).

• Network: Unusual volume of SSL VPN traffic from a single authenticated session; repeated or abnormally large packets directed at the Remote Access SSL VPN server endpoint.
• Logs: Cisco ASA/FTD system logs showing memory allocation failures or memory exhaustion warnings prior to a device reload; unexpected device reload events logged in syslog or SNMP traps.
• Device Behavior: Sudden device reload or crash without a clear hardware or configuration cause; VPN service becoming unresponsive while other interfaces remain briefly active; management interface becoming temporarily unresponsive.
• SNMP/Monitoring: Memory utilization alerts spiking to 100% on the affected appliance; availability monitoring detecting unexpected device reboots during active VPN sessions.

Cisco has released fixed software versions that fully remediate this vulnerability. There are no workarounds available. Organizations should upgrade to the following fixed releases as soon as possible:

• ASA Software: 9.16.4.85 or later, 9.18.4.66 or later, 9.20.4 or later, 9.22.2.4 or later, 9.23.1.7 or later
• FTD Software: 7.0.9 or later, 7.2.11 or later, 7.4.3 or later, 7.6.4 or later, 7.7.11 or later

As interim risk-reduction measures, administrators should implement strict access controls to limit VPN connections to trusted users and known IP ranges, monitor VPN server logs for memory exhaustion events or anomalous session behavior, and consider restricting remote VPN access if not immediately required for business operations (Cisco Advisory, Feedly).

The vulnerability was disclosed as part of Cisco's March 2026 Semiannual Cisco Secure Firewall ASA, Secure FMC, and Secure FTD Software Security Advisory Bundled Publication, which covered five related CVEs (CVE-2026-20100 through CVE-2026-20106). The Belgian Centre for Cybersecurity (CCB) issued a warning about multiple critical vulnerabilities in Cisco products from this bundle. The Center for Internet Security (CIS) also published an advisory noting that multiple vulnerabilities in Cisco products could allow for remote code execution and DoS conditions. Community and social media coverage was moderate, with aggregators such as CVEFeed, VulDB, and Mastodon security accounts noting the disclosure, but no significant researcher commentary or controversy was observed given the authenticated-only exploitation requirement (Cisco Advisory).