Introduction
Remote access for distributed workforces can grind to a halt when VPN gateways become unresponsive. CVE-2025-20133 directly impacts business continuity by allowing unauthenticated attackers to disrupt Cisco Secure Firewall ASA and FTD appliances running Remote Access SSL VPN. With a CVSS score of 8.6, this vulnerability is particularly relevant for organizations that depend on Cisco's VPN solutions for secure connectivity.
Cisco's ASA (Adaptive Security Appliance) and Firepower Threat Defense (FTD) are cornerstone products in enterprise network security, deployed by thousands of organizations worldwide. These platforms provide firewall, VPN, and advanced threat protection services, making them critical to the security and availability of modern business networks.
Technical Information
CVE-2025-20133 is a denial of service vulnerability rooted in the Remote Access SSL VPN feature of Cisco ASA and FTD software. The core issue is ineffective validation of user-supplied input during the VPN authentication process. An unauthenticated remote attacker can exploit this by sending specially crafted requests to the SSL VPN service. The flaw is classified under CWE-401 (missing release of memory after effective lifetime), indicating a memory management logic error.
When exploited, the crafted input triggers a memory leak or resource exhaustion scenario. This causes the device to stop responding to Remote Access SSL VPN authentication requests, effectively denying service to legitimate users. The vulnerability does not require authentication and can be triggered remotely, increasing its risk profile.
The issue is present in device configurations where Remote Access SSL VPN is enabled. No public code snippets or proof of concept details are available. The vulnerability is addressed by correcting the memory management logic that handles SSL VPN connections, preventing crafted SSL/TLS packets from causing device reloads.
Patch Information
To address the SSL VPN memory management vulnerability in Cisco Adaptive Security Appliance (ASA) Software and Firepower Threat Defense (FTD) Software, Cisco has released software updates that rectify the underlying issue. (cisco.com)
Understanding the Fix:
The vulnerability stemmed from a logic error in memory management during the handling of SSL VPN connections. This flaw could be exploited by sending crafted SSL/TLS packets to the SSL VPN server, potentially causing the device to reload unexpectedly, leading to a denial of service (DoS) condition.
How the Patch Works:
The software update corrects the memory management logic, ensuring that SSL VPN connections are processed securely without triggering unintended device reloads. By applying this patch, the system's resilience against such crafted packets is significantly enhanced, mitigating the risk of DoS attacks.
Applying the Patch:
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Identify Your Software Version:
- Determine the current version of your Cisco ASA or FTD Software to ascertain if it is among the affected releases.
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Obtain the Updated Software:
- Access the Cisco Support and Downloads page to download the appropriate software update for your device.
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Install the Update:
- Follow the installation instructions provided by Cisco to apply the update. Ensure that the device has sufficient memory and that the current hardware and software configurations are compatible with the new release.
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Verify the Update:
- After installation, confirm that the device is operating correctly and that the SSL VPN feature functions as expected without causing unexpected reloads.
By implementing this patch, administrators can safeguard their devices against potential exploitation of this vulnerability, thereby maintaining the integrity and availability of their network services.
Patch source: https://www.cisco.com/c/en/us/support/docs/csa/cisco-sa-asaftd-webvpn-dos-hOnB9pH4.html
Affected Systems and Versions
- Cisco Adaptive Security Appliance (ASA) Software with Remote Access SSL VPN enabled
- Cisco Firepower Threat Defense (FTD) Software with Remote Access SSL VPN enabled
Specific version numbers and ranges are not provided in the public advisory. Administrators should check their current ASA or FTD software version and confirm if Remote Access SSL VPN is enabled. All configurations with this feature enabled should be considered potentially vulnerable until patched.
Vendor Security History
Cisco has experienced a series of similar vulnerabilities in its ASA and FTD product lines, particularly affecting VPN and web services. Previous issues such as CVE-2024-20353 and CVE-2024-20481 also involved denial of service conditions via crafted requests targeting VPN or web interfaces. Cisco typically issues advisories and patches promptly but continues to face recurring challenges in input validation and memory management within complex features like SSL VPN.