Cisco Adaptive Security Appliance Software SSL VPN Denial of Service Vulnerability

A vulnerability in the Secure Sockets Layer (SSL) VPN feature of Cisco Adaptive Security Appliance (ASA) Software could allow an authenticated, remote attacker to cause a denial of service (DoS) condition that prevents the creation of new SSL/Transport Layer Security (TLS) connections to an affected device.

The vulnerability is due to incorrect handling of Base64-encoded strings. An attacker could exploit this vulnerability by opening many SSL VPN sessions to an affected device. The attacker would need to have valid user credentials on the affected device to exploit this vulnerability. A successful exploit could allow the attacker to overwrite a special system memory location, which will eventually result in memory allocation errors for new SSL/TLS sessions to the device, preventing successful establishment of these sessions. A reload of the device is required to recover from this condition. Established SSL/TLS connections to the device and SSL/TLS connections through the device are not affected.

Note: Although this vulnerability is in the SSL VPN feature, successful exploitation of this vulnerability would affect all new SSL/TLS sessions to the device, including management sessions.

Cisco has released software updates that address this vulnerability. There are no workarounds that address this vulnerability.

This advisory is available at the following link:
https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20191002-asa-ssl-vpn-dos [“https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20191002-asa-ssl-vpn-dos”]
This advisory is part of the October 2019 Cisco ASA, FMC, and FTD Software Security Advisory Bundled Publication, which includes 10 Cisco Security Advisories that describe 18 vulnerabilities. For a complete list of the advisories and links to them, see Cisco Event Response: October 2019 Cisco ASA, FMC, and FTD Software Security Advisory Bundled Publication [“https://sec.cloudapps.cisco.com/security/center/viewErp.x?alertId=ERP-72541”].