Cisco Adaptive Security Appliance (ASA) software and Cisco Firepower Threat Defense (FTD) software fails to properly parse SIP traffic, which can allow an attacker to trigger high CPU usage, resulting in a denial-of-service condition on affected devices. This vulnerability is exposed if SIP Inspection is enabled on affected devices, which is the default configuration on ASA devices. The Cisco SIP Inspection feature is advertised to "... enforce the sanity of the SIP messages, as well as detect SIP-based attacks."

By causing an affected Cisco device to parse specially-crafted SIP traffic, a remote, unauthenticated attacker may be able to trigger a denial-of-service condition on affected devices.

The CERT/CC is currently unaware of a practical solution to this problem. Please consider the following workarounds:

Disable SIP Inspection

According to the Cisco advisory:

Disabling SIP inspection will completely close the attack vector for this vulnerability. However, it may not be suitable for all customers. In particular, disabling SIP inspection would break SIP connections if either NAT is applied to SIP traffic or if not all ports required for SIP communication are opened via ACL.

To disable SIP inspection, configure the following:

Cisco ASA Software and Cisco FTD Software Releases 6.2 and later (in FTD 6.2 and later use Cisco FMC to add the following via FlexConfig policy):

    policy-map global_policy
     class inspection_default
      no inspect sip

Cisco FTD Software Releases prior to 6.2:

configure inspection sip disable