A vulnerability in the DNS resolution function of the Cisco Web Security Appliance (WSA) could allow an unauthenticated, remote attacker to cause a partial denial of service (DoS) condition due to DNS name resolution failing through the device.
 
The vulnerability is due to the handling of DNS requests awaiting a DNS response when new, incoming DNS requests are received. An attacker could exploit this vulnerability by sending TCP proxy traffic to the WSA at a high rate. An exploit could allow the attacker to cause a partial DoS condition because DNS name resolution fails, which results in the client receiving a HTTP 503 ”Service Unavailable” error.

Cisco has confirmed the vulnerability and released software updates.

To exploit the vulnerability, the attacker may need access to trusted or internal networks to send DNS queries to the targeted system. This access requirement could limit the likelihood of a successful exploit.

Cisco indicates through the CVSS score that functional exploit code exists; however, the code is not known to be publicly available.

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