CVE-2022-20760: Cisco Firepower Threat Defense DoS Vulnerability

CVE-2022-20760 Overview

CVE-2022-20760 is a denial of service vulnerability in the DNS inspection handler of Cisco Adaptive Security Appliance (ASA) Software and Cisco Firepower Threat Defense (FTD) Software. An unauthenticated, remote attacker can send crafted DNS requests at a high rate to an affected device and cause it to stop responding. The flaw stems from improper processing of incoming DNS requests, categorized as uncontrolled resource consumption [CWE-400]. Cisco published advisory cisco-sa-asaftd-dos-nJVAwOeq describing the issue. The vulnerability affects perimeter security devices, which are typically internet-facing and central to network availability.

Critical Impact

A successful exploit allows a remote, unauthenticated attacker to render Cisco ASA or FTD devices unresponsive, disrupting firewall, VPN, and inspection services for the entire protected network.

Affected Products

• Cisco Adaptive Security Appliance (ASA) Software (multiple versions)
• Cisco Firepower Threat Defense (FTD) Software (multiple versions, including 7.1.0)
• Cisco ASA and FTD platforms with DNS inspection enabled

Discovery Timeline

• 2022-05-03 - CVE-2022-20760 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2022-20760

Vulnerability Analysis

The vulnerability resides in the DNS inspection handler used by Cisco ASA and FTD Software. DNS inspection is an application-layer protocol inspection feature that parses DNS traffic traversing the firewall. The handler does not properly process incoming requests when they arrive at high volume. Attackers exploit this gap by transmitting crafted DNS packets at sustained high rates against the affected device.

The weakness maps to [CWE-400], Uncontrolled Resource Consumption. Resource consumption flaws in inline security devices are particularly impactful because the device itself is the choke point for protected traffic. An exploit does not yield code execution or data disclosure, but it removes the firewall as an availability dependency for downstream services.

The EPSS score for this CVE is 2.588%, placing it in the 85th percentile for likelihood of exploitation activity across the CVE corpus.

Root Cause

The DNS inspection handler lacks adequate flow control and request validation for high-rate inbound DNS traffic. When request volume exceeds what the handler can process, internal resources are exhausted and the device halts packet processing. Cisco's advisory attributes the condition to improper processing of incoming requests rather than a specific parser bug.

Attack Vector

Exploitation requires only network reachability to a DNS inspection-enabled interface. The attacker sends a high-rate stream of crafted DNS requests toward the affected ASA or FTD device. No authentication, user interaction, or prior foothold is required. Because DNS inspection commonly runs on outside-facing interfaces, the attack surface frequently extends to the public internet.

The vulnerability is described in prose because no public proof-of-concept code has been released. Refer to the Cisco Security Advisory cisco-sa-asaftd-dos-nJVAwOeq for full technical details, affected versions, and fixed releases.

Detection Methods for CVE-2022-20760

Indicators of Compromise

• Sudden unresponsiveness or reboot of an ASA or FTD device with DNS inspection enabled
• Spikes in inbound DNS query rate (UDP/53 or TCP/53) directed at firewall interfaces
• Loss of management plane connectivity coinciding with elevated DNS traffic volume
• High CPU utilization on the inspection process prior to device hang

Detection Strategies

• Monitor show service-policy inspect dns counters and DNS inspection drop statistics for anomalous growth
• Baseline normal DNS traffic volume hitting firewall interfaces and alert on deviations
• Correlate device syslog messages indicating resource exhaustion with NetFlow records showing DNS surges
• Forward firewall telemetry to a SIEM or data lake to correlate DNS volume spikes with availability events

Monitoring Recommendations

• Enable SNMP polling of CPU and memory utilization on ASA and FTD devices with alerting thresholds
• Capture NetFlow or IPFIX from upstream routers to identify the source of high-rate DNS floods
• Track device health via Cisco Firepower Management Center health monitoring policies
• Configure syslog forwarding for inspection engine errors to a centralized logging platform

How to Mitigate CVE-2022-20760

Immediate Actions Required

• Identify all ASA and FTD devices with DNS inspection enabled and prioritize them for patching
• Apply the fixed software releases listed in the Cisco security advisory cisco-sa-asaftd-dos-nJVAwOeq
• Restrict DNS traffic permitted to ingress firewall interfaces from untrusted networks where feasible
• Verify upstream rate-limiting or DDoS scrubbing protections are in place for perimeter devices

Patch Information

Cisco has released fixed software for both ASA and FTD. Consult the Cisco Security Advisory for the specific fixed-release versions that correspond to each affected train. There are no Cisco-provided workarounds; upgrading to a fixed release is the remediation path.

Workarounds

• No official workarounds are available from Cisco for CVE-2022-20760
• Where patching is not immediately possible, consider disabling DNS inspection if it is not required for the deployment
• Apply infrastructure access control lists (iACLs) to limit which sources can send DNS traffic to the device
• Deploy upstream rate limiting on edge routers to constrain inbound DNS request volume

# Example: disable DNS inspection in the default global policy as a temporary mitigation
configure terminal
policy-map global_policy
 class inspection_default
  no inspect dns preset_dns_map
end
write memory