CVE-2025-20136: Cisco Secure Firewall DoS Vulnerability

CVE-2025-20136 Overview

A denial of service vulnerability exists in the IPv4 and IPv6 Network Address Translation (NAT) DNS inspection functionality of Cisco Secure Firewall Adaptive Security Appliance (ASA) Software and Cisco Secure Firewall Threat Defense (FTD) Software. This vulnerability allows an unauthenticated, remote attacker to cause the affected device to reload unexpectedly, resulting in a complete denial of service condition.

The vulnerability is triggered by an infinite loop condition that occurs when a Cisco Secure ASA or Cisco Secure FTD device processes specially crafted DNS packets with DNS inspection enabled while the device is configured for NAT44, NAT64, or NAT46. An attacker could exploit this vulnerability by sending crafted DNS packets that match a static NAT rule with DNS inspection enabled through an affected device.

Critical Impact

Successful exploitation causes an infinite loop that forces the firewall device to reload, disrupting all network traffic flowing through the appliance and potentially leaving network segments unprotected during the reboot cycle.

Affected Products

• Cisco Secure Firewall Adaptive Security Appliance (ASA) Software with NAT DNS inspection enabled
• Cisco Secure Firewall Threat Defense (FTD) Software with NAT DNS inspection enabled
• Devices configured with NAT44, NAT64, or NAT46 and DNS inspection

Discovery Timeline

• August 14, 2025 - CVE-2025-20136 published to NVD
• August 15, 2025 - Last updated in NVD database

Technical Details for CVE-2025-20136

Vulnerability Analysis

This vulnerability falls under CWE-835 (Loop with Unreachable Exit Condition), commonly known as an Infinite Loop vulnerability. The core issue resides in the DNS inspection function responsible for processing DNS packets when Network Address Translation is configured.

When the firewall processes DNS traffic that matches a static NAT rule with DNS inspection enabled, specific characteristics in the crafted DNS packet cause the processing logic to enter a loop that lacks a proper exit condition. This algorithmic flaw results in the device consuming all available processing resources, ultimately triggering a device reload to recover from the hung state.

The attack is particularly concerning because it can be initiated by an unauthenticated remote attacker who can send traffic through the firewall. The vulnerability affects both IPv4 and IPv6 NAT configurations (NAT44, NAT64, and NAT46), expanding the potential attack surface across different network architectures.

Root Cause

The root cause is an infinite loop condition (CWE-835) in the DNS inspection processing logic. When specific DNS packet structures are processed in conjunction with NAT rules, the exit condition for the processing loop is never satisfied, causing the device to enter an infinite loop state.

The affected code path handles DNS packet inspection during NAT translation, where the DNS payloads need to be examined and potentially modified to reflect translated addresses. The improper handling of certain DNS packet formats prevents the loop from terminating normally.

Attack Vector

The attack is network-based and does not require authentication or user interaction. An attacker must be able to send DNS packets through the affected firewall that match a static NAT rule with DNS inspection enabled. The attack can be executed remotely from any network position that allows traffic to traverse the vulnerable device.

The attacker crafts malicious DNS packets designed to trigger the infinite loop condition. These packets must be structured to match a static NAT rule configuration while containing the specific characteristics that exploit the vulnerability in the DNS inspection logic. When the firewall attempts to inspect and process these packets, it enters the infinite loop and eventually reloads.

Detection Methods for CVE-2025-20136

Indicators of Compromise

• Unexpected firewall device reloads or reboots without administrative action
• System logs showing device crashes or watchdog timer expirations
• High CPU utilization on the firewall immediately before device reload
• Network connectivity interruptions coinciding with suspicious DNS traffic patterns
• Multiple reload events within a short time period, potentially indicating active exploitation attempts

Detection Strategies

• Monitor firewall system logs for unexpected reload events, crash dumps, or watchdog timer triggers
• Configure SNMP traps or syslog alerts for device reload events
• Implement network traffic analysis to identify anomalous DNS traffic patterns targeting the firewall
• Enable detailed DNS inspection logging where possible to capture traffic characteristics before crashes
• Deploy IDS/IPS rules to detect malformed DNS packets that could indicate exploitation attempts

Monitoring Recommendations

• Establish baseline metrics for DNS traffic volume and firewall CPU utilization
• Configure alerting for device availability monitoring with rapid notification of outages
• Review crash dump files and core dumps following any unexpected device reloads
• Implement redundant monitoring paths to detect firewall unavailability during DoS conditions
• Consider traffic mirroring to capture DNS traffic for forensic analysis during incident response

How to Mitigate CVE-2025-20136

Immediate Actions Required

• Review firewall configurations to identify devices with NAT44, NAT64, or NAT46 combined with DNS inspection
• Consult the Cisco Security Advisory for specific affected software versions
• Apply vendor patches as they become available from Cisco
• Implement compensating controls such as rate limiting DNS traffic if patching is delayed
• Ensure high availability configurations are properly functioning to minimize downtime during potential attacks

Patch Information

Cisco has published a security advisory for this vulnerability. Administrators should review the Cisco Security Advisory for detailed information on affected software versions and available patches. Organizations should prioritize applying the official patch to all affected ASA and FTD devices.

Workarounds

• Evaluate whether DNS inspection can be temporarily disabled on NAT rules until patches are applied (assess security trade-offs)
• Implement upstream DNS filtering or rate limiting to reduce exposure to crafted DNS packets
• Consider deploying DNS traffic through alternate paths that do not traverse vulnerable NAT configurations
• Enable high availability or failover configurations to minimize service disruption if exploitation occurs
• Restrict DNS traffic sources where possible using access control lists to reduce attack surface

# Configuration review example - identify NAT rules with DNS inspection
# On Cisco ASA, review NAT and DNS inspection configuration:
show run nat
show run policy-map
show service-policy inspect dns

# Review for static NAT rules that may be affected
show xlate