CVE-2025-20134: Cisco Secure Firewall DoS Vulnerability

CVE-2025-20134 Overview

CVE-2025-20134 is a denial of service vulnerability in Cisco Secure Firewall Adaptive Security Appliance (ASA) Software and Cisco Secure Firewall Threat Defense (FTD) Software. The flaw resides in the SSL/TLS certificate processing logic and allows an unauthenticated, remote attacker to force the affected device to reload. Exploitation requires sending crafted DNS packets that match a static Network Address Translation (NAT) rule with DNS inspection enabled. The vulnerability is tracked under [CWE-415] (Double Free) and carries a network-based attack vector with no privileges or user interaction required.

Critical Impact

An unauthenticated remote attacker can trigger an unexpected device reload on Cisco ASA and FTD firewalls, producing a sustained denial of service condition on perimeter security infrastructure.

Affected Products

• Cisco Secure Firewall Adaptive Security Appliance (ASA) Software
• Cisco Secure Firewall Threat Defense (FTD) Software
• Devices configured with static NAT rules and DNS inspection enabled

Discovery Timeline

• 2025-08-14 - CVE-2025-20134 published to NVD
• 2026-04-15 - Last updated in NVD database

Technical Details for CVE-2025-20134

Vulnerability Analysis

The vulnerability stems from improper parsing of SSL/TLS certificates within the certificate processing routines of Cisco ASA and FTD Software. When the device receives DNS traffic that matches a static NAT rule with DNS inspection enabled, the inspection engine processes certificate data along the traffic path. A malformed certificate structure triggers a memory management error classified as a double free condition [CWE-415]. Double free conditions corrupt heap metadata and lead to inconsistent allocator state. In this implementation, the corruption causes the device to reload, terminating all active sessions traversing the firewall.

Root Cause

The root cause is improper handling of SSL/TLS certificate fields during parsing. The code path frees the same memory region more than once when processing crafted input, which destabilizes the allocator. This memory safety defect surfaces only when the affected traffic flow reaches the certificate parser through the DNS inspection pipeline.

Attack Vector

An attacker exploits the flaw by sending crafted DNS packets through the affected device. The packets must match an existing static NAT rule with DNS inspection enabled. No authentication, credentials, or user interaction are required, and the attack originates from the network. Because Cisco ASA and FTD devices commonly sit at network perimeters, the attack surface is reachable from untrusted networks when matching NAT and inspection policies are present. Refer to the Cisco Security Advisory for vendor technical details.

Detection Methods for CVE-2025-20134

Indicators of Compromise

• Unexpected reload events on Cisco ASA or FTD devices logged in show crashinfo or system traceback output.
• Repeated traceback entries referencing SSL/TLS certificate parsing or DNS inspection modules.
• Bursts of anomalous DNS traffic targeting interfaces with static NAT and DNS inspection enabled immediately before a reload.

Detection Strategies

• Correlate firewall reload events with upstream DNS traffic patterns to identify reload-triggering flows.
• Inspect syslog messages for %ASA- reload or traceback entries that coincide with DNS inspection activity.
• Monitor SNMP coldStart and warmStart traps from ASA and FTD devices as early indicators of unexpected reloads.

Monitoring Recommendations

• Forward ASA and FTD syslog and crashinfo data to a centralized SIEM for correlation with network telemetry.
• Alert on more than one unscheduled reload per device within a short interval.
• Track DNS traffic volume and entropy against interfaces with DNS inspection enabled to identify crafted packet floods.

How to Mitigate CVE-2025-20134

Immediate Actions Required

• Identify all ASA and FTD devices running affected software versions and inventory those with static NAT rules combined with DNS inspection.
• Apply the fixed software releases listed in the Cisco Security Advisory during the next maintenance window.
• Restrict DNS traffic reaching affected interfaces to trusted resolvers where operationally feasible.

Patch Information

Cisco has published fixed software releases for ASA and FTD addressing this vulnerability. Consult the vendor advisory at Cisco Security Advisory cisco-sa-asaftd-ssltls-dos-eHw76vZe for the specific fixed versions corresponding to each affected train. There are no public proof-of-concept exploits or CISA KEV listings for this CVE at the time of publication.

Workarounds

• Disable DNS inspection on static NAT rules where the inspection is not strictly required by the security policy.
• Apply access control lists upstream of affected interfaces to limit DNS traffic to known, trusted sources.
• Implement rate limiting on DNS traffic traversing the firewall to reduce the chance of successful exploitation while patches are deployed.