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CVE-2025-20136: Denial of Service Vulnerability in Cisco Secure Firewall ASA and FTD Software

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Overview

A significant vulnerability has been discovered in the function that performs IPv4 and IPv6 Network Address Translation (NAT) DNS inspection for Cisco’s Secure Firewall Adaptive Security Appliance (ASA) Software and Secure Firewall Threat Defense (FTD) Software. This vulnerability affects a broad range of businesses and institutions that rely on these products for their cybersecurity infrastructure. The exploit can lead to a denial of service (DoS) condition, which could potentially compromise the system or result in data leakage.
The vulnerability, identified as CVE-2025-20136, is particularly concerning due to its potential to cause severe disruption to crucial network services. The exploit can lead to an unexpected device reload, disrupting the normal operation of the device, and potentially impacting the security and integrity of the network.

Vulnerability Summary

CVE ID: CVE-2025-20136
Severity: High (CVSS score 8.6)
Attack Vector: Network
Privileges Required: None
User Interaction: None
Impact: Denial of service, potential system compromise, and data leakage

Affected Products

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Product | Affected Versions

Cisco Secure Firewall ASA Software | All versions with NAT44, NAT64, or NAT46 and DNS inspection enabled
Cisco Secure Firewall FTD Software | All versions with NAT44, NAT64, or NAT46 and DNS inspection enabled

How the Exploit Works

The vulnerability is due to an infinite loop condition that occurs when a Cisco Secure ASA or Cisco Secure FTD device processes DNS packets with DNS inspection enabled and the device is configured for NAT44, NAT64, or NAT46. An attacker can exploit this vulnerability by sending crafted DNS packets that match a static NAT rule with DNS inspection enabled through an affected device. Successful exploitation could create an infinite loop, causing the device to reload and leading to a Denial of Service (DoS) condition.

Conceptual Example Code

While the exploit does not directly relate to a specific HTTP request or shell command, the conceptual implementation of this vulnerability might involve sending a DNS packet crafted to exploit the vulnerability. The code block below provides a conceptual example using a hypothetical DNS manipulation tool:

# Using a hypothetical tool 'dns-exploit'
dns-exploit --target 192.168.1.1 --natrule "static NAT rule" --payload "crafted DNS packet"

In the above example, the attacker uses a hypothetical tool (`dns-exploit`) to send a crafted DNS packet that matches a static NAT rule with DNS inspection enabled, targeting the IP address of the vulnerable device (`192.168.1.1`).

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Disclaimer:

The information and code presented in this article are provided for educational and defensive cybersecurity purposes only. Any conceptual or pseudocode examples are simplified representations intended to raise awareness and promote secure development and system configuration practices.

Do not use this information to attempt unauthorized access or exploit vulnerabilities on systems that you do not own or have explicit permission to test.

Ameeba and its authors do not endorse or condone malicious behavior and are not responsible for misuse of the content. Always follow ethical hacking guidelines, responsible disclosure practices, and local laws.

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