CVE-2025-20263: Cisco Secure Firewall Buffer Overflow

CVE-2025-20263 Overview

A buffer overflow vulnerability exists in the web services interface of Cisco Secure Firewall Adaptive Security Appliance (ASA) Software and Cisco Secure Firewall Threat Defense (FTD) Software. This vulnerability could allow an unauthenticated, remote attacker to cause a buffer overflow on an affected system, resulting in a denial of service (DoS) condition.

The vulnerability stems from insufficient boundary checks for specific data provided to the web services interface. An attacker could exploit this vulnerability by sending a crafted HTTP request to the affected system, causing the system to reload and disrupting network security operations.

Critical Impact

Unauthenticated remote attackers can cause affected Cisco ASA and FTD devices to reload, resulting in denial of service and potential disruption to network security infrastructure.

Affected Products

• Cisco Secure Firewall Adaptive Security Appliance (ASA) Software
• Cisco Secure Firewall Threat Defense (FTD) Software
• Cisco Firewall devices with web services interface enabled

Discovery Timeline

• August 14, 2025 - CVE-2025-20263 published to NVD
• August 15, 2025 - Last updated in NVD database

Technical Details for CVE-2025-20263

Vulnerability Analysis

This vulnerability is classified as CWE-680 (Integer Overflow to Buffer Overflow), indicating that the root cause involves an integer overflow condition that subsequently leads to a buffer overflow. The web services interface fails to properly validate the size or boundaries of specific input data, allowing an attacker to overflow memory buffers and corrupt adjacent memory structures.

The vulnerability is remotely exploitable without authentication, requiring only network access to the affected web services interface. No user interaction is required for exploitation. The scope of impact extends beyond the vulnerable component itself, as successful exploitation causes the entire system to reload, affecting all services running on the device.

Root Cause

The vulnerability exists due to insufficient boundary checks for specific data that is provided to the web services interface of affected systems. When processing certain HTTP request data, the software fails to properly validate input lengths before allocating or copying data to fixed-size buffers. This insufficient validation allows attackers to supply data that exceeds expected boundaries, triggering an integer overflow condition that results in undersized buffer allocation and subsequent buffer overflow during data processing.

Attack Vector

The attack is conducted remotely over the network by sending specially crafted HTTP requests to the web services interface of vulnerable Cisco ASA or FTD devices. The attacker does not require any authentication credentials or user privileges to exploit this vulnerability.

The exploitation flow involves:

1. The attacker identifies a Cisco ASA or FTD device with the web services interface accessible over the network
2. The attacker crafts a malicious HTTP request containing data designed to trigger the integer overflow and subsequent buffer overflow
3. The crafted request is sent to the target device's web services interface
4. The insufficient boundary validation allows the overflow to occur, corrupting memory structures
5. The memory corruption causes the device to crash and reload, resulting in denial of service

For specific technical details regarding the crafted HTTP request structure and exploitation methodology, refer to the Cisco Security Advisory.

Detection Methods for CVE-2025-20263

Indicators of Compromise

• Unexpected device reloads or reboots of Cisco ASA or FTD appliances
• Crash logs indicating memory corruption in web services components
• Unusual HTTP traffic patterns targeting the web services interface
• Multiple system reload events in a short time period

Detection Strategies

• Monitor Cisco ASA and FTD system logs for unexpected reload events and crashinfo files
• Implement network intrusion detection rules to identify anomalous HTTP requests to the web services interface
• Configure alerting for repeated device reloads that may indicate active exploitation attempts
• Deploy web application firewall rules to filter malformed HTTP requests targeting Cisco devices

Monitoring Recommendations

• Enable comprehensive logging on the web services interface of all affected devices
• Configure SNMP traps or syslog forwarding for device reload events
• Establish baseline traffic patterns to the web services interface for anomaly detection
• Monitor network flows for unusual HTTP request sizes or frequencies targeting Cisco firewalls

How to Mitigate CVE-2025-20263

Immediate Actions Required

• Review the Cisco Security Advisory for affected versions and available patches
• Assess exposure by identifying all Cisco ASA and FTD devices with web services interfaces accessible from untrusted networks
• Apply vendor patches as soon as they become available from Cisco
• Restrict access to the web services interface to trusted IP addresses only

Patch Information

Cisco has published a security advisory addressing this vulnerability. Administrators should consult the Cisco Security Advisory for detailed information on affected software versions and the availability of fixed releases. Organizations should plan to upgrade to patched versions according to Cisco's recommendations.

Workarounds

• Disable the web services interface if it is not required for operational purposes
• Implement access control lists (ACLs) to restrict access to the web services interface to trusted management networks only
• Deploy network-layer protections such as firewall rules to limit exposure of the vulnerable interface
• Consider placing vulnerable devices behind a reverse proxy with additional input validation capabilities

# Example ACL to restrict web services access to trusted management network
access-list MGMT_ACL extended permit tcp 10.0.0.0 255.255.255.0 host [ASA_IP] eq https
access-list MGMT_ACL extended deny tcp any host [ASA_IP] eq https