CVE-2021-40118: Cisco ASA DoS Vulnerability

CVE-2021-40118 Overview

A vulnerability in the web services interface of Cisco Adaptive Security Appliance (ASA) Software and Cisco Firepower Threat Defense (FTD) Software could allow an unauthenticated, remote attacker to trigger a denial of service (DoS) condition. This vulnerability is due to improper input validation when parsing HTTPS requests. An attacker could exploit this vulnerability by sending a malicious HTTPS request to an affected device. A successful exploit could allow the attacker to cause the device to reload, resulting in a DoS condition.

Critical Impact

Unauthenticated remote attackers can cause affected Cisco ASA and FTD devices to reload by sending specially crafted HTTPS requests, resulting in network security infrastructure disruption.

Affected Products

• Cisco Adaptive Security Appliance (ASA) Software
• Cisco Firepower Threat Defense (FTD) Software
• Cisco ASA 5505, 5512-X, 5515-X, 5525-X, 5545-X, 5555-X, 5580, and 5585-X Hardware Appliances

Discovery Timeline

• October 27, 2021 - CVE-2021-40118 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2021-40118

Vulnerability Analysis

This vulnerability resides in the web services interface of Cisco ASA and FTD software, specifically in the HTTPS request parsing functionality. The web services interface is commonly used for remote access VPN (WebVPN/AnyConnect) and management purposes. When the affected device processes incoming HTTPS requests, it fails to properly validate certain input parameters, leading to a stack-based buffer overflow condition (CWE-121) that can result in out-of-bounds memory writes (CWE-787).

The vulnerability is particularly concerning because it requires no authentication to exploit. Any network-accessible attacker who can reach the web services interface on TCP port 443 can send malicious requests to trigger the condition. The attack is relatively simple to execute with low complexity requirements and no user interaction needed.

Root Cause

The root cause of this vulnerability is improper input validation in the HTTPS request parsing code. When the web services interface receives specially crafted HTTPS requests, the parsing logic fails to properly check the boundaries of input data before writing to stack memory. This results in a stack-based buffer overflow (CWE-121) that can corrupt adjacent memory structures, ultimately causing the device to crash and reload.

The vulnerability is classified under CWE-787 (Out-of-bounds Write), indicating that the software writes data past the end or before the beginning of the intended buffer. Combined with CWE-121 (Stack-based Buffer Overflow), this suggests the overflow occurs in stack-allocated memory during request processing.

Attack Vector

The attack vector is network-based, requiring the attacker to have network connectivity to the web services interface of the vulnerable device. Exploitation involves sending a maliciously crafted HTTPS request to the device's web services interface (typically exposed on TCP port 443).

The attack methodology involves:

1. Identifying a vulnerable Cisco ASA or FTD device with the web services interface enabled
2. Crafting an HTTPS request with specially formatted data designed to trigger the buffer overflow
3. Sending the malicious request to the target device
4. The device's HTTPS parsing routine fails to properly validate the input
5. A stack-based buffer overflow occurs, corrupting memory and causing the device to crash
6. The device reloads, causing a denial of service condition

Since no authentication is required and the attack can be executed remotely over the network, this vulnerability poses a significant risk to organizations that expose their ASA or FTD web services interfaces to untrusted networks.

Detection Methods for CVE-2021-40118

Indicators of Compromise

• Unexpected device reloads or reboots of Cisco ASA or FTD appliances
• Crash dump files indicating memory corruption in the web services interface processes
• High volume of unusual HTTPS requests to the management interface from external sources
• Syslog messages indicating the device reloaded due to an unhandled exception or crash

Detection Strategies

• Monitor Cisco ASA/FTD syslog messages for unexpected reload events or crash notifications
• Implement network-based intrusion detection signatures to identify malformed HTTPS requests targeting Cisco web services interfaces
• Deploy web application firewalls or IPS rules to inspect and filter anomalous HTTPS traffic destined for management interfaces
• Analyze traffic patterns for unusual spikes in HTTPS connections to ASA/FTD devices

Monitoring Recommendations

• Enable comprehensive logging on Cisco ASA/FTD devices to capture crash events and reload reasons
• Configure SNMP traps or syslog alerts for device reload events
• Implement network flow analysis to detect anomalous traffic patterns to web services interfaces
• Regularly review crash dump files for signs of exploitation attempts

How to Mitigate CVE-2021-40118

Immediate Actions Required

• Review the Cisco Security Advisory for detailed guidance on affected versions and fixed releases
• Upgrade affected Cisco ASA and FTD devices to patched software versions as specified in the advisory
• Restrict network access to the web services interface to trusted IP addresses only using access control lists (ACLs)
• Consider disabling the web services interface if it is not required for business operations

Patch Information

Cisco has released software updates that address this vulnerability. Organizations should consult the Cisco Security Advisory (cisco-sa-asafdt-webvpn-dos-KSqJAKPA) to determine the appropriate fixed software release for their specific ASA or FTD version. The advisory provides a detailed mapping of affected versions to their corresponding fixed releases.

Administrators should plan and execute upgrades following Cisco's recommended procedures to minimize service disruption while ensuring protection against this vulnerability.

Workarounds

• Implement access control lists (ACLs) to restrict access to the web services interface to only authorized management hosts and VPN client networks
• Use Cisco's control plane protection mechanisms to rate-limit connections to the web services interface
• If feasible, disable the web services interface (WebVPN/AnyConnect) if remote access VPN functionality is not required
• Deploy the ASA/FTD devices behind additional network security controls that can inspect and filter malicious HTTPS traffic

# Example ACL configuration to restrict web services access on Cisco ASA
# Adjust IP addresses to match your trusted management networks

access-list OUTSIDE_MGMT_ACL extended permit tcp host 10.1.1.100 any eq https
access-list OUTSIDE_MGMT_ACL extended permit tcp 10.2.0.0 255.255.0.0 any eq https
access-list OUTSIDE_MGMT_ACL extended deny tcp any any eq https

# Apply to control plane or interface as appropriate
# Consult Cisco documentation for your specific configuration requirements