CVE-2021-40117 Overview
A vulnerability exists in the SSL/TLS message handler for Cisco Adaptive Security Appliance (ASA) Software and Cisco Firepower Threat Defense (FTD) Software that could allow an unauthenticated, remote attacker to cause a denial of service (DoS) condition on an affected device. This vulnerability stems from improper processing of incoming SSL/TLS packets. An attacker could exploit this vulnerability by sending a crafted SSL/TLS packet to an affected device, causing it to reload and resulting in a DoS condition.
Critical Impact
Remote unauthenticated attackers can cause network security appliances to reload, disrupting critical security perimeter defenses and potentially leaving networks unprotected during the outage.
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, 5585-X series appliances
Discovery Timeline
- October 27, 2021 - CVE-2021-40117 published to NVD
- November 21, 2024 - Last updated in NVD database
Technical Details for CVE-2021-40117
Vulnerability Analysis
This denial of service vulnerability affects the SSL/TLS message handler component within Cisco ASA and FTD software. The vulnerability is classified under CWE-119 (Improper Restriction of Operations within the Bounds of a Memory Buffer) and CWE-400 (Uncontrolled Resource Consumption), indicating that the flaw involves improper memory buffer operations when processing SSL/TLS traffic.
The attack can be executed remotely over the network without requiring any authentication or user interaction, making it particularly dangerous for internet-facing security appliances. When exploited, the affected device undergoes a reload, causing a complete service interruption that can leave protected network segments vulnerable.
Root Cause
The root cause of this vulnerability lies in the improper processing of incoming SSL/TLS packets within the message handler. The SSL/TLS stack fails to adequately validate or handle certain malformed or crafted packets, leading to a condition that triggers a device reload. This improper input validation in the cryptographic protocol handling allows attackers to craft specific packets that cause the device to enter an unstable state.
Attack Vector
The attack vector is network-based, requiring the attacker to send specially crafted SSL/TLS packets to an affected device. The attack requires no authentication or privileges, and no user interaction is needed for successful exploitation. The vulnerability affects devices that have SSL/TLS services enabled, which is common for VPN concentrators, management interfaces, and other secure communication endpoints.
The attack flow involves:
- Attacker identifies an internet-facing Cisco ASA or FTD device with SSL/TLS services enabled
- Attacker crafts a malicious SSL/TLS packet designed to trigger the vulnerability
- The malformed packet is sent to the target device
- The SSL/TLS message handler improperly processes the packet
- The device reloads, causing denial of service to all traffic passing through the appliance
Detection Methods for CVE-2021-40117
Indicators of Compromise
- Unexpected device reloads or reboots on Cisco ASA or FTD appliances
- Crash dump files indicating SSL/TLS handler failures
- Intermittent connectivity issues through the firewall
- Syslog messages indicating abnormal SSL/TLS processing errors
Detection Strategies
- Monitor Cisco ASA/FTD syslog messages for crash events and unexpected reloads
- Implement network traffic analysis to detect anomalous SSL/TLS handshake patterns
- Review device crash logs using show crashinfo command for SSL/TLS related crashes
- Deploy intrusion detection signatures for malformed SSL/TLS traffic patterns
Monitoring Recommendations
- Configure SNMP traps for device reload events on all Cisco ASA and FTD appliances
- Implement external uptime monitoring for critical firewall devices
- Enable detailed logging for SSL/TLS related events and forward to SIEM
- Monitor for repeated connection attempts to SSL/TLS services from single sources
How to Mitigate CVE-2021-40117
Immediate Actions Required
- Review the Cisco Security Advisory for affected version details
- Inventory all Cisco ASA and FTD devices in your environment
- Prioritize patching for internet-facing devices with SSL/TLS services enabled
- Implement network segmentation to limit exposure of affected devices
Patch Information
Cisco has released security patches to address this vulnerability. Organizations should consult the Cisco Security Advisory cisco-sa-asaftd-dos-4ygzLKU9 for detailed information on fixed software versions and upgrade paths specific to their deployed hardware and software combinations. Apply the appropriate fixed release for your ASA or FTD software version as documented in the advisory.
Workarounds
- Limit access to SSL/TLS services to trusted IP addresses using access control lists
- Implement rate limiting on SSL/TLS connection attempts where possible
- Consider deploying upstream DDoS protection services for critical appliances
- Enable high availability configurations to minimize impact of device reloads
# Example: Restrict SSL/TLS management access to trusted networks
access-list MGMT-ACCESS extended permit tcp 10.0.0.0 255.255.255.0 any eq 443
access-list MGMT-ACCESS extended deny tcp any any eq 443
# Apply to appropriate interface
access-group MGMT-ACCESS in interface outside
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
