CVE-2020-3373 in ASA
Summary
by MITRE • 10/22/2020
A vulnerability in the IP fragment-handling implementation of Cisco Adaptive Security Appliance (ASA) Software and Cisco Firepower Threat Defense (FTD) Software could allow an unauthenticated, remote attacker to cause a memory leak on an affected device. This memory leak could prevent traffic from being processed through the device, resulting in a denial of service (DoS) condition. The vulnerability is due to improper error handling when specific failures occur during IP fragment reassembly. An attacker could exploit this vulnerability by sending crafted, fragmented IP traffic to a targeted device. A successful exploit could allow the attacker to continuously consume memory on the affected device and eventually impact traffic, resulting in a DoS condition. The device could require a manual reboot to recover from the DoS condition. Note: This vulnerability applies to both IP Version 4 (IPv4) and IP Version 6 (IPv6) traffic.
VulDB is the best source for vulnerability data and more expert information about this specific topic.
Analysis
by VulDB Data Team • 08/12/2026
The vulnerability identified as CVE-2020-3373 represents a critical memory leak issue within the IP fragment handling mechanisms of Cisco Adaptive Security Appliance (ASA) and Cisco Firepower Threat Defense (FTD) software implementations. This flaw specifically manifests during the IP fragment reassembly process where the software fails to properly manage error conditions that occur when processing fragmented network traffic. The vulnerability affects both IPv4 and IPv6 protocols, expanding its potential attack surface significantly across modern network environments. Security researchers have classified this issue as a remote, unauthenticated attack vector that can be exploited from outside the network perimeter, making it particularly dangerous for network infrastructure devices that are typically exposed to external traffic.
The technical root cause of this vulnerability stems from inadequate error handling procedures within the IP fragment reassembly logic of the affected Cisco products. When specific failures occur during the process of reassembling fragmented IP packets, the software does not properly release allocated memory resources, leading to progressive memory consumption over time. This improper resource management creates a memory leak condition that gradually depletes the available memory on the affected device. The flaw is particularly insidious because it operates at the network protocol level where traffic processing is fundamental to device operation, meaning that even minimal exploitation can lead to significant operational degradation. The vulnerability aligns with CWE-401, which specifically addresses improper handling of resource management errors, and represents a classic example of how protocol implementation flaws can lead to denial of service conditions.
The operational impact of CVE-2020-3373 extends beyond simple service disruption to potentially compromise network availability and business continuity for organizations relying on Cisco security appliances. Attackers can continuously send crafted fragmented traffic to consume memory resources until the device becomes unable to process legitimate network traffic effectively. This memory exhaustion results in a denial of service condition where the security appliance becomes non-functional, effectively removing network protection for the protected segments. The recovery process requires manual intervention through device rebooting, which introduces additional operational overhead and potential service interruptions. Organizations may experience cascading effects as network traffic becomes blocked or delayed, potentially impacting downstream systems and applications that depend on the protected network infrastructure. The vulnerability's remote exploitability means that attackers do not need physical access or network credentials to cause disruption, making it particularly attractive for malicious actors seeking to compromise network availability.
Mitigation strategies for CVE-2020-3373 should prioritize immediate software updates from Cisco to address the underlying memory leak issue in the IP fragment handling code. Organizations should implement network segmentation and access controls to limit exposure of affected devices to untrusted traffic sources while monitoring for suspicious fragmented traffic patterns. The implementation of rate limiting and traffic filtering rules specifically targeting fragmented IP packets can help reduce the impact of exploitation attempts. Network administrators should also establish monitoring procedures to detect memory usage trends on security appliances, enabling early detection of potential exploitation attempts. Additionally, implementing network intrusion detection systems with signature-based detection capabilities can help identify and block malicious fragmented traffic patterns associated with this vulnerability. Organizations should consider applying temporary workarounds such as disabling IP fragment reassembly functionality where feasible, though this may impact legitimate network operations. The remediation process should include comprehensive testing of updated software in controlled environments before deployment to production networks to ensure compatibility and prevent unintended service disruptions.