CVE-2026-20021: Cisco Secure Firewall OSPF DoS Vulnerability

CVE-2026-20021 Overview

A vulnerability in the OSPF protocol implementation of Cisco Secure Firewall Adaptive Security Appliance (ASA) Software and Cisco Secure Firewall Threat Defense (FTD) Software could allow an authenticated, adjacent attacker to exhaust memory on an affected device, resulting in a denial of service (DoS) condition. This vulnerability is classified as CWE-401: Missing Release of Memory after Effective Lifetime (Memory Leak).

The vulnerability stems from improper validation of input by the OSPF protocol when parsing packets. An attacker could exploit this vulnerability by sending crafted OSPF packets to an affected device. A successful exploit could allow the attacker to exhaust memory on the affected device, resulting in a DoS condition that disrupts network routing and firewall services.

Critical Impact

Successful exploitation can cause memory exhaustion on Cisco ASA and FTD devices, leading to denial of service conditions that affect network routing and firewall protection capabilities.

Affected Products

• Cisco Secure Firewall Adaptive Security Appliance (ASA) Software
• Cisco Secure Firewall Threat Defense (FTD) Software
• Devices running OSPF protocol configurations

Discovery Timeline

• 2026-03-04 - CVE-2026-20021 published to NVD
• 2026-03-05 - Last updated in NVD database

Technical Details for CVE-2026-20021

Vulnerability Analysis

This vulnerability affects the OSPF (Open Shortest Path First) routing protocol implementation in Cisco's firewall products. OSPF is a widely-used interior gateway protocol that enables routers to dynamically exchange routing information within an autonomous system.

The core issue lies in how the OSPF protocol handler processes incoming packets. When malformed or specially crafted OSPF packets are received, the software fails to properly release allocated memory after processing, leading to a memory leak condition (CWE-401). Over time, repeated exploitation causes memory resources to become exhausted, degrading device performance and eventually causing a denial of service.

The attack requires adjacency to the target network, meaning the attacker must be on the same network segment or have routing adjacency with the affected device. This constraint limits the attack surface to scenarios where an attacker has already gained some level of network access, such as through compromised internal systems or physical access to network infrastructure.

Root Cause

The root cause is improper memory management in the OSPF packet parsing routines. Specifically, when the OSPF protocol receives and processes packets, memory is allocated to store packet data and state information. Due to missing or inadequate memory deallocation logic, certain packet sequences or malformed packets cause allocated memory to never be freed. This constitutes a memory leak vulnerability (CWE-401) where memory is not properly released after its effective lifetime.

Attack Vector

The attack requires adjacent network access, meaning the attacker must be on the same network segment or have OSPF routing adjacency with the target device. The exploitation process involves:

1. The attacker gains access to a network segment adjacent to the target Cisco ASA or FTD device
2. The attacker crafts malicious OSPF packets designed to trigger the memory leak condition
3. These packets are sent to the target device's OSPF-enabled interface
4. The device processes the packets, allocating memory that is never released
5. Continued packet transmission exhausts available memory on the device
6. The device becomes unresponsive or experiences degraded performance, resulting in DoS

The attack does not require authentication to the device itself, but does require the attacker to be in a position to send OSPF packets to the affected interface.

Detection Methods for CVE-2026-20021

Indicators of Compromise

• Abnormal memory utilization patterns on Cisco ASA or FTD devices with OSPF enabled
• Unexpected OSPF neighbor state changes or flapping
• Unusual volume of OSPF packets from unexpected sources
• System logs indicating memory allocation failures or resource exhaustion
• Device performance degradation coinciding with OSPF traffic spikes

Detection Strategies

• Monitor device memory utilization for gradual increases without corresponding traffic growth
• Implement OSPF authentication to reject packets from unauthorized sources
• Configure SNMP traps for memory threshold alerts on ASA and FTD devices
• Review OSPF neighbor tables for unexpected or unauthorized adjacencies
• Deploy network monitoring to baseline and alert on anomalous OSPF traffic patterns

Monitoring Recommendations

• Enable logging for OSPF events and memory allocation warnings
• Configure memory utilization thresholds with automated alerting
• Implement NetFlow or similar traffic analysis to identify abnormal OSPF packet volumes
• Establish baseline OSPF neighbor counts and alert on deviations
• Monitor device CPU and memory metrics through Cisco's built-in monitoring tools

How to Mitigate CVE-2026-20021

Immediate Actions Required

• Review and apply available patches from Cisco for affected ASA and FTD software versions
• Enable OSPF authentication (MD5 or SHA) to prevent unauthorized OSPF packet injection
• Audit OSPF configurations to ensure only necessary interfaces participate in OSPF
• Implement access control lists to restrict OSPF traffic to known, legitimate sources
• Establish monitoring for memory utilization on affected devices

Patch Information

Cisco has released security updates to address this vulnerability. Administrators should consult the Cisco Security Advisory for specific version information and upgrade guidance. The advisory provides details on affected versions and the corresponding fixed releases for both ASA Software and FTD Software.

Workarounds

• Enable OSPF authentication using MD5 or SHA to prevent unauthorized packet injection
• Restrict OSPF participation to only required interfaces using passive interface configurations
• Implement interface access control lists to filter OSPF traffic from untrusted sources
• Segment networks to limit attacker adjacency opportunities
• Consider using static routing on critical paths if OSPF can be temporarily disabled

# OSPF Authentication Configuration Example (ASA)
router ospf 1
 area 0 authentication message-digest
!
interface GigabitEthernet0/0
 ospf message-digest-key 1 md5 <strong-password>
 ospf authentication message-digest