CVE-2026-20335: Cisco Firewall Calculation Error Vulnerability

CVE-2026-20335 is an incorrect calculation flaw in Cisco Secure Adaptive Security Appliance, Firewall Threat Defense, and Management Center Software that could impact system operations. This article covers technical details, affected versions, potential impact, and recommended mitigation strategies.

CVE-2026-20335 Overview

Cisco disclosed CVE-2026-20335 as part of an internal security review of Cisco Secure Adaptive Security Appliance (ASA) Software, Cisco Secure Firewall Threat Defense (FTD) Software, and Cisco Secure Firewall Management Center (FMC) Software. The vulnerability results from an incorrect calculation weakness classified under [CWE-682]. Cisco released a software hardening update to address the issue alongside other internally identified defects. The flaw is network-reachable and requires no authentication or user interaction, though exploitation carries high attack complexity.

Critical Impact
A successful exploit can compromise the confidentiality, integrity, and availability of affected Cisco firewall and management appliances exposed on the network.

Affected Products

Cisco Secure Adaptive Security Appliance (ASA) Software
Cisco Secure Firewall Threat Defense (FTD) Software
Cisco Secure Firewall Management Center (FMC) Software

Discovery Timeline

2026-09-16 - CVE-2026-20335 published to NVD
2026-09-16 - Last updated in NVD database

Technical Details for CVE-2026-20335

Vulnerability Analysis

CVE-2026-20335 is an incorrect calculation vulnerability affecting core Cisco firewall software. The issue was uncovered during an internal Cisco security review focused on product hardening, rather than through external exploitation reporting. Because it is classified under the [CWE-682] pillar, the root defect lies in arithmetic or logical computation performed on untrusted or boundary input. Errors in these calculations can influence downstream memory operations, protocol parsing, or access-control decisions handled by the ASA, FTD, and FMC code paths.

Cisco groups multiple internally discovered issues under this advisory, indicating the affected component performs calculations on network-received data during normal traffic handling. The vulnerability is remotely reachable across the network without credentials, but successful exploitation depends on conditions outside the attacker's direct control.

Root Cause

The root cause is an incorrect calculation defect in the affected Cisco Secure firewall codebase. Under [CWE-682], the software computes a value that is not consistent with its intended logical or numeric result. When that miscalculated value drives subsequent operations, such as buffer sizing, offset computation, or state tracking, an attacker able to shape the input can push the system into an unsafe state.

Attack Vector

The attack vector is network-based. An unauthenticated remote attacker can send crafted traffic to an affected ASA, FTD, or FMC deployment. High attack complexity indicates that exploitation depends on timing, configuration, or environmental factors documented by Cisco. A successful exploit affects confidentiality, integrity, and availability of the appliance. No public proof-of-concept code is available, and the vulnerability is not listed in the CISA Known Exploited Vulnerabilities catalog. Refer to the Cisco Security Advisory: ASA Hardening for component-specific details.

Detection Methods for CVE-2026-20335

Indicators of Compromise

Unexpected process restarts, crashes, or traceback events on ASA or FTD devices logged to syslog.
Anomalous management-plane or data-plane traffic patterns preceding device instability.
Configuration drift or unexplained changes on FMC-managed policies without a corresponding administrative action.

Detection Strategies

Correlate device syslog and SNMP traps for reload events, memory allocation failures, or watchdog-triggered restarts.
Baseline management-plane connections to FMC and alert on new source IPs or unusual protocol behavior.
Ingest firewall telemetry into a centralized analytics platform to identify traffic patterns matching Cisco's advisory guidance.

Monitoring Recommendations

Forward ASA, FTD, and FMC logs to a SIEM or data lake for continuous review.
Enable NetFlow and management access logging to capture the source of anomalous requests.
Monitor firmware and software versions across the fleet and alert on devices running vulnerable releases.

How to Mitigate CVE-2026-20335

Immediate Actions Required

Inventory all ASA, FTD, and FMC deployments and identify software versions in use.
Apply the Cisco hardening release referenced in the advisory as soon as maintenance windows allow.
Restrict management-plane access to trusted administrative networks only.
Review firewall logs for signs of exploitation attempts or device instability.

Patch Information

Cisco addressed CVE-2026-20335 in a software hardening release covering ASA, FTD, and FMC. Fixed release information is documented in the Cisco Security Advisory: ASA Hardening. Administrators should consult the advisory for the specific fixed versions applicable to each platform and deployment model.

Workarounds

No vendor-approved workarounds are documented; patching is the recommended remediation path.
Where immediate patching is not feasible, apply strict access-control lists limiting who can reach the affected appliances over the network.
Segment management interfaces onto an out-of-band administrative network to reduce exposure.

bash

# Configuration example - restrict management access on Cisco ASA
# Replace 10.10.10.0/24 with your trusted administrative subnet
access-list MGMT_ACL extended permit tcp 10.10.10.0 255.255.255.0 any eq 443
access-list MGMT_ACL extended deny  ip any any log
access-group MGMT_ACL in interface management