CVE-2026-20316: Cisco Secure Firewall Auth Bypass Flaw

CVE-2026-20316 Overview

Cisco Secure Firewall Management Center (FMC) Software contains a static credentials vulnerability in its web interface. An unauthenticated, remote attacker can log in to an affected device using a hardcoded low-privileged account. Successful exploitation grants access to sensitive data as that low-privileged user.

Cisco assigned this advisory a Security Impact Rating of High rather than Medium. The reason is that attackers can chain this flaw with other Cisco Secure FMC Software vulnerabilities to elevate privileges. The vulnerability is tracked as [CWE-259: Use of Hard-coded Password]. CISA has added CVE-2026-20316 to its Known Exploited Vulnerabilities catalog.

Critical Impact
Remote attackers can authenticate to FMC devices without valid credentials and read sensitive data, enabling further privilege escalation when combined with additional FMC vulnerabilities.

Affected Products

Cisco Secure Firewall Management Center (FMC) Software
FMC deployments with the web interface exposed on the management network
FMC appliances with management interfaces reachable from the public internet (increased attack surface)

Discovery Timeline

2026-07-29 - CVE-2026-20316 published to NVD
2026-07-30 - Last updated in NVD database

Technical Details for CVE-2026-20316

Vulnerability Analysis

The flaw exists in the web interface of Cisco Secure Firewall Management Center Software. The product ships with static user credentials tied to a low-privileged account. Any attacker who knows the credentials can authenticate over the network without prior access.

On its own, the vulnerability exposes sensitive data readable by the low-privileged account. Cisco warns that adversaries can combine this authenticated foothold with additional FMC weaknesses to escalate to higher-privileged roles. That chaining potential is why Cisco elevated the Security Impact Rating to High.

FMC is the centralized management plane for Cisco firewalls, intrusion prevention policies, and threat intelligence. A foothold on FMC can expose firewall configurations, network topology, event logs, and policy data across the entire managed estate.

Root Cause

The root cause is the presence of hard-coded credentials shipped with the product ([CWE-259]). The account exists by default and cannot be removed through normal administrative workflows. Because the credential is static across installations, once it is known publicly, every unpatched instance becomes trivially accessible.

Attack Vector

The attack requires only network reachability to the FMC web interface. No user interaction and no prior authentication are needed. The attacker submits the static credentials to the login endpoint and receives an authenticated session as the low-privileged user.

Cisco notes that FMC management interfaces without public internet exposure have a reduced attack surface. Instances reachable from the internet or from broad internal segments face the highest risk. Refer to the Cisco Security Advisory for authoritative technical details.

Detection Methods for CVE-2026-20316

Indicators of Compromise

Successful web interface logins to FMC from unexpected source IP addresses, particularly external or non-administrative subnets.
Repeated login attempts against the FMC web interface using the same low-privileged username.
Session activity from the affected low-privileged account outside normal operational hours or from geographies inconsistent with administrator locations.
Read access to sensitive FMC data (policy exports, event queries) initiated by an account that historically performs no such actions.

Detection Strategies

Enable and forward FMC web interface audit logs to a centralized SIEM for correlation against known-good administrator IP ranges.
Baseline the accounts that normally authenticate to FMC and alert on any deviation, particularly usage of default or service-tier accounts.
Correlate FMC login events with firewall and VPN telemetry to identify unauthorized ingress paths reaching the management network.

Monitoring Recommendations

Continuously monitor management-plane network segments for unauthorized inbound connections to FMC on HTTPS.
Alert on any successful authentication by the account identified in the Cisco Security Advisory as containing static credentials.
Track privilege changes, role assignments, and configuration exports on FMC as leading indicators of chained privilege escalation.

How to Mitigate CVE-2026-20316

Immediate Actions Required

Apply the fixed Cisco Secure FMC Software release referenced in the Cisco Security Advisory as the primary remediation.
Remove any public internet exposure of the FMC management interface and restrict access to a dedicated management network.
Review authentication logs since the last patch cycle for evidence of exploitation and rotate credentials for any account that could have been reached from the compromised session.
Given inclusion in the CISA Known Exploited Vulnerabilities catalog, treat patching as time-critical and follow the CISA-mandated remediation deadline for federal environments.

Patch Information

Cisco has released fixed software addressing CVE-2026-20316. The corrected releases remove the static credentials and are documented in the Cisco Security Advisory cisco-sa-fmc-static-cred-BET3Cjh. Administrators should identify their running FMC version, cross-reference the fixed release table in the advisory, and schedule an upgrade. No official workaround eliminates the hard-coded account short of the patch.

Workarounds

Restrict inbound access to the FMC web interface to a small allowlist of administrator source addresses via network ACLs or firewall rules.
Place FMC behind a jump host or VPN that enforces multi-factor authentication before the web interface is reachable.
Disable or block external management access until the patch is applied, accepting the operational impact as a temporary trade-off.

bash

# Example ACL concept restricting FMC web interface to a management subnet
# Apply on the upstream firewall or router protecting the FMC appliance
access-list FMC-MGMT permit tcp 10.10.20.0 0.0.0.255 host <FMC_IP> eq 443
access-list FMC-MGMT deny   tcp any host <FMC_IP> eq 443
access-list FMC-MGMT permit ip any any