CVE-2026-76413: Cisco ASDM SSO Authentication Bypass Vulnerability

CVE-2026-76413 Overview

CVE-2026-76413 is a session token forgery vulnerability in the Cisco Adaptive Security Device Manager (ASDM) single sign-on (SSO) handler for Cisco Secure Firewall Management Center (FMC) Software. An unauthenticated, remote attacker can log in as the ASDM administrator user by exploiting improper management of the ASDM SSO token. Repeated exploitation can keep legitimate administrators locked out of ASDM indefinitely. The weakness is tracked as [CWE-1259] Improper Restriction of Security Token Assignment.

Critical Impact
Unauthenticated remote attackers can forge ASDM SSO tokens to authenticate as the administrator, gaining administrative control over the ASDM session and denying legitimate admin access.

Affected Products

Cisco Secure Firewall Management Center (FMC) Software
Cisco Adaptive Security Device Manager (ASDM) SSO handler component
Deployments where ASDM management is exposed to reachable networks

Discovery Timeline

2026-09-16 - CVE-2026-76413 published to NVD
2026-09-17 - Last updated in NVD database
Cisco published the corresponding Cisco Security Advisory

Technical Details for CVE-2026-76413

Vulnerability Analysis

The flaw resides in how the ASDM SSO handler within Cisco Secure FMC Software issues and validates SSO tokens. The handler does not adequately restrict token assignment, allowing an attacker to craft tokens that the server accepts as valid administrator credentials. Because the exploit path is entirely network-reachable and requires no user interaction or prior authentication, an attacker on any network with access to the ASDM interface can attempt exploitation. Successful exploitation grants administrator-level ASDM login. Repeatedly forging administrator sessions can force session invalidation for legitimate operators, producing a persistent denial of access to the management interface.

Root Cause

The root cause is improper management of the ASDM SSO token as classified under [CWE-1259]. The SSO handler assigns or accepts token material without sufficient integrity or binding controls, letting an attacker forge a valid-looking administrator token. This is a design and validation weakness in the token lifecycle rather than a memory-safety bug.

Attack Vector

The attack vector is network based with low complexity and no privileges or user interaction required. An attacker reaches the ASDM SSO endpoint exposed by Cisco Secure FMC Software and submits a forged SSO token to obtain an authenticated administrator session. Because the same technique can be replayed, the attacker can continually invalidate legitimate administrator sessions and maintain lockout conditions.

No public proof-of-concept code has been released. See the Cisco Security Advisory for vendor technical details.

Detection Methods for CVE-2026-76413

Indicators of Compromise

Unexpected ASDM administrator logins from unfamiliar source IP addresses or geographies
Legitimate administrators repeatedly forced out of ASDM sessions or unable to maintain a session
ASDM SSO authentication events lacking a matching upstream identity provider or MFA event
Bursts of SSO token validation requests to the FMC ASDM endpoint from a single source

Detection Strategies

Correlate ASDM administrator login events with identity provider logs to identify sessions with no corresponding upstream authentication
Alert on rapid session churn for privileged ASDM accounts, especially repeated login/logout cycles from different sources
Baseline normal ASDM management source addresses and flag deviations

Monitoring Recommendations

Forward FMC and ASDM audit logs to a centralized analytics platform for retention and correlation
Monitor administrative access to the FMC management interface for anomalies in timing, source, and frequency
Track failed and successful SSO token validations against expected identity provider traffic

How to Mitigate CVE-2026-76413

Immediate Actions Required

Apply the fixed Cisco Secure FMC Software release identified in the Cisco Security Advisory
Restrict network access to the FMC ASDM management interface to trusted administrative networks only
Rotate ASDM administrator credentials and invalidate active sessions after patching
Review recent ASDM administrator login history for unauthorized access

Patch Information

Cisco has published fixed software as part of the cisco-sa-fmc2-multivulns-HXgcqRG advisory. Administrators should consult the advisory for the specific fixed release trains that address CVE-2026-76413 and plan upgrade windows accordingly. No official workaround is listed as a substitute for the patched software.

Workarounds

Limit exposure of the FMC ASDM management plane using access control lists or management-only network segmentation
Require VPN or jump-host access before permitting connectivity to the ASDM interface
Disable ASDM access from untrusted interfaces where operationally feasible until patching is complete

bash

# Example: restrict management access to a trusted subnet on Cisco Secure FMC
# Replace 203.0.113.0/24 with your administrative network
configure network management-interface add-access-list 203.0.113.0/24
configure network management-interface remove-access-list any