CVE-2026-76439: Cisco ISE Authentication Bypass Vulnerability

CVE-2026-76439 Overview

CVE-2026-76439 is an authentication bypass vulnerability in the endpoint posture status reporting functionality of the Cisco Identity Services Engine (ISE) guest portal web application. The flaw allows an unauthenticated, remote attacker to submit forged posture status events into the endpoint posture pipeline. Exploitation requires only a crafted network request to the affected system.

The underlying weakness is classified as Missing Authentication for Critical Function [CWE-306]. A successful attack lets the adversary manipulate the posture status recorded for endpoints, potentially influencing network access decisions that rely on posture compliance.

Critical Impact
An unauthenticated remote attacker can inject forged endpoint posture status events into Cisco ISE, undermining the integrity of posture-based network access control.

Affected Products

Cisco Identity Services Engine (ISE) guest portal web application
Refer to the Cisco Security Advisory for the authoritative list of affected versions

Discovery Timeline

2026-09-16 - CVE-2026-76439 published to the National Vulnerability Database
2026-09-16 - Last updated in NVD database

Technical Details for CVE-2026-76439

Vulnerability Analysis

Cisco ISE exposes an internal interface through its guest portal web application for reporting endpoint posture status. The posture pipeline consumes these events and updates the compliance state associated with connected endpoints. That state is then referenced by authorization policies that gate network access.

The guest portal is intended to be reachable by unauthenticated users during the guest onboarding flow. Because the internal posture-reporting interface is exposed through this same portal without adequate authentication, an attacker can submit forged posture events without possessing credentials or a valid session. The result is an integrity impact against the posture data used by downstream policy decisions.

Root Cause

The root cause is insufficient authentication on an internal interface that is exposed through the guest portal [CWE-306]. The interface accepts posture status submissions without verifying that the requester is an authorized posture agent or authenticated endpoint. Trust is implicitly granted based on reachability rather than identity.

Attack Vector

The attack is performed over the network with no privileges and no user interaction. An attacker sends a crafted HTTP request to the guest portal endpoint that handles posture status reporting. The forged event is ingested by the posture pipeline and applied to the targeted endpoint identifier, allowing the attacker to manipulate reported compliance status.

No verified proof-of-concept code is publicly available. Refer to the Cisco Security Advisory for vendor-supplied technical details.

Detection Methods for CVE-2026-76439

Indicators of Compromise

Unexpected posture status changes for endpoints that did not run a posture assessment through a legitimate agent
Unauthenticated HTTP requests to the guest portal targeting posture reporting endpoints from unusual source addresses
Posture compliance state transitions that do not correlate with authentication or accounting records in ISE

Detection Strategies

Correlate ISE posture status change events with agent check-in logs to identify status updates lacking a corresponding agent session
Baseline expected sources of posture reporting traffic and alert on submissions from IP ranges outside guest onboarding subnets
Review ISE guest.log and posture subsystem logs for anomalous request patterns to internal posture interfaces

Monitoring Recommendations

Forward ISE syslog and posture pipeline events to a centralized analytics platform for correlation with authorization decisions
Alert on high-frequency posture status submissions or repeated updates for the same endpoint identifier in short time windows
Monitor network access logs for endpoints granted elevated access immediately following a posture status change

How to Mitigate CVE-2026-76439

Immediate Actions Required

Consult the Cisco Security Advisory and identify affected ISE deployments in your environment
Restrict network reachability to the guest portal to only the interfaces and client subnets required for guest onboarding
Audit recent posture status changes and authorization decisions that depended on posture compliance state

Patch Information

Apply the fixed software release identified in the Cisco Security Advisory cisco-sa-ise-multiauth-bypass-sgD2HbL4. Cisco is the authoritative source for fixed version numbers and upgrade guidance for this vulnerability.

Workarounds

Segment the guest portal on a dedicated interface exposed only to guest onboarding networks and block reachability from corporate or untrusted networks
Apply upstream access control lists to restrict which sources can reach the guest portal service ports
Where feasible, reduce reliance on posture status as the sole factor in authorization policies until the patch is deployed

bash

# Example: restrict guest portal exposure with an upstream ACL (adapt to your platform)
access-list GUEST_PORTAL_IN permit tcp <guest_subnet> host <ise_guest_portal_ip> eq 8443
access-list GUEST_PORTAL_IN deny   tcp any host <ise_guest_portal_ip> eq 8443
access-list GUEST_PORTAL_IN permit ip any any