CVE-2026-20028: Cisco TS Agent Auth Bypass Vulnerability

CVE-2026-20028 Overview

CVE-2026-20028 is a firewall bypass vulnerability in the network driver of Cisco Terminal Service (TS) Agent. An authenticated, remote attacker with at least user-level credentials can bypass firewall rules tied to their account by sending crafted network traffic to an affected device. The flaw stems from an incorrect mapping of network connections to user accounts, allowing an attacker to inherit firewall rules assigned to a different user.

The issue is classified under CWE-266: Incorrect Privilege Assignment and enables horizontal privilege escalation across firewall policy scopes.

Critical Impact
An authenticated attacker can inherit another user's firewall permissions, bypassing network access controls enforced by the Cisco TS Agent integration.

Affected Products

Cisco Terminal Service (TS) Agent — network driver component
Deployments integrating Cisco TS Agent with Cisco Firepower/Secure Firewall user-to-IP mapping
Refer to the Cisco Security Advisory for the complete list of affected releases

Discovery Timeline

2026-08-05 - CVE-2026-20028 published to NVD
2026-08-05 - Last updated in NVD database

Technical Details for CVE-2026-20028

Vulnerability Analysis

The Cisco TS Agent maps network connections from multi-user terminal servers to individual user identities. Downstream firewalls use these mappings to apply per-user access policies. When the mapping is incorrect, the firewall enforces the wrong policy on a connection.

An authenticated attacker sends crafted network traffic that the TS Agent associates with a different user account. The firewall then evaluates the traffic against the other user's policy set. If that user holds broader permissions, the attacker gains network access they were not authorized to receive.

The attack requires only low-privileged credentials on the affected device. It does not require user interaction, and the impact scope changes because policy enforcement crosses a trust boundary between accounts.

Root Cause

The root cause is an incorrect privilege assignment [CWE-266] in the connection-to-user mapping logic within the TS Agent network driver. The driver does not correctly bind an outbound connection to the originating session or user context. This mismatch propagates to the firewall as authoritative identity data.

Attack Vector

The attack vector is network-based and requires valid user credentials on the terminal server hosting the TS Agent. The attacker crafts traffic patterns that cause the agent to attribute the flow to a different logged-in user. No specialized tooling or memory corruption is required. Exploitation is deterministic when the required session conditions exist on the shared terminal host.

No public proof-of-concept or exploit code is available. See the Cisco Security Advisory for vendor-supplied technical details.

Detection Methods for CVE-2026-20028

Indicators of Compromise

Firewall logs showing connections attributed to a user whose active session did not originate the traffic
Access to network resources by an account that lacks matching policy entitlements in the identity source
Anomalous per-user connection counts on terminal servers running Cisco TS Agent

Detection Strategies

Correlate Cisco TS Agent user-to-IP mapping events with authentication logs from the terminal server to identify mismatches
Alert on firewall policy hits for destinations that the mapped user has never previously accessed
Baseline expected traffic per user on shared terminal hosts and flag deviations that involve sensitive network segments

Monitoring Recommendations

Ingest Cisco TS Agent and firewall syslog into a centralized analytics platform for cross-source correlation
Monitor terminal server session tables and compare active users against source ports reported by the agent
Track failed and successful access attempts to segmented resources for accounts that share a terminal host

How to Mitigate CVE-2026-20028

Immediate Actions Required

Apply the fixed Cisco TS Agent release identified in the Cisco Security Advisory
Inventory all terminal servers running Cisco TS Agent and confirm version levels
Restrict interactive logon rights on shared terminal hosts to reduce the pool of accounts that can be impersonated

Patch Information

Cisco has published a security advisory tracking this issue as cisco-sa-ts-agent-fw-bypass-MYBTMrev. Consult the advisory for fixed release numbers, upgrade paths, and any interim guidance. No workaround is documented that fully substitutes for the vendor patch.

Workarounds

Where feasible, avoid mixing users with differing firewall privilege tiers on the same terminal server
Enforce least privilege in firewall policies so that inherited rules provide minimal additional access
Segment high-value destinations behind identity-aware proxies that perform independent authentication

bash

# Example: verify installed Cisco TS Agent version on Windows terminal server
wmic product where "Name like 'Cisco Terminal Services Agent%%'" get Name,Version