CVE-2026-20028 in Terminal Services Agent
Summary
by MITRE • 08/05/2026
A vulnerability in the network driver of Cisco Terminal Service (TS) Agent could allow an authenticated, remote attacker to bypass firewall rules that are associated with the account of the attacker.
This vulnerability is due to an incorrect mapping of network connections to user accounts. An attacker with at least user-level credentials could exploit this vulnerability by sending crafted network traffic to an affected device. A successful exploit could allow the attacker to inherit the firewall rules associated with a different user in the system.
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Analysis
by VulDB Data Team • 08/05/2026
This vulnerability resides within the network driver component of Cisco Terminal Service Agent, representing a significant security flaw that undermines fundamental access control mechanisms. The issue stems from improper mapping between network connections and user accounts, creating a pathway for authenticated remote attackers to manipulate firewall rule enforcement. Such a vulnerability fundamentally compromises the principle of least privilege and user isolation that forms the cornerstone of network security architectures.
The technical implementation flaw manifests as an incorrect association between network sessions and user contexts within the terminal service agent's networking stack. When an attacker with valid user credentials establishes a connection to an affected device, the malformed connection mapping allows them to effectively impersonate or inherit the security context of other users within the system. This misconfiguration enables attackers to bypass firewall rules that should normally restrict their network access based on their authenticated account permissions. The vulnerability operates at the protocol level where network traffic is processed and user contexts are validated, creating a direct attack vector that can be exploited without requiring elevated privileges beyond standard user authentication.
The operational impact of this vulnerability extends far beyond simple privilege escalation, as it enables attackers to circumvent network segmentation controls that are critical for protecting sensitive resources. An attacker could potentially access systems or data that should only be reachable by specific user groups, effectively breaking down the firewall protections that organizations rely upon for network security. This compromise allows for lateral movement within networks where firewall rules would normally prevent such access patterns, making it particularly dangerous in enterprise environments where multiple user contexts and security zones are common.
Mitigation strategies must address both the immediate vulnerability and underlying architectural weaknesses. Organizations should implement strict network segmentation controls that limit the impact of such privilege inheritance scenarios, while also applying the latest security patches from Cisco to address the specific network driver flaw. Network monitoring solutions should be enhanced to detect anomalous connection patterns that might indicate exploitation attempts, and access control policies should be reviewed to ensure that user account permissions align with the principle of least privilege. The vulnerability aligns with CWE-284 Access Control Issues and could be categorized under ATT&CK technique T1078 Valid Accounts for maintaining persistent access while bypassing firewall restrictions. Regular security assessments of network service agents and their associated privilege handling mechanisms are essential to prevent similar vulnerabilities from emerging in other critical infrastructure components.