CVE-1999-0894 in Linux
Summary
by MITRE
Red Hat Linux screen program does not use Unix98 ptys, allowing local users to write to other terminals.
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Analysis
by VulDB Data Team • 04/20/2026
The vulnerability described in CVE-1999-0894 affects the Red Hat Linux screen program, which is a terminal multiplexer used to manage multiple terminal sessions within a single window. This issue stems from the screen program's failure to utilize Unix98 pseudoterminals properly, creating a significant security flaw that allows local users to gain unauthorized access to other terminal sessions. The problem specifically relates to the improper implementation of terminal device management within the Unix98 standard, which defines the expected behavior for pseudoterminal allocation and access control.
The technical flaw manifests when the screen program creates terminal sessions without properly implementing the Unix98 pty allocation mechanisms. This allows malicious local users to exploit the lack of proper access controls and write to other users' terminals through the screen program's improper handling of pseudoterminal devices. The vulnerability essentially creates a path for privilege escalation and information disclosure, as users can potentially interfere with or monitor other active terminal sessions. This behavior violates fundamental security principles of isolation and access control that should be maintained between different user sessions.
From an operational impact perspective, this vulnerability enables local users to perform unauthorized actions on other users' terminal sessions, potentially leading to data interception, session hijacking, or denial of service conditions. Attackers could use this flaw to monitor sensitive information being entered into other terminals, inject malicious commands, or disrupt ongoing user sessions. The vulnerability particularly affects multi-user environments where multiple users share the same system and rely on terminal sessions for their work activities. This creates a significant risk for environments where confidential data processing occurs, as the security boundary between user sessions becomes compromised.
The vulnerability aligns with CWE-264, which addresses permissions, privileges, and access control issues in software implementations. It also maps to ATT&CK technique T1068, which involves local privilege escalation through exploitation of system vulnerabilities. The improper implementation of Unix98 pty mechanisms represents a failure to properly enforce mandatory access controls and user isolation, which are fundamental requirements for maintaining system security. Organizations using screen programs in production environments should consider this vulnerability as a critical security risk that could lead to unauthorized information access and session manipulation.
Mitigation strategies should focus on upgrading to screen versions that properly implement Unix98 pty allocation mechanisms and ensure that all terminal session management code adheres to established security standards. System administrators should also implement additional monitoring and logging of terminal session activities to detect unauthorized access attempts. The recommended approach includes applying security patches from Red Hat, implementing proper access controls for terminal sessions, and conducting regular security audits of terminal multiplexer implementations. Additionally, organizations should consider implementing network segmentation and monitoring solutions to detect and prevent unauthorized terminal access attempts that could exploit this vulnerability.