CVE-2023-24626 in Screen
Summary
by MITRE • 04/08/2023
socket.c in GNU Screen through 4.9.0, when installed setuid or setgid (the default on platforms such as Arch Linux and FreeBSD), allows local users to send a privileged SIGHUP signal to any PID, causing a denial of service or disruption of the target process.
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Analysis
by VulDB Data Team • 01/28/2026
The vulnerability identified as CVE-2023-24626 resides within the socket.c component of GNU Screen version 4.9.0 and earlier, representing a critical local privilege escalation and denial of service flaw. This issue manifests when GNU Screen is installed with setuid or setgid permissions, which is the default configuration on several operating systems including Arch Linux and FreeBSD. The vulnerability stems from improper signal handling mechanisms that fail to validate the privileges of processes attempting to send SIGHUP signals to arbitrary process identifiers, creating a fundamental security gap in the Unix permission model implementation.
The technical flaw exploits the inherent trust model within setuid applications by allowing unprivileged local users to manipulate signal delivery to any process running on the system. When GNU Screen operates with elevated privileges due to its setuid bit, it processes signals without adequate verification of the originating process's capabilities, enabling malicious actors to send SIGHUP signals to target processes regardless of their ownership or privilege level. This behavior violates the principle of least privilege and creates a pathway for unauthorized process manipulation that can result in system instability and service disruption.
The operational impact of this vulnerability extends beyond simple denial of service scenarios to encompass potential system compromise and service availability degradation. An attacker with local access can leverage this flaw to disrupt critical system processes, terminate user sessions, or cause cascading failures within the operating system's process management framework. The vulnerability is particularly concerning in multi-user environments where unauthorized process termination could lead to data loss or service interruption for legitimate users. According to CWE classification, this represents a weakness in the design of signal handling mechanisms, specifically CWE-252, which addresses improper handling of signals and their associated privileges.
Mitigation strategies for CVE-2023-24626 should prioritize immediate patching of GNU Screen installations to versions 4.9.1 or later where the vulnerability has been addressed through enhanced signal validation and privilege checking mechanisms. System administrators should also consider implementing additional security controls such as disabling setuid installation of GNU Screen where possible, monitoring for suspicious signal delivery patterns, and employing process monitoring tools to detect unauthorized process termination attempts. The ATT&CK framework categorizes this vulnerability under privilege escalation techniques, specifically targeting the T1068 - Exploitation for Privilege Escalation tactic, and the T1489 - Service Stop technique for denial of service operations. Organizations should conduct comprehensive vulnerability assessments to identify systems running vulnerable versions of GNU Screen and implement network segmentation to limit local access where possible, as the vulnerability requires local execution privileges to exploit effectively.
This vulnerability demonstrates the persistent challenges in maintaining secure setuid applications and highlights the importance of proper privilege separation in Unix-like operating systems. The flaw underscores the need for rigorous security auditing of legacy applications that continue to operate with elevated privileges, particularly those that handle system-level operations such as process management and signal handling. Security teams must remain vigilant in identifying similar issues within other setuid applications and ensure that proper input validation and privilege checking mechanisms are implemented throughout system software components.