CVE-2000-0901 in Screen
Summary
by MITRE
Format string vulnerability in screen 3.9.5 and earlier allows local users to gain root privileges via format characters in the vbell_msg initialization variable.
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Analysis
by VulDB Data Team • 06/24/2024
The vulnerability described in CVE-2000-0901 represents a critical format string flaw within the screen terminal multiplexer version 3.9.5 and earlier. This issue arises from improper handling of user-supplied input during the initialization of the vbell_msg variable, which controls the visual bell message display functionality. The flaw exists in the screen application's configuration processing mechanism where the system fails to properly sanitize or validate format specifiers provided by users. When screen processes the vbell_msg initialization variable, it directly passes user-controlled data to printf-style functions without adequate input validation, creating an exploitable condition that can be leveraged by local attackers.
The technical exploitation of this vulnerability occurs through the manipulation of the vbell_msg variable in screen's configuration files or environment variables. Attackers can inject format specifiers such as %x, %s, or %n into the variable value, which when processed by the vulnerable printf-family functions, can lead to memory corruption and arbitrary code execution. The privilege escalation aspect of this vulnerability stems from the fact that screen typically runs with elevated privileges, particularly when invoked through sudo or setuid binaries. When a local user manipulates the vbell_msg variable to contain malicious format specifiers, the application's processing of this input can result in the execution of arbitrary code with the privileges of the screen process, which often runs as root or with elevated system permissions.
The operational impact of CVE-2000-0901 extends beyond simple privilege escalation to encompass potential system compromise and data integrity breaches. This vulnerability aligns with CWE-134, which specifically addresses the use of format strings inappropriately, and demonstrates how improper input handling can lead to severe security consequences. The attack vector is particularly concerning because it requires only local access and does not depend on network connectivity, making it exploitable in environments where local users have access to the system. The vulnerability can be leveraged by attackers to execute arbitrary commands with elevated privileges, potentially leading to complete system compromise. In enterprise environments where screen is commonly used for terminal multiplexing and remote access management, this vulnerability creates a significant attack surface that can be exploited to establish persistent access or to escalate privileges within the system.
Mitigation strategies for this vulnerability should include immediate patching of screen to version 3.9.6 or later, which contains the necessary fixes for the format string handling. Organizations should also implement strict input validation procedures for configuration variables and avoid using user-supplied data in printf-style function calls without proper sanitization. The remediation process should include comprehensive system audits to identify any instances of the vulnerable screen version and ensure all installations are updated. Additionally, security configurations should enforce proper access controls and privilege separation to minimize the impact of potential exploitation, aligning with the principle of least privilege as recommended in security frameworks such as those outlined in the MITRE ATT&CK framework. System administrators should also monitor for unusual processes or unauthorized configuration changes that might indicate exploitation attempts, as this vulnerability can be used as a stepping stone for further attacks within the compromised environment.