CVE-2018-0502 in zsh
Summary
by MITRE
An issue was discovered in zsh before 5.6. The beginning of a #! script file was mishandled, potentially leading to an execve call to a program named on the second line.
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Analysis
by VulDB Data Team • 05/07/2023
The vulnerability identified as CVE-2018-0502 represents a critical flaw in the zsh shell interpreter affecting versions prior to 5.6. This issue stems from improper handling of shebang lines at the beginning of script files, creating a potential privilege escalation vector through malicious script execution. The vulnerability specifically impacts how zsh processes the initial shebang line when executing scripts, allowing for unexpected program execution paths that could be exploited by attackers.
The technical implementation of this flaw occurs within zsh's script execution mechanism where the interpreter fails to properly validate or process the shebang line syntax at the beginning of script files. When a script file begins with a shebang line, the system typically uses the specified interpreter to execute the script content. However, in vulnerable versions of zsh, the interpreter's parsing logic incorrectly handles the initial shebang line, potentially causing the system to execute a program named on the second line of the script file rather than the intended interpreter specified in the first line. This misinterpretation creates a path where an attacker could manipulate script content to execute arbitrary programs with elevated privileges.
The operational impact of this vulnerability extends beyond simple script execution issues, as it could enable attackers to perform privilege escalation attacks when scripts are executed with elevated privileges or when the system processes scripts in privileged contexts. This flaw particularly affects environments where zsh is used as the default shell and where users execute scripts with elevated permissions. The vulnerability can be exploited in scenarios where scripts are executed by system services or administrative users, potentially allowing an attacker to execute arbitrary commands with the privileges of the executing user. This represents a significant concern for system administrators who rely on zsh for scripting and automation tasks.
Mitigation strategies for CVE-2018-0502 focus primarily on upgrading to zsh version 5.6 or later, which contains the necessary patches to properly handle shebang line parsing. System administrators should conduct comprehensive vulnerability assessments to identify all systems running vulnerable versions of zsh and prioritize updates accordingly. Additionally, organizations should implement strict script execution policies that limit the execution of untrusted scripts and ensure proper file permissions are enforced. The vulnerability aligns with CWE-22, which addresses improper limitation of a pathname to a restricted directory, and relates to ATT&CK technique T1059.004 for scripting, where adversaries may exploit shell vulnerabilities to execute malicious code. Organizations should also consider implementing additional monitoring and logging mechanisms to detect anomalous script execution patterns that could indicate exploitation attempts.