CVE-2018-13259 in zsh
Summary
by MITRE
An issue was discovered in zsh before 5.6. Shebang lines exceeding 64 characters were truncated, potentially leading to an execve call to a program name that is a substring of the intended one.
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Analysis
by VulDB Data Team • 05/07/2023
The vulnerability identified as CVE-2018-13259 represents a critical flaw in the zsh shell's handling of shebang lines, specifically affecting versions prior to 5.6. This issue stems from a buffer size limitation that truncates shebang lines exceeding 64 characters, creating a potential security risk that could be exploited by malicious actors to execute unintended programs. The vulnerability is particularly concerning because it directly impacts the execution flow of shell scripts and could be leveraged to bypass security controls or execute arbitrary code with elevated privileges.
The technical implementation of this flaw occurs within zsh's script execution mechanism where the shell parses shebang lines to determine which interpreter should execute a script file. When a shebang line exceeds the 64-character threshold, the system truncates the line, resulting in an execve system call that references a program name derived from the truncated substring rather than the intended executable. This truncation behavior creates a path traversal scenario where an attacker could manipulate the shebang line to point to a malicious program while maintaining the appearance of legitimate execution. The flaw essentially allows for command injection through script execution mechanisms, as the truncated shebang may resolve to a different program than originally intended.
The operational impact of this vulnerability extends beyond simple script execution issues, as it can be exploited in various attack scenarios including privilege escalation, code injection, and malicious payload delivery. Attackers could craft shebang lines that, when truncated, point to system utilities or binaries that have been compromised or modified to execute malicious code. This vulnerability particularly affects environments where zsh is used for executing user scripts or automated processes, and could be exploited in scenarios involving web applications that utilize zsh for server-side script execution or in containerized environments where shell scripts are executed with elevated privileges. The vulnerability's impact is amplified in systems where script execution is not properly validated or where users can influence the content of executable files.
Mitigation strategies for CVE-2018-13259 primarily focus on upgrading to zsh version 5.6 or later, which contains the fix for the shebang line truncation issue. System administrators should implement comprehensive patch management processes to ensure all zsh installations are updated promptly, particularly in environments where script execution is common or where security is a primary concern. Additional protective measures include implementing strict file permission controls on script files, validating shebang lines before execution, and monitoring for unusual script execution patterns that might indicate exploitation attempts. Organizations should also consider implementing application whitelisting policies that restrict which programs can be executed through shebang mechanisms, and establish monitoring protocols to detect potential truncation attacks that might attempt to exploit this vulnerability.
This vulnerability aligns with CWE-128, which describes "Wrap-around Error" and relates to situations where input validation fails to properly handle boundary conditions. The flaw also connects to ATT&CK technique T1059.004, which covers "Command and Scripting Interpreter: PowerShell" and similar execution techniques where command injection occurs through malformed input. The vulnerability demonstrates how seemingly minor buffer size limitations can create significant security implications in shell execution environments, highlighting the importance of proper input validation and boundary checking in security-critical applications. Organizations should also consider implementing security controls that monitor for unusual shebang patterns and maintain awareness of similar vulnerabilities in other shell implementations that might exhibit similar truncation behaviors.