CVE-2017-18206 in zsh
Summary
by MITRE
In utils.c in zsh before 5.4, symlink expansion had a buffer overflow.
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Analysis
by VulDB Data Team • 02/16/2023
The vulnerability identified as CVE-2017-18206 represents a critical buffer overflow flaw within the zsh shell's utility functions, specifically in the utils.c file of versions prior to 5.4. This issue arises during symbolic link expansion operations where the shell fails to properly validate input lengths before copying data into fixed-size buffers. The buffer overflow occurs when zsh processes symbolic links that contain excessively long paths or recursive references, creating a scenario where attacker-controlled input can overwrite adjacent memory regions. Such vulnerabilities are particularly dangerous in shell environments where privilege escalation opportunities often exist, as the shell typically runs with elevated permissions depending on how it is invoked or used.
The technical implementation of this flaw stems from inadequate bounds checking during symlink resolution processes within the zsh codebase. When the shell encounters a symbolic link, it attempts to expand the path by following the link to its target location. However, the code does not sufficiently validate the length of the expanded path against the buffer size allocated for storing the result. This oversight allows for potential memory corruption when the expanded path exceeds the allocated buffer boundaries, leading to arbitrary code execution possibilities. The vulnerability manifests as a classic stack-based buffer overflow, where the overflow can overwrite return addresses, function pointers, or other critical control data structures within the program's memory space.
The operational impact of CVE-2017-18206 extends beyond simple denial of service conditions, as it presents a potential path for remote code execution when zsh is used in privileged contexts or when users can influence symbolic link creation. Systems running vulnerable versions of zsh are susceptible to exploitation through carefully crafted symbolic links that trigger the buffer overflow during normal shell operations. Attackers could leverage this vulnerability in scenarios where they have the ability to create or manipulate symbolic links in directories accessible to the shell process, such as in web server environments where zsh might be used for scripting or in system administration tasks. The vulnerability aligns with CWE-121, which describes stack-based buffer overflow conditions, and represents a significant concern for systems where zsh is used as a default shell or in automated processes.
Mitigation strategies for this vulnerability require immediate patching of affected zsh installations to version 5.4 or later, where the buffer overflow has been resolved through proper input validation and bounds checking. Organizations should conduct comprehensive inventory assessments to identify all systems running vulnerable versions of zsh, particularly those where shell access is available to untrusted users or where zsh is used in automated scripts with elevated privileges. Additionally, system administrators should implement proper access controls and file permissions to limit the ability of untrusted users to create or modify symbolic links in directories where zsh operations occur. The remediation process should also include monitoring for potential exploitation attempts through log analysis and implementing intrusion detection systems that can identify suspicious symbolic link creation patterns or shell execution anomalies. This vulnerability demonstrates the critical importance of input validation in shell environments and aligns with ATT&CK technique T1059.004 for executing shell commands, highlighting how buffer overflows can enable attackers to gain unauthorized access to systems through shell-based attack vectors.