CVE-2017-18205 in zsh
Summary
by MITRE
In builtin.c in zsh before 5.4, when sh compatibility mode is used, there is a NULL pointer dereference during processing of the cd command with no argument if HOME is not set.
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Analysis
by VulDB Data Team • 02/16/2023
The vulnerability identified as CVE-2017-18205 represents a critical NULL pointer dereference flaw in the zsh shell's builtin.c component affecting versions prior to 5.4. This issue specifically manifests when the shell operates in sh compatibility mode and processes the cd command without any arguments while the HOME environment variable remains unset. The flaw stems from inadequate input validation and error handling within the shell's command processing logic, creating a scenario where a null pointer reference can occur during normal shell operation. The vulnerability is particularly concerning as it can lead to arbitrary code execution or denial of service conditions when exploited by malicious actors.
The technical root cause of this vulnerability lies in the improper handling of environment variables within the zsh shell's built-in command processing mechanism. When the cd command is executed without arguments in sh compatibility mode, the shell attempts to access the HOME environment variable to determine the default directory for navigation. However, when HOME is not properly set in the environment, the shell fails to properly validate this condition before attempting to dereference a null pointer that would normally contain the HOME directory path. This behavior violates fundamental security principles of input validation and error handling, creating an exploitable condition that can be triggered by manipulating the shell's execution environment. The vulnerability aligns with CWE-476 which specifically addresses NULL pointer dereference conditions in software implementations.
The operational impact of this vulnerability extends beyond simple denial of service scenarios, as it can potentially enable privilege escalation attacks when the shell is executed with elevated privileges. An attacker who can control the shell environment and trigger the cd command without arguments in sh compatibility mode could cause the shell to crash or potentially execute arbitrary code, depending on the specific system configuration and execution context. This vulnerability affects system administrators and end users who rely on zsh for command execution, particularly in environments where shell compatibility modes are enabled or when legacy scripts expect specific behavior from the cd command. The flaw is particularly dangerous in automated systems or environments where zsh is used as a default shell or in security-sensitive contexts where predictable shell behavior is required.
Mitigation strategies for CVE-2017-18205 should prioritize immediate patching of affected zsh installations to version 5.4 or later where the vulnerability has been addressed through proper null pointer validation. System administrators should also implement environment variable validation checks to ensure that HOME is properly set before shell execution, particularly in automated or security-sensitive contexts. Additional defensive measures include monitoring for unusual shell behavior patterns that might indicate exploitation attempts, implementing proper input sanitization for shell command processing, and maintaining up-to-date system security configurations. Organizations should also consider implementing runtime protections and application whitelisting to prevent unauthorized shell command execution that could trigger this vulnerability. The remediation process should include thorough testing of patched systems to ensure that the fix does not introduce regressions in shell functionality while maintaining the security improvements necessary to address the NULL pointer dereference condition.