CVE-2018-7548 in zsh
Summary
by MITRE
In subst.c in zsh through 5.4.2, there is a NULL pointer dereference when using ${(PA)...} on an empty array result.
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Analysis
by VulDB Data Team • 02/16/2023
The vulnerability described in CVE-2018-7548 represents a critical NULL pointer dereference flaw within the zsh shell's parameter expansion mechanism. This issue specifically manifests when utilizing the ${(PA)...} parameter expansion syntax on empty array results, creating a scenario where the shell's internal handling of such operations fails to properly validate array states before attempting memory access. The flaw exists in the subst.c file of zsh versions up to and including 5.4.2, indicating a fundamental design oversight in how the shell processes parameter expansions with empty arrays. The vulnerability stems from the shell's inability to gracefully handle edge cases where array expansion operations result in empty states, leading to uncontrolled memory access patterns that can cause immediate program termination.
The technical execution of this vulnerability involves the shell's parameter expansion subsystem attempting to process an array that has been explicitly defined as empty, yet the underlying code does not properly check for null or empty array conditions before proceeding with memory dereference operations. This particular flaw falls under the category of improper input validation and memory management issues, aligning with CWE-476 which addresses NULL pointer dereference conditions. When an attacker can manipulate shell commands to trigger this specific parameter expansion syntax with empty arrays, the shell process will crash with a segmentation fault, effectively causing a denial of service condition that prevents legitimate users from utilizing the shell functionality.
The operational impact of CVE-2018-7548 extends beyond simple denial of service scenarios, as it represents a potential vector for more sophisticated attacks within environments where zsh is used for automated scripting or privileged operations. Since zsh is commonly used in Unix-like systems for both interactive and automated shell scripting, an attacker could exploit this vulnerability to disrupt system services that depend on shell execution, potentially affecting system stability and availability. The vulnerability affects all versions of zsh through 5.4.2, making it particularly concerning for systems that have not been updated to newer releases, and it demonstrates how seemingly minor parameter expansion syntax can create significant security implications when not properly validated. This flaw is particularly relevant in environments where shell injection attacks are possible, as the predictable crash behavior could be leveraged to create more targeted denial of service attacks against shell-based applications.
Mitigation strategies for CVE-2018-7548 primarily involve updating to zsh versions that have patched this vulnerability, specifically zsh 5.5 and later releases which contain the necessary code modifications to properly handle empty array parameter expansions. System administrators should prioritize patching affected systems, particularly in environments where zsh is used for critical automation tasks or where shell-based services are exposed to untrusted inputs. Additionally, implementing proper input validation and sanitization in shell scripts that might be exposed to user-controlled data can help reduce the attack surface, though this does not address the core vulnerability in the shell itself. Organizations should also consider monitoring for abnormal shell process termination patterns that might indicate exploitation attempts, as the predictable nature of this crash makes it potentially detectable through system monitoring tools. The vulnerability serves as a reminder of the importance of thorough testing of edge cases in shell implementations and the critical need for proper null pointer validation in parameter expansion mechanisms that are widely used across Unix-like systems.