CVE-2018-0502
📋 TL;DR
A vulnerability in zsh before version 5.6 mishandles the beginning of #! (shebang) script files, potentially causing the execve system call to execute a program named on the second line instead of the intended interpreter. This affects users running vulnerable zsh versions on Unix-like systems, particularly when processing untrusted scripts.
💻 Affected Systems
- zsh
📦 What is this software?
Ubuntu Linux by Canonical
Ubuntu Linux by Canonical
Ubuntu Linux by Canonical
Zsh by Zsh
⚠️ Risk & Real-World Impact
Worst Case
Remote code execution with root privileges if a vulnerable system processes a malicious script from an untrusted source, leading to complete system compromise.
Likely Case
Local privilege escalation or arbitrary code execution when users run malicious scripts, potentially gaining unauthorized access or control.
If Mitigated
Limited impact if proper input validation and script source verification are enforced, restricting execution to trusted sources only.
🎯 Exploit Status
Exploitation requires user interaction to run a malicious script; proof-of-concept details are publicly available in references.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: zsh 5.6 and later
Vendor Advisory: https://sourceforge.net/p/zsh/code/ci/1c4c7b6a4d17294df028322b70c53803a402233d
Restart Required: No
Instructions:
1. Update zsh to version 5.6 or later using your system's package manager. 2. For Debian/Ubuntu: sudo apt update && sudo apt upgrade zsh. 3. For RHEL/CentOS: sudo yum update zsh. 4. Verify the update with zsh --version.
🔧 Temporary Workarounds
Avoid Untrusted Scripts
allDo not execute scripts from untrusted sources; verify script integrity before running.
Use Alternative Shell
linuxTemporarily switch to a non-vulnerable shell like bash for script processing.
chsh -s /bin/bash
🧯 If You Can't Patch
- Restrict script execution to trusted users and sources only.
- Implement strict file permissions and monitor for unusual script execution patterns.
🔍 How to Verify
Check if Vulnerable:
Run 'zsh --version' and check if version is below 5.6.Check Version:
zsh --versionVerify Fix Applied:
After update, run 'zsh --version' to confirm version is 5.6 or higher.📡 Detection & Monitoring
Log Indicators:
- Unusual execve calls from zsh processes
- Script execution errors related to shebang parsing
SIEM Query:
Process execution logs where parent process is zsh and command includes unexpected binary names.🔗 References
- https://bugs.debian.org/908000
- https://lists.debian.org/debian-lts-announce/2020/12/msg00000.html
- https://security.gentoo.org/glsa/201903-02
- https://sourceforge.net/p/zsh/code/ci/1c4c7b6a4d17294df028322b70c53803a402233d
- https://usn.ubuntu.com/3764-1/
- https://www.zsh.org/mla/zsh-announce/136
- https://bugs.debian.org/908000
- https://lists.debian.org/debian-lts-announce/2020/12/msg00000.html
- https://security.gentoo.org/glsa/201903-02
- https://sourceforge.net/p/zsh/code/ci/1c4c7b6a4d17294df028322b70c53803a402233d
- https://usn.ubuntu.com/3764-1/
- https://www.zsh.org/mla/zsh-announce/136
