CVE-2021-21706 Explained : Impact and Mitigation

This article provides detailed information about CVE-2021-21706, a vulnerability in PHP versions 7.3.x, 7.4.x, and 8.0.x that can be exploited to write files outside the target directory when extracting a ZIP file on Windows platforms.

Understanding CVE-2021-21706

CVE-2021-21706 is a security vulnerability in PHP versions 7.3.x below 7.3.31, 7.4.x below 7.4.24, and 8.0.x below 8.0.11 that affects the ZipArchive::extractTo function in a Windows environment.

What is CVE-2021-21706?

In PHP versions 7.3.x below 7.3.31, 7.4.x below 7.4.24, and 8.0.x below 8.0.11 on Windows, ZipArchive::extractTo can be manipulated to write files outside the intended directory when extracting a ZIP file. This could lead to unauthorized file creation or overwriting depending on the operating system permissions.

The Impact of CVE-2021-21706

The vulnerability allows malicious actors to potentially write files in unintended locations, compromising the integrity of the system. Successful exploitation could result in unauthorized access or data loss.

Technical Details of CVE-2021-21706

The vulnerability is due to improper input validation in the ZipArchive::extractTo function, allowing attackers to traverse directories and write files outside the target directory.

Vulnerability Description

The flaw in ZipArchive::extractTo could be exploited to bypass directory traversal protections and write files to arbitrary locations on the system, leading to unauthorized file operations.

Affected Systems and Versions

PHP versions 7.3.x, 7.4.x, and 8.0.x running on Windows platforms are impacted by this vulnerability.

Exploitation Mechanism

Attackers can craft a malicious ZIP file containing directory traversal sequences to trick ZipArchive::extractTo into writing files to locations outside the intended directory.

Mitigation and Prevention

To address CVE-2021-21706, users are advised to take immediate action to secure their systems and prevent potential exploitation of the vulnerability.

Immediate Steps to Take

Update PHP to versions 7.3.31, 7.4.24, or 8.0.11 to mitigate the vulnerability.
Restrict access to PHP scripts and directories to trusted users only.

Long-Term Security Practices

Implement input validation mechanisms to prevent directory traversal attacks.
Regularly monitor and audit file system activities for any unauthorized changes.

Patching and Updates

PHP Group has released patches for versions 7.3.31, 7.4.24, and 8.0.11 to address this vulnerability. Users are strongly encouraged to apply the latest updates to protect their systems.