CVE-2021-21956: Cloudlinux Imunify360 RCE Vulnerability

CVE-2021-21956 Overview

A PHP unserialize vulnerability exists in the Ai-Bolit functionality of CloudLinux Inc Imunify360 5.10.2. A specially-crafted malformed file can lead to potential arbitrary command execution. An attacker can provide a malicious file to trigger this vulnerability.

Critical Impact

This insecure deserialization vulnerability allows attackers to achieve arbitrary command execution on systems running vulnerable versions of Imunify360, potentially compromising server security and enabling further malicious activities.

Affected Products

• CloudLinux Imunify360 version 5.8
• CloudLinux Imunify360 version 5.9
• CloudLinux Imunify360 version 5.10.2

Discovery Timeline

• 2022-04-14 - CVE CVE-2021-21956 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-21956

Vulnerability Analysis

This vulnerability is classified as CWE-502 (Deserialization of Untrusted Data), a critical weakness that occurs when an application deserializes untrusted data without proper validation. In the context of Imunify360's Ai-Bolit malware scanner functionality, the vulnerability arises from improper handling of serialized PHP objects during file scanning operations.

The Ai-Bolit component is designed to scan files for malware signatures and suspicious patterns. However, when processing specially-crafted files containing malicious serialized PHP data, the scanner's unserialize operation can be exploited to instantiate arbitrary objects and trigger dangerous methods through PHP's magic methods (__wakeup(), __destruct(), etc.).

The local attack vector requires user interaction, meaning an attacker must convince a user or automated process to scan a malicious file. Once triggered, the vulnerability can lead to complete compromise of confidentiality, integrity, and availability of the affected system.

Root Cause

The root cause of CVE-2021-21956 lies in the unsafe use of PHP's unserialize() function within the Ai-Bolit scanning engine. When the scanner processes file contents or metadata, it deserializes data without implementing proper safeguards such as allowlisting acceptable classes or validating the serialized data structure before deserialization.

PHP object injection through unserialize is particularly dangerous because attackers can leverage existing classes in the application (known as "gadget chains") to chain method calls that ultimately lead to arbitrary code execution, file manipulation, or other malicious outcomes.

Attack Vector

The attack requires local access and user interaction—an attacker must craft a malicious file containing serialized PHP payload and ensure it gets scanned by the Ai-Bolit component. This could occur through:

1. Uploading a malicious file to a web server protected by Imunify360
2. Triggering an automated scan of a directory containing the malicious file
3. Social engineering a system administrator to manually scan a suspicious file

The serialized payload would contain crafted object properties that, when deserialized, trigger a chain of method calls resulting in command execution. The vulnerability does not require authentication, making any system running the vulnerable Imunify360 versions a potential target.

For detailed technical information about the exploitation mechanism, see the Talos Vulnerability Report TALOS-2021-1383.

Detection Methods for CVE-2021-21956

Indicators of Compromise

• Unexpected PHP process spawning with unusual command-line arguments following Ai-Bolit scan activities
• Presence of files containing serialized PHP objects with suspicious class names or gadget chain patterns
• Anomalous system calls or network connections originating from the Imunify360 process
• Log entries indicating Ai-Bolit processing files with malformed or oversized serialized data

Detection Strategies

• Monitor Imunify360 and Ai-Bolit processes for unexpected child processes or command execution
• Implement file integrity monitoring on critical system directories to detect post-exploitation changes
• Analyze incoming files for PHP serialization patterns (O:, a:, s:) combined with suspicious class names
• Review Imunify360 scan logs for errors or unusual behavior during file processing

Monitoring Recommendations

• Configure alerting for any command execution originating from the Ai-Bolit scanner process
• Enable verbose logging for Imunify360 to capture detailed scan activities and potential exploitation attempts
• Deploy endpoint detection and response (EDR) solutions to monitor for behavioral indicators of PHP object injection exploitation
• Implement network segmentation to limit lateral movement if exploitation occurs

How to Mitigate CVE-2021-21956

Immediate Actions Required

• Update CloudLinux Imunify360 to the latest patched version immediately
• Review scan logs for any suspicious activity that may indicate prior exploitation attempts
• Temporarily disable automated scanning of untrusted file sources until patching is complete
• Implement additional access controls to restrict who can upload files to scanned directories

Patch Information

CloudLinux has addressed this vulnerability in subsequent releases of Imunify360. Administrators should update to the latest version available through the official CloudLinux update channels. Consult the Talos Vulnerability Report TALOS-2021-1383 for additional technical details and remediation guidance.

Verify your current Imunify360 version and ensure it is newer than version 5.10.2 to confirm the vulnerability has been patched.

Workarounds

• Restrict file upload capabilities to trusted users only until the patch can be applied
• Implement strict input validation on all uploaded files before they reach the Imunify360 scanner
• Configure network-level controls to prevent outbound connections from the scanning process
• Consider temporarily disabling the Ai-Bolit functionality if immediate patching is not feasible

# Check current Imunify360 version
imunify360-agent version

# Update Imunify360 to latest version (CentOS/RHEL)
yum update imunify360-firewall

# Update Imunify360 to latest version (Ubuntu/Debian)
apt-get update && apt-get upgrade imunify360-firewall

# Verify update was successful
imunify360-agent version