A critical command injection vulnerability found in the popular POHPMailer PHP library.

A critical command injection vulnerability in PHPMailer—a widely used PHP library for sending emails—has recently come under renewed scrutiny following reports of active exploitation. Identified as CVE-2016-10033, this vulnerability poses a significant risk to web applications that rely on PHPMailer for email functionality.

Vulnerability Overview

The vulnerability stems from improper sanitization of user-supplied input, particularly in email sender fields. When exploited, this flaw allows unauthenticated attackers to execute arbitrary commands on the target server, potentially leading to full system compromise.

Technical Details

PHPMailer, in vulnerable versions, passes user-controlled input directly to the underlying mail() function, which in turn invokes the system’s sendmail binary. By injecting specially crafted input—such as malicious characters or command-line arguments—an attacker can break out of the intended command structure and execute arbitrary commands.

Example Attack Scenario:

1. An attacker submits a maliciously crafted email address or sender field.
2. PHPMailer, failing to properly sanitize this input, passes it to the mail() function.
3. The attacker’s payload is executed by the server, often resulting in the creation of a web shell or other malicious files.
4. The attacker gains remote code execution capabilities, potentially compromising the entire server.

Impact and Risk

• Remote Code Execution: Successful exploitation grants attackers the ability to run arbitrary commands on the server, leading to possible data theft, defacement, or further compromise.
• Denial of Service: Exploitation attempts may also destabilize or crash vulnerable applications.
• Widespread Exposure: Any PHP application using PHPMailer versions up to and including 5.2.17, and allowing user input in email fields, is at risk.

Affected Versions

• PHPMailer versions up to and including 5.2.17 are confirmed to be vulnerable.
• Subsequent releases have addressed this specific issue, though users are urged to remain vigilant for other potential vulnerabilities in older versions.

Recommended Actions

Organizations and developers are strongly advised to take the following steps:

1. Upgrade Immediately: Update PHPMailer to the latest available version to ensure all known vulnerabilities are patched.
2. Sanitize User Input: Implement strict input validation and sanitization, especially for any data passed to email-related functions.
3. Monitor Official Advisories: Stay informed by following official PHPMailer and cybersecurity advisories for updates and guidance.

Official Guidance

The U.S. Cybersecurity and Infrastructure Security Agency (CISA) has added CVE-2016-10033 to its Known Exploited Vulnerabilities (KEV) Catalog, underscoring the urgency of remediation. PHPMailer’s maintainers have also published security updates and best practice recommendations.