Source: cve@mitre.org
The isMail transport in PHPMailer before 5.2.20 might allow remote attackers to pass extra parameters to the mail command and consequently execute arbitrary code by leveraging improper interaction between the escapeshellarg function and internal escaping performed in the mail function in PHP. NOTE: this vulnerability exists because of an incorrect fix for CVE-2016-10033.
PHPMailer versions prior to 5.2.20 are vulnerable to a critical remote code execution (RCE) flaw. This vulnerability allows attackers to inject arbitrary commands into the mail function via the isMail transport, potentially leading to complete system compromise.
Step 1: Payload Injection: An attacker crafts a malicious payload, typically including shell metacharacters, designed to be injected into the email parameters. This payload is crafted to exploit the isMail transport in PHPMailer.
Step 2: Parameter Passing: The attacker's crafted payload is passed to the PHPMailer's mail function via the isMail transport. This could be through a web form, API call, or other means of interacting with the vulnerable application.
Step 3: Escaping and Sanitization (Attempted): The escapeshellarg function attempts to sanitize the attacker's input before passing it to the PHP mail function. However, the escaping is insufficient.
Step 4: Command Execution: The PHP mail function, due to the flawed escaping, interprets the attacker's payload as valid command arguments. This allows the attacker to execute arbitrary commands on the server. For example, the attacker could use the -f flag to specify a different sender, or other flags to execute arbitrary code.
The vulnerability stems from an incorrect fix for CVE-2016-10033. The isMail transport in PHPMailer uses escapeshellarg to sanitize user-supplied input before passing it to the mail function in PHP. However, the interaction between escapeshellarg and the internal escaping performed by the mail function is flawed. Specifically, the escapeshellarg function, intended to prevent shell injection, doesn't adequately handle all edge cases, allowing attackers to craft payloads that bypass the intended sanitization. This leads to a situation where attackers can inject malicious parameters into the mail command, such as -f to specify a different sender or other arguments that can be used to execute arbitrary commands. The root cause is a command injection vulnerability due to insufficient input validation and incorrect use of escapeshellarg in conjunction with the PHP mail function. The vulnerability leverages the way the PHP mail function handles parameters passed to it, especially when combined with the escapeshellarg function's limitations.
While no specific APTs are definitively linked, the ease of exploitation and the potential for complete system compromise make this a target for a wide range of attackers. CISA KEV status is likely, given the severity and ease of exploitation.
Monitor web server logs for suspicious activity related to email sending, such as unusual email headers or command-line arguments passed to the mail command.
Analyze network traffic for unusual email patterns, including large attachments or unusual sender addresses.
Implement file integrity monitoring to detect changes to critical system files.
Examine PHP error logs for errors related to the mail function, especially those involving unexpected parameters or command execution attempts.
Use intrusion detection/prevention systems (IDS/IPS) with signatures specifically designed to detect PHPMailer exploits.
Upgrade to PHPMailer version 5.2.20 or later. This is the primary and most effective remediation.
If upgrading is not immediately possible, apply a web application firewall (WAF) with rules specifically designed to detect and block PHPMailer exploits. This is a temporary measure.
Review and harden the server's PHP configuration, including disabling unnecessary PHP functions and restricting access to sensitive files.
Implement strong input validation and output encoding throughout the application to prevent other potential vulnerabilities.
Regularly scan the application for vulnerabilities using static and dynamic analysis tools.