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: The attacker crafts a malicious email address or other input containing shell commands, designed to be passed to the mail command via PHPMailer's isMail transport.
Step 2: Input Processing: The attacker's crafted input is passed to PHPMailer's isMail transport, where it's intended to be sanitized by escapeshellarg.
Step 3: Incomplete Sanitization: escapeshellarg fails to completely sanitize the input, leaving exploitable characters in place.
Step 4: Command Execution: The sanitized (but still malicious) input is passed to the PHP mail function. The mail function, due to its internal handling of arguments, interprets the injected commands as part of its execution, leading to arbitrary code execution on the server.
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 itself is flawed. Specifically, escapeshellarg does not fully prevent malicious characters from being passed to the underlying mail command. Attackers can craft a malicious email address or other input that, when processed by escapeshellarg and then the mail function, results in the execution of arbitrary shell commands. The root cause is a command injection vulnerability due to insufficient input validation and escaping, allowing for shell command injection through the mail function's parameters. This bypasses the intended security measures.