Source: cve@mitre.org
The mailSend function in the isMail transport in PHPMailer before 5.2.18 might allow remote attackers to pass extra parameters to the mail command and consequently execute arbitrary code via a \" (backslash double quote) in a crafted Sender property.
PHPMailer versions prior to 5.2.18 are vulnerable to a critical remote code execution (RCE) flaw. Attackers can leverage a crafted Sender property containing a backslash-escaped double quote to inject arbitrary commands into the mail server's execution, leading to complete system compromise.
Step 1: Payload Creation: The attacker crafts a malicious Sender property. This property includes a backslash-escaped double quote (") followed by the attacker's desired command and then a closing quote to terminate the injected argument. For example: evil@example.com" -oQ/tmp/ -X/var/www/shell.php
Step 2: Payload Delivery: The attacker sends an email through a vulnerable PHPMailer instance, setting the crafted Sender property in the email's headers.
Step 3: Vulnerability Trigger: The mailSend function in PHPMailer processes the email and attempts to send it using the mail() function. It passes the crafted Sender property as an argument to the mail() command.
Step 4: Command Injection: The operating system's mail server interprets the injected command as a valid argument to the mail() command. The attacker's command is executed.
Step 5: Code Execution: The attacker's command, such as writing a web shell to a publicly accessible directory, is executed on the server, granting the attacker remote control.
The vulnerability lies within the mailSend function of the isMail transport in PHPMailer. The code fails to properly sanitize the Sender property before passing it to the mail() function. Specifically, the use of a backslash-escaped double quote (") allows attackers to inject arbitrary command-line arguments. The mail() function, when invoked with these crafted arguments, executes the attacker-supplied commands on the server. This bypasses intended security measures and grants the attacker the ability to execute code with the privileges of the web server user. The root cause is a lack of proper input validation and escaping of the Sender property before it's used in a system call.
While no specific APT groups are definitively linked to this CVE, the ease of exploitation makes it a prime target for various threat actors. The vulnerability is likely exploited by both opportunistic attackers and more sophisticated groups. Not listed on CISA KEV.
Monitor web server logs for suspicious activity related to email sending, particularly unusual Sender addresses or command-line arguments passed to the mail command.
Analyze email headers for unusual characters or patterns in the Sender field, such as backslash-escaped double quotes or other special characters.
Implement file integrity monitoring to detect unauthorized changes to web server files, especially those in publicly accessible directories.
Network Intrusion Detection Systems (IDS) can be configured to detect malicious payloads in email traffic, specifically those targeting the PHPMailer vulnerability.
Review system logs for the execution of suspicious commands or processes related to email sending.
Upgrade to PHPMailer version 5.2.18 or later. This version includes a fix that properly sanitizes the Sender property.
Implement input validation and output encoding to prevent command injection vulnerabilities in other parts of the application.
Use a web application firewall (WAF) to filter malicious requests and block attempts to exploit the vulnerability.
Restrict the privileges of the web server user to the minimum necessary to perform its tasks. This limits the impact of a successful exploit.
Regularly update all software and libraries to the latest versions to patch known vulnerabilities.
Consider using a different email transport method if possible, such as SMTP, which may be less susceptible to this type of vulnerability.