Remote Code Execution vulnerability in PHPMailer 6.4.1 running on Windows

PHPMailer 6.4.1 contains a possible remote code execution vulnerability through the $lang_path parameter of the setLanguage() method. If the $lang_path parameter is passed unfiltered from user input, it can be set to a UNC path, and if an attacker is also able to create a remote mount on the server that the UNC path points to, a script file under their control may be executed.

Impact

Arbitrary code may be run by a remote attacker under the web server or PHP process running on Window hosts.

Patches

Mitigated in PHPMailer 6.5.0 by no longer treating translation files as PHP code, but by parsing their text content directly. This approach avoids the possibility of executing unknown code while retaining backward compatibility. This isn't ideal, so the current translation format is deprecated and will be replaced in the next major release.

Workarounds

Any of:

• Ensure that calling code does not pass unfiltered user-supplied data to the $lang_path parameter of the setLanguage() method.
• Block or filter the use of unknown UNC paths in this parameter (or altogether).
• Ensure that unauthorised users do not have the ability to read from unknown remote servers via UNC paths.
• Run on an OS that does not support UNC paths

References

CVE-2021-34551.

Reported by listensec.com via Tidelift.

For more information

If you have any questions or comments about this advisory:

• Open an issue in PHPMailer
• [Email the maintainers](mailto:phpmailer@synchromedia.co.uk)