Vulnerability Instance

PHPMailer untrusted code may be run from an overridden address validator
If a function is defined that has the same name as the default built-in email address validation scheme (`php`), it will be called in default configuration as when no validation scheme is provided, the default scheme's callable `php` was being called. If an attacker is able to inject such a function into the application (a much bigger issue), it will be called whenever an email address is validated, such as when calling `validateAddress()`.

### Impact
Low impact – exploitation requires that an attacker can already inject code into an application, but it provides a trigger pathway.

### Patches
This is patched in PHPMailer 6.5.0 by denying the use of simple strings as validator function names, which is a very minor BC break.

### Workarounds
Inject your own email validator function.

### References
Reported by [Vikrant Singh Chauhan](mailto:vi@hackberry.xyz) via [huntr.dev](https://www.huntr.dev/).
[CVE-2021-3603](https://web.nvd.nist.gov/view/vuln/detail?vulnId=CVE-2021-3603)

### For more information
If you have any questions or comments about this advisory:
* Open an issue in [the PHPMailer project](https://github.com/PHPMailer/PHPMailer)
* [Email us](mailto:phpmailer@synchromedia.co.uk).