Add the possibility of changing @phpmailer.0 string in asHtml method when composing image cid.

[Matslom]

Hello,

There is a hard coded string in https://github.com/PHPMailer/PHPMailer/blob/master/src/PHPMailer.php#L4418 and https://github.com/PHPMailer/PHPMailer/blob/master/src/PHPMailer.php#L4452 which leaks server/app details. For security reasons, we should be able to change this to a custom one, like other headers.

Currently, only way to do that, is to override the entire msgHTML method. Is there a chance to introduce a more flexible approach in the library?

[Synchro]

How do you see this as a risk? What threat model do you foresee as applying to it? Neither of these uses are headers. Also bear in mind that msgHTML is purely a convenience function that you don't need to use at all. How would you change the ID generation, bearing in mind that even a random string would be readily fingerprintable?

[mariuszkrzaczkowski]

Our system underwent a security audit and it was detected as a vulnerability (excerpt below)

Disclosure of redundant environment information in email headers
RECOMMENDATION
We strongly recommend removing redundant headers that reveal information about the technologies used.

I would suggest adding @phpmailer.0 to the variable, which will make it easy to overwrite it
Overwriting the entire function is problematic because it requires monitoring during updates