We got a realatively high bounce rate today, because someone decided to spread some links using our mail server. The implementation looked like this: He used the registration form and planted a link in the firstname field, which appears in the email's first line. Then he sent out like 1200 emails like this. And my question is what can we do to prevent this? We can use captcha for sure, but can we do more about it?
It's the verification email which is sent. Like if you register to any other sites, the website sends you an email to verify your email address. And what he did was to put the link in the firstname field of the registration field. so in the verification email in the place, when it would say -
Hello XY! pls verify your email... - there is this instead -
Hello <link>! pls verify your email...
I don't think that those email addresses belong to him, because in most of the case we didn't get a bounce, and also all the email messages had a different structure, plus there was more in the firstname field. Example:
Also the verification email has a template which starts like this:
I think it's might be also good to validate the field and check if it contains some characters which can't be part of a firstname as an extra. This might be better to be done with blacklisting some characters, because some languages has special characters in names, but those are not punctuations.
The recaptcha can't do much against request forgery, so it might be a good idea to also implement an IP filter limit, where you allow only X registration in an Y amount of time only from the same IP address.