It's also why I far-too-often run into forms that won't let me use a "+" in the username part of my email address, which I use to track who's responsible for sending my email account off to third parties (e.g., "rob+paypal@....").

Some kinds of email validation are better than others. Using regular expressions and strictly adhering to the RFC is the one that developers are usually talking about when they say not to do it. filter_var(..., FILTER_VALIDATE_EMAIL) is sort of okay, although there are lots of edge cases that it doesn't handle correctly.

Other than that: true, I hate incorrect validation. But I hate sloppy security practices even more.

To get around that, the service provider would have to verify identity further down the line, making the email format restriction redundant.

If the service value is not too high then even making people who want to abuse the service go through the process of registering a free email account works. Put a little bit of friction into the process and the script kiddies move onto an easier target.

I don't have any other email. Seems like a really bad idea.

In general I agree that PHP mail() function should take a part of blame here for not exposing a sane interface. Spammers have abused many bugs where programmers failed to sanitize "To:" or "From:" addresses and attackers could pass \r and \n characters (which allowed adding BCC fields, effectively sending e-mails to arbitrary addresses).

> Also the vuln does not apply to CLI sendmail...

Sure it does: >> ...will result in the followig list of arguments passed to sendmail program:

EDIT: you probably meant to say that the vulnerability is only triggered when CLI sendmail is called via mail(), which is true. Configuring PHPMail to use sendmail via CLI directly apparently doesn't expose this vulnerability.

you have to be careful with the meaning of "valid" though: What's valid in one context might not be in another context. Do you want to limit the character set to the least common denominator of non-special characters valid in all possible contexts your address might be used?

What if you don't exclude some character because it's not special in any of the current contexts but then you later add another thing to the mix that uses in-band signalling? Now you need to update your whitelist and remove even more "invalid" characters.

This won't end well for you.

I would recommend you don't put a restriction on the input (aside of what the RFC defines as valid) and instead correctly escape (or throw if your context doesn't support escaping) when moving the raw data to a new context.

In this case, I think the problem is in PHP's `mail()` function that put stuff in shell context without any escaping.

The fix should be happing in `mail()` (which is internally shelling out and thus switching context), not in the caller and certainly not in the frontend controller when it needs to decide whether a given email address is valid or not.

You should still properly escape data when passed to another context, no doubt about it.

I am also not suggesting "dirty" practices like replacing double quotes (Wordpress) or magically escaping quotes on input (PHP prior to... 5?). But if you know the input should be a number, allow only digits and '.' and clean everything else. And validate the range too! If you need a database ID, validate form and existence. If you need an e-mail address, run it through filter_var. Always. There is no reason not to.

That's not correct. PHPMailer can be configured to send mail through raw SMTP, by directly invoking sendmail, or by calling PHP's mail() function (which is itself a wrapper around sendmail). This vulnerability affects only the last mode, when PHP's mail() calls sendmail. If you have PHPMailer configured to call sendmail directly, this vulnerability does not apply.

Understatement of the year!

If you were to try and actually validate according to the RFC, chances are you'll introduce new vectors of attack in your code simply because of the complexity of what you're trying to achieve.

Honestly, the email address RFC is a litany of insane choices and kludged-together standards to account for the wild west free-for-all that existed before the modern internet emerged, and most of it has no bearing on the reality of how people use and create email addresses today. Better to just check if the address supplied has an @ in it somewhere and be done with it.

So yes, I believe that in 2016, a match on ^[^@]+@([^.@]+\.)+[^.@]+\.?$ does not have any actual false negatives, although allowing false positives. That's not entirely helpful in this CVE, though.

  filter_var($app->request()->post('email'), FILTER_VALIDATE_EMAIL)