Cannot follow user with email address as name

[Downes]

In a WordPress environment with ActivityPub installed, I can successfully follow @Downes@leftish.media from Mastodon, but cannot follow @stephen@downes.ca@leftish.media

The problem may be related to the behaviour of the Web Finger plugin, as 'Downes' resolves nicely: https://leftish.media/.well-known/webfinger?resource=acct:Downes@leftish.media
while 'stephen@downes.ca' does not: https://leftish.media/.well-known/webfinger?resource=acct:stephen@downes.ca@leftish.media

The error appears to be caused by the parsing of username and website URL from the identifier. It appears to be splitting at the first '@' which would result in a username of 'stephen' and a website of 'downes.ca@leftish.media' whereas it should be split at the last '@', which would result in a username of 'stephen@downes.ca' and 'leftish.media'.

This issue may exist in several plugins. It may also exist in ActivityPub clients as well, such as Mastodon.

[mediaformat]

Interesting edge case

While the spec is meant to support this:

A common scenario is for a user to register with a service provider
using an identifier (such as an email address) that is associated
with some other service provider. For example, a user with the email
address "juliet@capulet.example" might register with a commerce
website whose domain name is "shoppingsite.example". In order to use
her email address as the localpart of the 'acct' URI, the at-sign
character (U+0040) needs to be percent-encoded as described in
[RFC3986]. Thus, the resulting 'acct' URI would be
"acct:juliet%40capulet.example@shoppingsite.example".

Even webfinger.net, which seems to be the reference implementation tries to fetch the resource from https://downes.ca%40leftish.media

15:48:11 Looking up WebFinger data for acct:stephen@downes.ca@leftish.media
15:48:11 GET https://downes.ca%40leftish.media/.well-known/webfinger?resource=acct%3Astephen%40downes.ca%40leftish.media
15:48:11 Error getting JRD: parse https://downes.ca%40leftish.media/.well-known/webfinger?resource=acct%3Astephen%40downes.ca%40leftish.media: invalid URL escape "%40"

I've opened a ticket there to start: webfinger/webfinger.net#28