Trying to log in using my username ____ doesn’t work, it displays this error (this bug has existed since I created the account):

Today I discovered a workaround, I can log in if I prepend a \ to my username:

It also works if I write my username like this \_\_\_\_

Using a single _ for the username (and removing the minlength of 3 of the field with the html inspector) logs me in too, but not into this account, I get logged in into my @Lee account that has the same password:

I haven’t read the code (I don’t think I can, I never managed to learn Rust) but my theory is that _ is being used as a placeholder or matchall value when comparing usernames and emails. If I escape it with \ it works because it matches my username. If I write a single unescaped _ it logs me into my other account because the email of that account is set to a single character “@” which should match a single matchall underscore.

Edit: the autocomplete field in the block users page seems to confirm my theory as it matches any username as long or longer than the number of underscores I write:

  • ​OP
    5•2 years ago

    You need to know the exact length of the account name (it seems that % is filtered because it is not allowed in usernames and only underscores can be used as placeholders). The risk is minimal, the only possible exploit that comes to mind is trying a list of compromised/common passwords and testing each with underscore usernames of different lengths. That way you will be able to log in as the first person (by database query sort order) using a compromised/common password whose username (or email) has the same amount of characters as underscores you tried. So the usual advice applies: don’t use a compromised or common password and you will be safe, use a password manager and let it generate a random password for you if you can. Also this is easy to detect server side and if there is any kind of rate limiting the attack won’t work, I wouldn’t worry about this bug.