This was a vulnerability in the past, but was never properly fixed. About a month ago I contacted the AO admin's about it and provided a PoC, however I never received a response after its acknowledgement and the hole was never fixed. I decided while going over this thread that I owe it to the community to publicize what I found:

Overview:Cross Site Scripting (XSS) attacks are possible in the username field of karma.php. XSS attacks are scripts injected
through user provided information to carry sensitive information into another zone. In this case, JavaScript can be
injected into any the username field. Although there are precautions taken by to prevent this, input
sanitization is incomplete.
By inserting malicious JavaScript into karma.php an attacker can forward the cookies of members that click a malicious
link. allows users to authenticate themselves through cookies, allowing attackers to impersonate victims
through stolen cookies.
Solution: Disable JavaScript in browsers and/or disable cookie authentication.
mnstrgrl: An addslashes function isn't enough to sanitize fields.