The zero-day exploit is present in versions 11.x of the Yahoo Messenger client - including the very last released version - and allows remote attackers to very easily hijack the victim's status.
"The status message change occurs when an attacker simulates sending a file to a user," explains BitDefender. "This action manipulates the $InlineAction parameter (responsible for the way the Messenger form displays the accept or deny the transfer) in order to load an iFrame which, when loaded, swaps the status message for the attacker's custom text."
The changed message can include links to malicious sites or - a lesser evil - affiliate links. And unfortunately, the attack is possible even if the attacker is not included in the victim's contact list.
Users can currently partly protect themselves from such an attack if they change their Yahoo Messenger settings so that anyone who is not already on their contact list is ignored. And they should, since the attack has been spotted in the wild. But that doesn't protect them if one of their contacts fell for the scheme and his/her account is now sporting a malicious message.
The researchers say that they have contacted Yahoo! and sent the proof-of-concept code and the documentation to them, so let's hope the bug will be fixed soon.