[NT] Yahoo! Messenger URL Handler Remote DoS

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  Yahoo! Messenger URL Handler Remote DoS
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SUMMARY

A denial of service vulnerability exists in the way Yahoo! Messenger 
processes arguments in their YMSGR: URL handler links. By crafting the 
links with certain characters after the first colon or after the third 
colon (after YMSGR:) attackers can create malformed packets to be sent to 
Yahoo!'s YMSG servers. When these packets are sent Yahoo! will immediately 
disconnect us from our current chat session.

DETAILS

Vulnerable Systems:
 * Yahoo! Messenger version 6.0
 * Yahoo! Messenger version 5.0

By crafting YMSGR: links specifically after the first or third colons, 
preceding with an ampersand (&), we can force Yahoo! Messenger to generate 
room login packets that are malformed with whatever data we would like to 
send to the Yahoo! YMSG servers causing a disconnect upon receipt.

Proof of Concept:
Example of a 'legit' use of the YMSGR: URL handler to join a room
YMSGR:Chat?ChatterBox:2::21748078

The above link would instruct Yahoo! Messenger to send a join room request 
packet to the server, the room in this example being ChatterBox:2. 
Breaking down the arguments we have the room name, room # and room space 
#, all needed in the complete YMSGR: "chat?" link (or Messenger 6.0 won't 
send any packets if this syntax isn't followed). All of this together 
would be used to specifically enter a given room through invoking the 
handler.

Interesting to point out that after the room name, room # and rmspace # 
are supplied the room # and rmspace #s aren't even used in the request 
packet so even though we're specifying a specific room to join the packets 
don't reflect that and instead we're sent to a ChatterBox room # at random 
by Yahoo! This apparently is a bug in itself since the only way to 
actually have Messenger send up the room request packet is to include the 
three colons even though the arguments behind them aren't used (until 
now).

Proof of Concept:
Example of a malicious use of the YMSGR: URL handler to disconnect a 
Messenger user:
YMSGR:Chat?:::&&&<(*_*)>

When created and used in this manner Yahoo! Messenger will accidentally 
"corrupt" the room login and/or room join request packets with whatever 
data we'd like to add, injected after the last ampersand in the link.

This example here would insert a smiley face into a 0x00 0x96 room login 
request packet and will be rejected by the server immediately 
disconnecting the target:

59 4D 53 47 00 0C 00 00 00 46 YMSG.....F
00 96 00 00 00 00 9D 9E 1F F9 31 30 39 C0 80 6B .-.... . 109 k
65 6E 5F 74 68 6F 6D 70 73 6F 6E 33 39 C0 80 31 en_thompson39 1
C0 80 3C 28 2A 5F 2A 29 3E C0 80 36 C0 80 61 62 <(*_*)> 6 ab
63 64 65 C0 80 39 38 C0 80 75 73 C0 80 31 33 35 cde 98 us 135
C0 80 79 6D 36 2C 30 2C 30 2C 31 39 32 32 C0 80 ym6,0,0,1922 

The smiley face in this packet, between the YMSG delimiters " 1 " and " 6 
", should really have been the id again, 'ken_thompson39'.

By embedding this into IFRAMEs and links in web pages/e-mails we can 
remotely disconnect the target. Since link's contents sometimes look 
obvious (when hovering over them with the mouse pointer) we could possibly 
get around the suspicion (or add to it?) by encoding the handler arguments 
as hex chars.

Proof of Concept:
Two obfuscated link examples:
<a href="YMSGR:%63%68%61%74%3F:::%26%26%26%26">Click Here</a>
<a href="YMSGR:Chat?:::%26%26%26%26">Click Here</a>

An IFRAME example:
<iframe src="ymsgr:chat?:::&&&&">

Note: If target is not in a chat room when the link is clicked or IFRAME 
containing the handler link is launched an ad may pop up in the 
"Connecting to Yahoo! Chat" window. After the ad loads clicking on "Enter 
Chat" will cause you to disconnect. If the target is already in chat at 
the time or if an ad doesn't pop up when they're not in chat they'll be 
disconnected immediately.

Workaround:
In the Windows registry delete the string value: 
c:\progra~1\yahoo!\messenger\ypager.exe %1 under 
HKEY_CLASSES_ROOT\ymsgr\shell\open\command, or point to another file or 
location (preferably a file that won't be ran in multiple instances). As a 
result all future YMSGR: links will cease to operate under Yahoo! 
Messenger.


ADDITIONAL INFORMATION

The information has been provided by  <mailto:[email protected]> Torseq 
Tech.



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