October 3rd, 2001


AIM vulnerabilities

i use AIM a lot (deuxmacht, mail me if you wanna be on my buddy list). i thought other people might find this interesting as well:


(Note: I wasn't going to release this until the 8th in order to give AOL some time to release a fix/workaround, but since exploit scripts have already been posted to bugtraq...)

Anyone who can send instant messages to a user signed on to
the AOL Instant Messenger service can crash that user's AOL
Instant Messenger. The default settings allow everyone to
send the user messages. This bug does not appear to be
exploitable for running arbitrary code.
Confirmed Vulnerable:
AOL Instant Messenger/Win32 4.7.2480
AOL Instant Messenger/Win32 4.3.2229
Confirmed Not Vulnerable:
aimirc (all versions)
AIM Express
AOL Instant Messenger/Linux 1.5.234
All other AOL Instant Messenger clients

Reported to AOL on October 1st, 2001. No reply received.

It is possible for any remote user to crash the AOL Instant Messenger for Windows, at least version 4.7.2480. The target user's visibility settings must allow the exploiter to send him or her IMs. When a message with the HTML open-comment tag (omitted here because it fucks up the journal entry) is repeated approximately 640 or more times, AIM crashes with the following
AIM caused in invalid page fault in module ATK32.DLL at
EAX=00000000 CS=015f EIP=12023f63 EFLGS=00010246
EBX=0063ea94 SS=0167 ESP=0063e9dc EBP=0063ea24
ECX=0043dab0 DS=0167 ESI=0043051c FS=0e87
EDX=00000000 KS=0167 KDI=0063ea8c GS=0000
Bytes at CS:EIP:
83 78 28 00 74 08 c7 07 ff 7f 00 00 eb 06 8b 40
Stack dump:
00000000 0043051c 00000409 218f0004 8a120000
17df0b04 00010000 00000000 00000000 00000002
00000000 00000302 0000000c 00000001 0000000c

Note that it does not appear to be possible to send this message from AOL's Windows AOL Instant Messenger client, both because it imposes tighter length restrictions than the OSCAR protocol mandates and
because it will translate < into <

If the "Show 'Accept Message' dialog for messages from users not in Buddy List" preference is turned on and the exploiter is not in the target's buddylist, that dialog will appear and then AIM will immediately crash. If that preference is not turned on or if the exploiter is in the target's buddylist, an IM dialog will be created (if one does not already exist), and then AIM will immediately crash.

This bug is already being exploited in the wild. It initially came to my attention through a post to the vuln-dev@securityfocus.com mailing list as well as, simultaneously, in traffic observed in the AIM sessions of users of my network.

Suggested workaround:
If possible, modify your privacy settings so that only users
on your buddylist can contact you.
However, this still makes
it possible for people on your buddylist to use this
bug against you. Until AOL releases a fix, the only other
option is to switch to a non-vulnerable client.
Alternatively, one can simply live with the occasional crash
and simply restart AOL Instant Messenger. Of course,
malicious persons could set up scripts to automatically send
a crash-inducing message to the user as soon as he or she
signed on to the AOL Instant Messenger service.


(i've had that setting on my buddy list for months and i'm now glad it's there.)
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