(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...)

Scope:
        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
        QuickBuddy
        AOL Instant Messenger/Linux 1.5.234
Unknown:
        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 text "<!-- " (without the quotes) is repeated
approximately 640 or more times, AIM crashes with the following
error.
        AIM caused in invalid page fault in module ATK32.DLL at
015f:12023f63.
        Registers:
        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
        00000000

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 &lt;

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-devsecurityfocus.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.

-- 
Matthew Sachs, the original nonstandard deviant
matthewgzevils.com	http://www.zevils.com/
GPG key: 0x600A0342	PGP key: 0x93EA1151

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