AOL ICQ Pro 2003b heap overflow vulnerability

AOL ICQ Pro 2003b heap overflow vulnerability

AOL ICQ Pro 2003b heap overflow vulnerability

Core Security Technologies - CoreLabs Advisory

Date Published: 2006-09-07

Last Update: 2006-09-06

Advisory ID: CORE-2006-0321

Bugtraq ID: 19897

CVE Name: CVE-2006-4662

Title: AOL ICQ Pro 2003b heap overflow vulnerability

Class: Boundary Error Condition

Remotely Exploitable: Yes

Locally Exploitable: Yes

Advisory URL:

Vendors contacted:

America Online Inc.
. 2006-07-27: Initial notification sent to vendor, advisory release date set for Aug. 14th.
. 2006-07-27: Vendor response acknowledging notification.
. 2006-08-11: Request for an update sent to vendor asking for an estimated date for fix availability.
. 2006-08-14: Request for an update sent to vendor asking for an estimated date for fix availability, advisory release date now set for Aug. 22nd.
. 2006-08-15: Vendor response received. Still determining when a fix will be available. A new update from the vendor forthcoming before Aug. 22nd.
. 2006-08-16: Vendor email received requesting further technical details or proof-of-concept code.
. 2006-08-17: Core response vendor: proof-of-concept for the ICQ client bug can not be made available as standalone program without incurring in a substantial development effort.
. 2006-08-21: Vendor email describing coordination issues with ICQ development team. No fix schedule provided
. 2006-08-17: Core response vendor: proof-of-concept can not be made available as standalone program without incurring in a substantial development effort.
. 2006-08-21: Vendor email describing coordination issues with ICQ development team. No fix schedule provided.
. 2006-08-21: In liue of proof-of-concept, Core provides succinct technical explanation of the problem in the ICQ 2003b client.
. 2006-08-29: Updated advisory sent to vendor requesting comments and fix availability information. Advisory release date now set for Aug. 31st.
. 2006-08-30: Vendor response received stating that 30 days is insufficient to fix bugs and reiterating the previously noted coordination and communications problems with engineering team at remote facilities. No tentative fix schedule made available, earliest date for an official vendor statement about fixes is Sept. 1st
. 2006-08-30: Core response to vendor, publication of advisories will be delayed until Sept. 6th in order to receive offical statement from vendor. Baring a precise schedule that demonstrates an imminent release of fixes the publication date is final.
. 2006-08-30: Vendor provides an official statement.
. 2006-09-06: Advisory published.

Release Mode: USER RELEASE

*Vulnerability Description:*

A vulnerability in AOL's ICQ Pro 2003b instant messenger client could lead to denial of service attacks and remote compromise of systems running vulnerable versions of the client.

The AOL/Mirabilis ICQ client is a popular Instant Messaging (IM) program that enables users to communicate through instant messaging, chat, e-mail, SMS and wireless-pager messages as well as transferring files and URLs, among other features.

In 1998 America Online Inc. acquired Mirabilis Ltd., the company responsible for the development of the ICQ instant messenger and all associated services at that time. [1] Since then, AOL's ICQ unit continued to develop and maintain the ICQ client program.

The ICQ Pro2003b client was officially launched on October 30th, 2003 and included capabilities to interoperate with AOL's Instant Messenger AIM) and AOL services. The press release with the ICQ Pro 2003b announcement indicated that, at the time, ICQ had over 160 million registered users that spent - when connected - an average of 4.5 hours on the service. [2]

The latest release of this particular IM client, ICQ Pro 2003b Build #3916, is still one of the officially available options for users who want to download an ICQ client from ICQ’s website (

Even though by its name the IM client may seem to be a "veteran" client, the ICQ team has been updating it up until -at least- Build #3916 released on October 2005. [3]

A vulnerability found in the way the ICQ Pro 2003b client handles incoming message lengths could lead to denial of service attacks and remote compromise of systems running vulnerable versions of the client.

Attacks that leverage this vulnerability would be difficult to identify and isolate as exploit traffic does not present any features that makes it easily distinguishable from normal IM communications.

*Vulnerable Packages:*

The following AOL/ICQ software products are affected by this issue:
- ICQ Pro 2003b Build #3916 and previous.

*Non-vulnerable Packages:*

- ICQ 5.1
- ICQ2Go!

*Solution/Vendor Information:*

Statement provided by AOL Product Vulnerabilities team:
"AOL has recently been made aware of a vulnerability in the ICQ 2003b
client build #3916. Successful exploitation of the vulnerability may
allow an attacker to remotely execute commands.

AOL and ICQ recommend that users upgrade to the latest version of the
ICQ client: ICQ 5.1"


Luciana Tabo, Lucas Lavarello, Sebastian Cufre, Ezequiel Gutesman and Javier Garcia Di Palma from Core Security Technologies discovered and tested this vulnerability during Bugweek 2006.

This vulnerability was found using synaptic-based fuzzing.

*Technical Description - Exploit/Concept Code:*

A heap overflow vulnerability was found in the ICQ Pro 2003b build #3916 IM client. The problem derives from the way the vulnerable client handles the length of a specific type of message received from other clients.

The ICQ protocol supports exchange of IM messages both using servers as well as with direct client-to-client connections, where data is sent without a need for an intermediate ICQ server to process it.

The vulnerability was tested using the client-server-client model, presenting a high-risk scenario since exploitation does not require the establishment of a direct client-to-client connection with the victim system. In the tested case, ICQ communications servers will pass malicious traffic to unsuspecting clients without inspecting it first and without enforcing strict sanity checks on the data fields.

To understand the technical description that follows, a few terms from common ICQ message communication terminology will be defined:

FLAP: A 6 bytes structure, used to identify the channel (login[1], connected[2], errors[3], logout[4], ...) for the packet being sent.

It also contains a sequence number and the length of the whole packet.

SNAC: A 10 bytes header used to identify the purpose of the packet. SNACs identify packet types through a family type (Word) as well as a SubType (Word).

TLV: Type-Length-Value, a container structure where the first two fields are a Type (Word) and a Length (Word), followed by the data.

LNTS: A null terminated string preceded by a word (Little Endian), indicating the length of the NTS, including the terminating null character.

The vulnerability is triggered when a specific packet is received by a vulnerable client on FLAP Channel 2, the channel in which most of the packets are sent during a successful connection.

There are 3 main types of messages at the time of exchanging data between ICQ clients when communicating through servers:
[Type 1] - Simple, plaintext messages.
[Type 2] - Messages, extended to support rtf, colors, etc.
[Type 4] - Utility messages, used for URLs, contacts, etc..

The issue resides inside a Type 2 message. Messages are stored inside the Channel 2 FLAP with a SNAC of family-type 4, subtype 6.

Here is the outlook of ICQ communications packet so far:
[FLAP channel 2
[ SNAC type 4 - subtype 6
[message type 2]

There are two other encapsulation layers within the described packet that need to be inspected in order to identify malicious data that could trigger or exploit the described bug. Inside the Type 2 Message, a TLV of Type 5 will include a set of information such as client capabilities and sequence numbers. These are split in different Sub-TLVs within the type 5 TLV (carried within a Type-2 message of SNAc type4, subtype 6).

There is one Sub-TLV in particular that we want to look at: TLV Type 0x2711.

TLV Type 0x2711 will hold, among other things, a Message structure that includes LNTs.

So, let's look at an updated version of the previous outline:

[FLAP channel 2
[ SNAC type 4 - subtype 6
[message type 2
[ TLV type 5
[TLV type 0x2711
[Message - LNTS ]

It is inside the TLV type 0x2711 where a LNTS field resides with the contents of the [Message]. AS explained above, the first word of a LNTS determines the length of the message, followed by a null-terminated string.

The ICQ Pro 2003b client does not perform any sanity check on this length field and does not compare it to the actual size of the 0x2711 TLV or the size of the entire received packet. Unlike with other packet fields, an intermediate server does not perform any sanitation on the contents of this field either and therefore passes potentially malformed data to connected clients, making a fully controllable attack vector available to using potentially malicious IM client programs.

The nature of the bug can be understood by attaching a debugger to the ICQ Pro 2003b client and tracing down the issue to find the problem inside a routine called “MCRegEx__Search”, which calls memset to clear the contents of a heap allocated buffer, directly using our length field (described above) as the third argument to the memset function. [4]

The following short disassembly should provide more detail:

First breakpoint is set inside ICQCUtl!ReadStringBCStreamFormat:

20002108 ff152cb00020 call dword ptr [ICQCUtl!MCRegEx__Search+0x89d4
(2000b02c)]{ICQRT!Ordinal360 (21382b39)} ds:0023:2000b02c=21382b39

The reason the initial breakpoint is set inside ReadStringBCStreamFormat is because MCRegEx__Search is constantly called from several different locations.

It is inside this routine that a call to ICQRT!Ordinal116+0x1af ends up calling memset and using our length value directly:

213821ea 0fbe442414 movsx eax,byte ptr [esp+0x14]
213821ef 53 push ebx (length specified in the LNTS)
213821f0 50 push eax (character being written, 0)
213821f1 8b4604 mov eax,[esi+0x4]
213821f4 034608 add eax,[esi+0x8]
213821f7 50 push eax (destination buffer)
213821f8 e8b5300000 call ICQRT!Ordinal116+0x1af (213852b2)

ICQRT!Ordinal116+0x1af is the stub for memset that contains a direct jmp to the msvcrt.


Switch to ICQ 5.1, which is (at the moment of writing the advisory) the latest build for the alternative non-vulnerable ICQ official client.

ICQ 5.1 is available at



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The contents of this advisory are copyright (c) 2006 CORE Security Technologies and (c) 2006 CoreLabs, and may be distributed freely provided that no fee is charged for this distribution and proper credit is given.

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