Microsoft Outlook Express NNTP Response Parsing Buffer Overflow
Vulnerability

iDEFENSE Security Advisory 06.14.05
www.idefense.com/application/poi/display?id=263&type=vulnerabilities
June 14, 2005

I. BACKGROUND

Microsoft Outlook Express is an e-mail and newsgroup client shipped with
the Microsoft Windows operating system.

II. DESCRIPTION

Remote exploitation of a buffer overflow vulnerability in the news
reader functionality of Microsoft Corp.'s Outlook Express allows
attackers to execute arbitrary code under the current users privileges.

The vulnerability specifically exists when parsing Network News Transfer
Protocol (NNTP, RFC 977) server responses after issuing a "LIST"
command. A stack-based buffer overflow occurs within a routine in
MSOE.dll, found in "C:\Program Files\Outlook Express\MSOE.DLL." The
addresses and offsets that follow are based on MSOE.DLL version
5.50.4927.1200 as shipped with Microsoft Windows 2000 SP4. When parsing
a server response of the following form:

    alt.12hr 0<LONG STRING>000001325 0000001322 y\r\n
    FIELD1   FIELD2                  FIELD3     FIELD4 TERMINATOR

Various string parsing loops are used that call the CharNext() and
IsSpace() routines to determine the length of the whitespace delimited
fields. An assembly equivalent StrCpy() is used to copy FIELD2 into a
static (16 byte) stack-based buffer here:

    SUB_6AED247A()
    ...
    6AED268B mov eax, ebx                    ; eax = start of FIELD2
    6AED268D lea edi, [ebp+buff]             ; edi = stack variable
    6AED2690 sub eax, esi                    ; esi = end of FIELD2
    6AED2692 mov ecx, eax                    ; ecx = length of FIELD2
    6AED2694 mov edx, ecx                    ; edx = length of FIELD2
    6AED2696 shr ecx, 2
    6AED2699 rep movsd                       ; *** overflow occurs here
    6AED269B mov ecx, edx
    6AED269D and ecx, 3
    6AED26A0 rep movsb                       ; copy remaining bytes
    6AED26A2 and byte ptr [ebp+eax+buff], 0  ; null terminate the string

The copied buffer is later passed to the routine StrToIntA(). The
"rep movsd" instruction located at 0x6AED2699 causes a stack-based
buffer overflow to occur with user-supplied data. An attacker can
overwrite a stack-stored Structured Exception Handler (SEH) to alter
instruction flow and eventually execute arbitrary code.

Outlook Express as shipped with Windows XP SP2 is not affected. The
following excerpt from MSOE.DLL version 6.0.2900.2180 shows that a
check has been added that limits the length of FIELD2 to 14 bytes:

    SUB_60354869()
    ...
    60354A7B mov edi, ebx                    ; edi = current location
    60354A7D sub edi, [ebp+var_11C]          ; edi = length of field2
    60354A83 cmp edi, 14                     ; if length of field2 > 14
    60354A86 jbe short loc_60354A8D
    60354A88 push 14
    60354A8A pop edi                         ; length of field2 = 14

Furthermore, a StrCpy() or equivalent instruction sequence is not used
to generate a new buffer to pass to the StrToIntA() routine. Instead
FIELD2 is null terminated in memory and a pointer to the start of FIELD2
is passed to StrToIntA().

III. ANALYSIS

Exploitation of the described vulnerability allows remote attackers to
execute arbitrary code under the privileges of the currently logged on
user. An attacker must create a malicious server and either force or
convince a target user to connect to it with Outlook Express. On at
least Windows 2000 and Windows XP systems the default handler for the
"news://" URI is Outlook Express. An attacker can force a target user to
connect to a malicious server through a link such as:

    news://attack.server.ip

The impact of this vulnerability is mitigated by the fact that when the
target user connects to the malicious server he or she will be presented
with a yes/no dialog box containing the following text:

    "You are not subscribed to any newsgroups in this account.
    Would you like to view a list of available newsgroups now?"

The user must select "yes" for exploitation to occur.

IV. DETECTION

iDEFENSE has confirmed the existence of this vulnerability in Microsoft
Outlook Express 5 as shipped with Windows 2000 SP4
(MSOE.DLL v5.50.4927.1200) and Outlook Express 6 as shipped with Windows
XP SP1 (MSOE.DLL v6.0.2800.1437). It is suspected that earlier versions
of Outlook Express are also vulnerable.

iDEFENSE has confirmed that Outlook Express 6 as shipped with Windows XP
SP2 (MSOE.DLL v6.0.2900.2180) is not vulnerable.

V. WORKAROUND

Disable Outlook Express as the default "news://" URI handler and connect
to untrusted news sources with an unaffected news reader. The URI
handler can be removed by deleting the following registry key:

    HKEY_CLASSES_ROOT\news\shell\open\command

Alternatively, Windows XP users can install service pack 2.

VI. VENDOR RESPONSE

The vendor security advisory and appropriate patches are available at:

   http://www.microsoft.com/technet/security/Bulletin/MS05-030.mspx

VII. CVE INFORMATION

The Common Vulnerabilities and Exposures (CVE) project has assigned the
names CAN-2005-1213 to these issues. This is a candidate for inclusion
in the CVE list (http://cve.mitre.org), which standardizes names for
security problems.

VIII. DISCLOSURE TIMELINE

11/16/2004  Initial vendor notification
11/18/2004  Initial vendor response
06/14/2004  Coordinated public disclosure

IX. CREDIT

An anonymous contributor is credited with discovering this
vulnerability.

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X. LEGAL NOTICES

Copyright (c) 2005 iDEFENSE, Inc.

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Disclaimer: The information in the advisory is believed to be accurate
at the time of publishing based on currently available information. Use
of the information constitutes acceptance for use in an AS IS condition.
There are no warranties with regard to this information. Neither the
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this information.