[NT] Microsoft Outlook Express NNTP "LIST" Buffer Overflow (MS05-030)

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Date: 15 Jun 2005 16:17:26 +0200

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  Microsoft Outlook Express NNTP "LIST" Buffer Overflow (MS05-030)
------------------------------------------------------------------------

SUMMARY

 <http://www.microsoft.com/> Microsoft
<http://www.microsoft.com/windows/oe/> Outlook Express is an email and
news client bundled with operating systems and the Internet Explorer web
browser by Microsoft.

The NNTP news reader embedded in Outlook Express is vulnerable to buffer
overflow, exploiting this vulnerability allows remote attacker to execute
malicious code with the rights of Outlook's user.

DETAILS

Vulnerable Systems:
 * Microsoft Outlook Express shipped with Microsoft Windows 2000 SP4
(MSOE.DLL version 5.50.4927.1200)

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

The vulnerability specifically exists when parsing Network News Transfer
Protocol ( <http://www.ietf.org/rfc/rfc977.txt> 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:

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().

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.

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.

Disclosure Timeline:
 * 11.16.04 - Initial vendor notification
 * 11.18.04 - Initial vendor response
 * 06.14.04 - Coordinated public disclosure

Vendor Status:
The vendor security advisory and appropriate patches are available at:
 <http://www.microsoft.com/technet/security/Bulletin/MS05-030.mspx>
http://www.microsoft.com/technet/security/Bulletin/MS05-030.mspx

CVE Information:
 <http://www.cve.mitre.org/cgi-bin/cvename.cgi?name=CAN-2005-1213>
CAN-2005-1213

ADDITIONAL INFORMATION

The information has been provided by
<mailto:idlabs-advisories@idefense.com> iDEFENSE.
The original article can be found at:
<http://www.idefense.com/application/poi/display?id=263&type=vulnerabilities> http://www.idefense.com/application/poi/display?id=263&type=vulnerabilities

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