Microsoft Outlook Express NNTP Response Parsing Buffer Overflow Vulnerability iDEFENSE Security Advisory 06.14.05 www.idefense.com/application/poi/display?id=263&type=vulnerabilitie s 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
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. Get paid for vulnerability research http://www.idefense.com/poi/teams/vcp.jsp Free tools, research and upcoming events http://labs.idefense.com X. LEGAL NOTICES Copyright (c) 2005 iDEFENSE, Inc. Permission is granted for the redistribution of this alert electronically. It may not be edited in any way without the express written consent of iDEFENSE. If you wish to reprint the whole or any part of this alert in any other medium other than electronically, please email email@example.com for permission. 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 author nor the publisher accepts any liability for any direct, indirect, or consequential loss or damage arising from use of, or reliance on, this information.