Jul 19 2010

Microsoft Validates Shortcut Vulnerability

Last Thursday I read a posting over at   http://krebsonsecurity.com referencing  a potential vulnerability that relates to the way how Windows parses shortcuts. Since Microsoft didn’t confirm this at the time it was just another  interesting read. Now one day later Microsoft did validate this claim and now its yet another Windows zero-day without a proper workaround.

Issue:

The vulnerability exists because Windows incorrectly parses shortcuts in such a way that malicious code may be executed when the user clicks the displayed icon of a specially crafted shortcut. This vulnerability is most likely to be exploited through removable drives. For systems that have AutoPlay disabled, customers would need to manually browse to the root folder of the removable disk in order for the vulnerability to be exploited.

Workaround:

Microsoft has tested the following workarounds and states in the discussion whether a workaround reduces functionality:

Disable the displaying of icons for shortcuts

Note Using Registry Editor incorrectly can cause serious problems that may require you to reinstall your operating system. Microsoft cannot guarantee that problems resulting from the incorrect use of Registry Editor can be solved. Use Registry Editor at your own risk. For information about how to edit the registry, view the “Changing Keys And Values” Help topic in Registry Editor (Regedit.exe) or view the “Add and Delete Information in the Registry” and “Edit Registry Data” Help topics in Regedt32.exe.

1. Click Start, click Run, type Regedit in the Open box, and then click OK
2. Locate and then click the following registry key:

HKEY_CLASSES_ROOT\lnkfile\shellex\IconHandler

3. Click the File menu and select Export
4. In the Export Registry File dialog box, enter LNK_Icon_Backup.reg and click Save

Note This will create a backup of this registry key in the My Documents folder by default

5. Select the value (Default) on the right hand window in the Registy Editor. Press Enter to edit the value of the key. Remove the value, so that the value is blank, and press Enter.
6. Restart explorer.exe or restart the computer.

Impact of workaround.Disabling icons from being displayed for shortcuts prevents the issue from being exploited on affected systems. When this workaround is implemented, shortcut files and Internet Explorer shortcuts will no longer have an icon displayed.

Disable the WebClient service

Disabling the WebClient service helps protect affected systems from attempts to exploit this vulnerability by blocking the most likely remote attack vector through the Web Distributed Authoring and Versioning (WebDAV) client service. After applying this workaround, it will still be possible for remote attackers who successfully exploited this vulnerability to cause Microsoft Office Outlook to run programs located on the targeted user’s computer or the Local Area Network (LAN), but users will be prompted for confirmation before opening arbitrary programs from the Internet.

To disable the WebClient Service, follow these steps:

1. Click Start, click Run, type Services.msc and then click OK.
2. Right-click WebClient service and select Properties.
3. Change the Startup type to Disabled. If the service is running, click Stop.
4. Click OK and exit the management application.

Impact of workaround. When the WebClient service is disabled, Web Distributed Authoring and Versioning (WebDAV) requests are not transmitted. In addition, any services that explicitly depend on the Web Client service will not start, and an error message will be logged in the System log. For example, WebDAV shares will be inaccessible from the client computer.

How to undo the workaround.

To re-enable the WebClient Service, follow these steps:

1. Click Start, click Run, type Services.msc and then click OK.
2. Right-click WebClient service and select Properties.
3. Change the Startup type to Automatic. If the service is not running, click Start.
4. Click OK, and exit the management application.

This brings up the same concern I spoke about in this post , now that Microsoft is no longer supporting SP2 we can add this to the list of exploits that we can always be certain will work on the SP2 platform. In the end, if the workaround is not hindering any mission critical application  I think everyone should apply it.

References:

http://krebsonsecurity.com/2010/07/experts-warn-of-new-windows-shortcut-flaw/

http://www.computerworld.com/s/article/9179299/Microsoft_confirms_nasty_Windows_zero_day_bug?taxonomyId=17

http://www.microsoft.com/technet/security/advisory/2286198.mspx

http://clubhouse.microsoft.com/Public/Post/48631f1c-c884-4d3e-a19b-c9994bcd4637

Click OK and exit the management application.

3 Responses to “Microsoft Validates Shortcut Vulnerability”

  1. Nett0yan7 says:

    I love the fact that Metasploit has this exploit available

  2. [...] few days ago I posted a blog entry called Microsoft Validates Shortcut Vulnerability, this entry basically explains what the issue is and also listed a few basic mitigation [...]

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