Software Engineering Institute

A buffer overflow exists in the "ResDLL" parameter of the MSN Chat ActiveX control that may permit a remote attacker to execute arbitrary code on the system with the privileges of the current user. This vulnerability affects MSN Messenger and Exchange Instant Messenger users. Since the control is signed by Microsoft, users of Microsoft's Internet Explorer (IE) who accept and install Microsoft-signed ActiveX controls are also affected. The Microsoft MSN Chat control is also available for direct download from the web.

The <object> tag could be used to embed the ActiveX control in a web page. This vulnerability could be exploited if an attacker can trick the user into visiting a malicious site or the attacker sends the victim a web page as an HTML-formatted email message or newsgroup posting. This acceptance and installation of the control can occur automatically within IE for users who trust Microsoft-signed ActiveX controls. When the web page is rendered, either by opening the page or viewing the page through a preview pane, the ActiveX control could be invoked. Likewise, if the ActiveX control is embedded in a Microsoft Office (Word, Excel, etc.) document, it may be executed when the document is opened.

According to the Microsoft Advisory (MS02-022):

It's important to note that this control is used for chat rooms on several MSN sites in addition to the main MSN Chat site. If you have successfully used chat on any MSN-site, you have downloaded and installed the chat control.

A remote attacker can execute arbitrary code on the target machine with the privileges of the current user.

On June 11, 2002, Microsoft updated Microsoft Advisory (MS02-022) and released a new patch that remedies the vulnerability for users that downloaded and accepted the control. The previous solution did not fully protect against this action and it was possible for an attacker to load the vulnerable control, even though the previous patch and updated versions had been installed.

The new patch is available at http://www.microsoft.com/Downloads/Release.asp?ReleaseID=39632. All users should apply this patch, even if you previously installed an updated version of your software. This patch supercedes the patch information below.

Microsoft has released a patch as an interim solution and upgrades to address this issue.

If you have updated your software prior to June 11, 2002, you should reinstall the software from the following locations:

Updated version of MSN Chat control

Updated version of MSN Messenger with the corrected control

Updated version of Exchange Instant Messenger with the corrected control

Microsoft also suggests that the email attack vector is blocked by the following Microsoft mail products: Outlook 98 and Outlook 2000 with the Outlook Email Security Update, Outlook 2002, and Outlook Express. All of these products will open HTML email in the Restricted Sites zone.

Other mitigation strategies include opening web pages and email messages in the Restricted Sites zone and using email clients that permit users to view messages in plain-text. Likewise, it is important for users to realize that a signed control only authenticates the origin of the control and does not imply any information with regard to the security of the control. Therefore, downloading and installing signed controls through an automated process is not a secure choice.