A Browser Helper Object (BHO) is a DLL module designed as a plugin for Microsoft's Internet Explorer web browser to provide added functionality. BHOs were introduced in October 1997 with the release of version 4 of Internet Explorer. Most BHOs are loaded once by each new instance of Internet Explorer. However, in the case of the Windows File Explorer, a new instance is launched for each window.

Some modules enable the display of different file formats not ordinarily interpretable by the browser. The Adobe Acrobat plugin that allows Internet Explorer users to read PDF files within their browser is a BHO.

Other modules add toolbars to Internet Explorer, such as the Alexa Toolbar that provides a list of web sites related to the one you are currently browsing, or the Google Toolbar that adds a toolbar with a Google search box to the browser user interface.

However, the BHO API exposes hooks that allow the BHO to access the Document Object Model (DOM) of the current page and to control navigation. Because BHOs have unrestricted access to the Internet Explorer event model, some forms of malware have also been created as BHOs. For example, the Download.ject exploit installed a BHO that would activate upon detecting a secure HTTP connection to a financial institution, record the user's keystrokes (intending to capture passwords) and transmit the information to a website used by Russian computer criminals. Other BHOs such as the MyWay Searchbar track users browsing patterns and pass the information they record to third-parties.

In response to the problems associated with BHOs and similar extensions to Internet Explorer, Microsoft added an Add-on Manager to Internet Explorer with the release of Service Pack 2 for Windows XP. This displays a list of all installed BHOs, Browser Extensions and ActiveX controls, and allows the user to enable or disable them at will.

For users that are not using Windows XP, there exist free tools (such as BHODemon) that list installed BHOs and allow the user to disable malicious extensions.

External links

Microsoft sites

• Browser Helper Objects: The Browser the Way You Want It - a January 1999 MSDN article by Dino Esposito
• IEHelper-Attaching to Internet Explorer 4.0 by Using a Browser Helper Object
• Control Internet Explorer Add-ons with Add-on Manager - an article on microsoft.com that explains this new feature of Windows XP Service Pack 2

Listings and examples

• CLSID List - a list created by Tony Klein and others, that attempts to record and indentify every BHO available
• Example of code for a BHO
• PestPatrol - has listings and classifications of BHOs

Removal tools

• BHODemon - a free tool for listing and disabling malicious BHOs
• HijackThis
• A2HijackFree