Concerns
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 malware installs 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.
Many BHOs introduce visible changes to a browser's interface, such as installing toolbars in Internet Explorer and the like, but others run without any change to the interface. This renders it easy for malicious coders to conceal the actions of their browser add-on, especially since, after being installed, the BHO seldom requires permission before performing further actions. For instance, variants of the ClSpring trojan use BHOs to install scripts to provide a number of instructions to be performed such as adding and deleting registry values and downloading additional executable files, all completely transparently to the user. The DyFuCA spyware even replaces Internet Explorer's general error page with an ad page.
In response to the problems associated with BHOs and similar extensions to Internet Explorer, Microsoft debuted an Add-on Manager in Internet Explorer 6 with the release of Service Pack 2 for Windows XP (updating it to IE6 Security Version 1, a.k.a. SP2). This utility displays a list of all installed BHOs, browser extensions and ActiveX controls, and allows the user to enable or disable them at will. There are also free tools (such as BHODemon) that list installed BHOs and allow the user to disable malicious extensions. Spybot S&D advanced mode has a similar tool built in to allow the user to disable installed BHOs.
In IE9 Beta, BHOs and toolbars are not loaded when a link pinned to the taskbar is accessed.
Read more about this topic: Browser Helper Object
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