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A Browser Helper Object (BHO) is a DLL module designed by Microsoft to allow developers to create plug-ins for its Internet Explorer browser. BHOs can display content that was not originally written in HTML, such as a PDF file, or it can allow developers to add toolbars to the browser. BHOs are supported in Internet Explorer version 4 and up.
The Browser Helper Object was created by Microsoft in 1997 to make it easy for developers to create plug-ins for Internet Explorer and Windows Explorer. They are implemented as DLL modules.
BHOs first appeared in Internet Explorer 4.0 and are still supported, even though plug-ins like Flash are falling out of use and Microsoft itself has discontinued development of Internet Explorer in favor of Edge. Some major plug-ins implemented as BHOs include Adobe Reader and the Alexa toolbar.
As with any browser plug-in mechanism, there are always security risks. Some versions of malware have been implemented as BHOs. Other BHO-based plug-ins can track browsing habits and keystrokes.