Yahoo! Inc. (NASDAQ:YHOO) Tech recently stumbled upon Sell Hack, an extension for Safari, Firefox, and Google Inc (NASDAQ:GOOG)’s Chrome browsers. Once the extension is installed, users simply need to visit the LinkedIn Corp (NYSE:LNKD) site and find a user whose email you would like. A little button will appear labeled “Hack In,” once clicked you will have access to their email address in mere seconds.

LinkedIn Corporation

At least for now, you needn’t worry about your email being revealed to marketers and others. The extension has seen so many people install it in the last couple of days that it has overloaded Sell Hack’s servers. That will likely be fixed, and then yes, your email address is there for anyone to see.

In addition to finding someone’s email address, you will be working for Sell Hack. The extension “follows” you while mining data from the pages you visit. Presumably, the emails you find will be packaged by Sell Hack and sold to marketers and spammers.

Isn’t it illegal?

Nefarious? Perhaps. Illegal? No. Sell Hack is not stealing private data but rather using public data to cross-reference the most likely email address, and it is surprisingly accurate.

According to Sell Hack:

The data we process is all publicly available. We just do the heavy lifting and complicated computing to save you time. We aren’t doing anything malicious to the LinkedIn website. We think browser extensions are the best way to personalize an individuals web experience. We love LinkedIn and are trying to make it better for the community.

LinkedIn likely not pleased with development

Suffice is to say, LinkedIn Corp (NYSE:LNKD) is not thrilled with the existence of the extension and has got its legal team on the case. While its being reported that a cease and desist order has been issued to Sell Hack, it may fall on deaf ears or blind eyes. Right now, it looks like Sell Hack has found a “loophole” and is doing nothing illegal. That being the case, it’s difficult to see them shutting it down.

UPDATE: (April 3rd, 3:00 ET): Complying with LinkedIn’s cease and desist notice, Sell Hack has been taken down as of this writing.