Following Yahoo's lawsuit against Facebook last month, Facebook's gone on the counter-attack.
Last month, Yahoo filed a suit alleging that Facebook was violating ten patents covering messaging, news feed generation, social commenting, advertising display, preventing click fraud and privacy controls.
But Facebook's filed back - very quickly - with a counter-claim, in which it accuses Yahoo of infringing ten of its own patents in the areas of advertising, online recommendations and photo-tagging.
"Yahoo! infringes the Facebook patents-in-suit through, by way of example and notlimitation, the Yahoo! Home Page, Yahoo!’s Content Optimization and Relevance Engine(“C.O.R.E.”), the Yahoo! Flickr photo sharing service, and advertisements displayed throughoutYahoo! including on My Yahoo!, Yahoo! Finance, Yahoo! Sports, Yahoo! News, Yahoo! Games,Yahoo! Movies, Yahoo! Shopping, Yahoo!Travel, Yahoo! Autos, and Flickr," reads the complaint.
Five of the patents relate to online advertising, which Facebook says accounts for 80 percent of Yahoo's business.
Yahoo owns around 1,000 patents, while Facebook has about 800 - and most of those are recent acquisitions, following a deal with IBM last month.
Yahoo rejects Facebook's claims, telling Reuters that they're 'without merit and nothing more than a cynical attempt to distract from the weakness of its defence'.
Yahoo's been widely criticized for using its patents as a weapon, in a move seen as especially cynical given Facebook's approaching IPO.
""If Yahoo thought Facebook was a soft target due to its IPO, today's 10 patent infringement counterclaims show it barked up the wrong tree," says patent expert Florian Mueller.