Just three weeks after after Yahoo filed a patent infringement lawsuit against Facebook, the social giant struck back on Tuesday with a counterclaim alleging Yahoo is infringing on 10 of Facebook’s own software patents.

“From the outset, we said we would defend ourselves vigorously against Yahoo’s lawsuit, and today we filed our answer as well as counterclaims against Yahoo for infringing ten of Facebook’s patents,” said Ted Ullyot, general counsel of Facebook, in a statement. “While we are asserting patent claims of our own, we do so in response to Yahoo’s short-sighted decision to attack one of its partners and prioritize litigation over innovation.”

Facebook’s counterclaim (.pdf) is similar to Yahoo’s original complaint. It alleges that Yahoo infringed on methods of providing basic web functionality such as search, headline feeds, photo tagging, and, of course, advertising.

The software patent skirmish comes at a sensitive time for both companies, though in opposite ways. Yahoo filed its lawsuit just six weeks after Facebook announced its planned IPO.  Facebook filed its counterclaim Tuesday, the day before Yahoo is expected to enact massive job cuts. AllThingsD was first to report the lawsuit.