The White House released their International Strategy for Cyberspace document, which lays out a general platform for all things internet, but it also addresses their stance on enemies hacking the U.S. and its allies: attack if necessary.
From the document:
"States have an inherent right to self-defense that may be triggered by certain aggressive acts in cyberspace," says the policy. Indeed, such aggressive acts might compel a country like the US to act even when the hacking is targeted at an allied country."
According to Ars Technica, the document says that an attack would be a last resort if diplomatic channels fail, but that the U.S. will not stand idle should that happen. And the White House also emphasizes the desire to catch cybersecurity threats early via an implemented warning system, so that corporations and agencies are at less risk.