The latest revelation of how government spies tap into the personal data that U.S. consumers so blithely place into the control of the Internet’s advertising giants is the most profound yet.

The Washington Post today outed a National Security Agency data snooping program, code-named MUSCULAR, that copies all traffic flowing between two of the largest online advertising giants: Google and Yahoo.

In the latest installment of revelations from Edward Snowden, the Post is reporting that NSA partnered with its British counterpart, GCHQ, to carry out MUSCULAR.

“This is the first real evidence of deep intrusions by NSA and GCHQ into the internal networks of major Internet companies,” says Dave Jevans, chief technology officer of mobile security firm Marble Security. “By essentially copying all traffic that flows through these networks, the intelligence agencies can see everything that happens at these companies.”