So did the Russians hijack Yahoo or did the CIA do it and blame it on the Russians?
This tidbit below is buried in the Muscular writeup by Wikipedia.
Operational details:
According to the leaked document the NSA’s acquisitions directorate sends millions of records every day from internal Yahoo! and Google networks to data warehouses at the agency’s headquarters at Fort Meade, Maryland. The programme operates via an access point known as DS-200B, which is outside the United States, and it relies on an unnamed telecommunications operator to provide secret access for the NSA and the GCHQ.[3]
According to a post-it style note from the presentation, the exploitation relied on the fact that (at the time at least) data was transmitted unencrypted inside Google's private cloud, with "Google Front End Servers" stripping and respectively adding back SSL from/to external connections.
According to the Washington Post: "Two engineers with close ties to Google exploded in profanity when they saw the drawing." After the information about MUSCULAR was published by the press, Google announced that it was working on deploying encrypted communication between its datacenters.[2]
https://en.wikipedia.org/wiki/MUSCULAR_(surveillance_program)