But, a source speaking to Reuters said the outage was not caused by an outside attack -- rather, glitchy update code.
The source said the company struggled to immediately roll-back the code, which would have potentially cut the outage time down considerably. After the roll-back failed, engineers were forced to shut down its groups of linked servers to get back to a safe point where the sites were fully operational.
Bing, which powers Yahoo Search, was down for about 20 minutes.
Microsoft said in a statement Friday that its sites suffered a "brief, isolated services outage," but did not elaborate further.
The company could not be reached for comment Saturday.