Stamos had been the chief technology officer of the security firm Artemis. He succeeds Yahoo's former CISO Justin Somaini, taking a seat that had been vacant for more than a year.
Stamos was also a key organizer of TrustyCon, a security industry conference held last month that was billed as counter programming to the RSA conference because of RSA's supposed involvement with the NSA.
He will report to Jay Rossiter, Yahoo's senior vice president of platforms and personalization products, and not CEO Marissa Mayer, as was previously rumored. He'll lead Yahoo's security team, internally referred to as "Paranoids." "This is a broad role which includes implementing top-to-bottom security for our products and systems but also to lead the company and the industry in not just how security works today but how it needs to work in the future," Rossiter wrote in a blog post.
The appointment comes at a critical time for Yahoo. Last month, The Guardian reported that the GCHQ had captured millions of images from Yahoo users' Webcam chats, many of them sexually explicit. The company, along with Google, had also reportedly been targeted by the NSA and GCHQ, which infiltrated data stores belonging to the companies by zeroing in on information in transit between data centers. In January, the company made secure searching the default option for its home page, meaning that Yahoo's searches would automatically go through a secure server.