SAN FRANCISCO — Yahoo has become the worst-case example of an unnerving but increasingly common phenomenon — massive hacks that steal secrets and other potentially revealing information from our personal digital accounts, or from big organizations that hold sensitive data on our behalf.

On Wednesday, Yahoo disclosed a gargantuan breach affecting more than a billion user accounts, the largest such attack in history. The company said that attack happened in August 2013, although Yahoo only discovered it recently. Worse, the company’s announcement followed a similar announcement last September of a 2014 hack — one Yahoo ascribed to an unnamed foreign government — that affected 500 million accounts.

Neither Yahoo breach has yet been linked to online fraud or any specific repercussions for Yahoo users. But their disclosure closely follows U.S. intelligence concerns about Russian hacking of Democratic emails during the presidential campaign – not to mention other recent attacks on a major health insurer, a medical lab-test company and the government office that manages millions of federal employees.

“The lesson is clear:  no organization is immune to compromise,” said Jeff Hill, director of product management for cybersecurity consultant Prevalent. “And since most of us are dependent on big organizations that hold our digital lives in their hands, in a broad sense that effectively means no one is safe.”