That is where Mr. Krebs comes in. Unlike physical crime — a bank robbery, for example, quickly becomes public — online thefts are hushed up by companies that worry the disclosure will inflict more damage than the theft, allowing hackers to raid multiple companies before consumers hear about it.

“There’s a lot going on in this industry that impedes the flow of information,” Mr. Krebs said. “And there’s a lot of money to be made in having intelligence and information about what’s going on in the underworld. It’s big business but most people don’t want to pay for it, which explains why they come to someone like me.”

Mr. Krebs is “doing the security industry an enormous favor by disseminating real-time threat information,” said Barmak Meftah, chief executive of AlienVault, a threat-detection service. “We are only as strong as our information. Unless we are very specific and effective about exchanging threat data when one of us gets breached, we will always be a step behind the attackers.”

The tally of victims from the breaches at Target, Neiman Marcus and others now exceeds one-third of the United States population — a grim factoid that may offer Mr. Krebs a strange sense of career vindication.

He first developed an interest in computers because his father, an Air Force engineer, was obsessed with the latest devices. But he did little about it until 1998, when he began writing about technology for The Post, after working his way up from the mailroom. Cybersecurity became a bit of a focus after his own computer was infected by that worm in 2001. “I learned there’s this whole underworld that seemed really fascinating,” he said.

In 2005, he started The Post’s Security Fix blog, occasionally frustrating editors with hacker jargon and unnerving some who worried he was becoming too close to sources.

“A lot of what Brian does would scare the hell out of traditional newsroom editors,” said Russ Walker, Mr. Krebs’s former editor at The Post. “I don’t think he crossed the lines journalistically, but he was living a different type of experience.”