But doxxing has emerged from subculture websites like 4Chan and Reddit to become something of a mainstream phenomenon since a white supremacist march on Charlottesville, Va., earlier this month.

“Originally it was little black-hat hacker crews who were at war with each other — they would take docs, like documents, from a competing group and then claim they had ‘dox’ on them,” said Gabriella Coleman, a professor at McGill University who wrote a book about the hacker vigilante group Anonymous. “There was this idea that you were veiled and then uncovered.”

Now the online hunt to reveal extremists has raised concerns about unintended consequences, or even collateral damage. A few individuals have been misidentified in recent weeks, including a professor from Arkansas who was wrongly accused of participating in the neo-Nazi march. And some worry that the stigma of being outed as a political extremist can only reinforce that behavior in people who could still be talked out of it.

Doxxing was on the minds of a number of protesters on the streets of San Francisco on Saturday. In the Castro and Mission neighborhoods and Alamo Square, the home of the famous row of houses known as the Painted Ladies, thousands participated in counter-demonstrations to the right-wing rally. There was the energy of a street party — children and dogs joined in, protesters shared baked goods, and the bars nearby were full.

Marla Wilson, 35, of San Francisco, said she was appalled when she saw white supremacists marching so brazenly in Charlottesville. Doxxing, she believed, was an effective way to make people think twice about being so bold with their racism.