Experts are speculating that the FBI may be closely guarding a secret vulnerability in the Firefox browser that it can exploit for future law enforcement purposes, according to a Motherboard report yesterday.

The article refers to a network investigative technique that the FBI used to hack visitors of the Playpen child pornography website. That site runs on the encrypted Tor network, but an exploit that works on the Tor browser would also work just as effectively on Firefox, upon which Tor was built.

So far the U.S. Department of Justice has resisted a U.S. district court order to disclose the technique. In an interview with Motherboard, Christopher Soghoian, principal technologist at the American Civil Liberties Union (ACLU), said that by not disclosing the alleged vulnerability in such a popular global browser, “The government is essentially choosing to keep hundreds of millions of people vulnerable in case a few of them turn out to be criminals later.”