Twitter now requires users connecting via the online anonymity service Tor to provide their phone number in order to log on. The change, which renders Tor's anonymity useless, is meant to prevent serial trolls, but it could also affect the ability of ISIS and other terrorist organizations to use Twitter to broadcast propaganda.

Twitter unveiled new security measures last week that make it easier for users to report abuse and easier for the company to identify which users are frequently called out for trolling, or harassing others. Now, if a user is suspended, he must provide the company with his phone number to log back on. But users connecting with Tor – the software that cloaks a user's Internet connection, obscuring their IP address and physical location – need to provide their phone number immediately, even before they're accused of any wrongdoing.

The change, first reported by TechCrunch, means that human rights activists, government critics and security researchers could also lose their ability to use Twitter safely. Use of Tor also skyrockets among law-abiding citizens when restrictive governments are put into place, but the new requirement could be the death knell for ISIS' recruiting and propaganda strategy.

Twitter has been engaged in a constant game of whack-a-mole against the terrorist group, which started giving its followers instructions on how to use Tor last year, according to terrorist monitoring groups observing ISIS' social media strategy. One Twitter user, according to the Cyber and Jihad Lab, called for a “fierce campaign” against users providing information about what's “truly” happening in Iraq and Syria, specifically citing the “repetitive closure” of Twitter accounts.

More than 3,000 Westerners have joined ISIS since the beginning of the group's terror campaign, according to Director of National Intelligence James Clapper.