Facebook is warning some 800,000 users that a bug caused the system to unblock some people that they had previously blocked.

It meant that the supposedly-blocked users could view the profiles of people who had blocked them, and send them messages — which isn't supposed to happen after you block them.

The block tool is sometimes used by people to protect themselves from abusive or harassing behaviour.



Facebook is warning some 800,000 users that a bug caused the system to temporarily unblock some people that they had previously blocked.

Facebook disclosed the bug in a blog post published on Monday, and said it meant blocked users could again see the profiles of people who had blocked them or send them messages — both things that a blocked user shouldn't want to do. It affected both the main Facebook platform and its messaging app, Messenger.

The bug was live between May 29 and June 5, Facebook said, and has since been fixed. Users who were affected will receive a message from the company warning them about it.

Of the 800,000-plus affected users, 83% (about 664,000 people) had one person they had blocked unblocked by the bug, while the remaining 17% (about $136,000 people) had more than one unblocked. Blocked users couldn't see posts shared by the person who blocked them that were only visible to their friends — but "someone who was unblocked might have been able to contact people on Messenger who had blocked them," the company says.

A Facebook spokesperson declined to comment on how many of these temporarily-unblocked users took advantage of the bug to contact the person who had originally blocked them, or to view profile information that they shouldn't have been able to access.

This is a significant error from Facebook. Victims of harassment or of abusive ex-partners will sometimes use the block feature to prevent aggressors from contacting them or viewing their online activities. This bug could have given these bad actors an avenue back into their victims' online lives that shouldn't have been open to them.

The news is yet another misstep from Facebook as it attempts to recover from a string of bruising scandals. The company has come under intense criticism over its role in the spread of Russian propaganda and misinformation. And more recently it was revealed that political research firm Cambridge Analytica misappropriated the personal data of tens of millions of users.