As TechCrunch notes, Jayaram isn’t the first to report this issue. WaBetaInfo pointed it out in February. While the issue seems to be fixed, it’s a pretty big security flaw and apparently it’s been a problem for at least several months.
According to Facebook, it was already working to fix the issue and the phone numbers found by Jayaram were old results cached by the search engine. Those should be removed as the site continues to re-index websites and finds the no-index tag.
As you may remember, in 2018, Facebook stopped letting people search for users by their phone numbers. WhatsApp has also taken steps to improve chat security by letting users block group chat invites from strangers or select contacts.
Update 6/9/2020 3:25PM ET: This story was updated to note that Facebook was already working to fix this issue before it was revealed by Athul Jayaram.