Bug-hunters have told how they uncovered a significant security flaw that affected the likes of Tinder, Yelp, Shopify, and Western Union – and potentially hundreds of millions of folks using these sites and apps.

The software sniffers said they first came across the exploitable programming blunder while digging into webpage code on dating websites. After discovering a Tinder.com subdomain – specifically, go.tinder.com – that had a cross-site scripting flaw, they got in touch with the hookup app’s makers to file a bug report.

As it turned out, the vulnerability they discovered went far beyond one subdomain on a site for lonely hearts. The team at VPNMentor said the since-patched security hole had left as many as 685 million netizens vulnerable to cross-site-scripting attacks, during which hackers attempt to steal data and hijack accounts. To pull off one of these scripting attacks, a victim would have to click on a malicious link or open a booby-trapped webpage while logged into a vulnerable service.

That staggering nine-figure number is because the security issue was actually within a toolkit, called branch.io, that tracks website and app users to figure out where they’ve come from, be it Facebook, email links, Twitter, etc. With the bug lurking in branch.io’s code and embedded in a ton of services and mobile applications, the number of people potentially at risk of being hacked via cross-site scripting soared past the half-a-billion mark, we’re told.

Source: Now this might be going out on a limb, but here’s how a branch.io bug left ‘685 million’ netizens open to website hacks • The Register