In a blog post on Tuesday, Anton Lazarev, research engineer at Brave Software, and senior privacy researcher Peter Snyder, explain that online tracking scripts may use canonical name DNS records, known as CNAMEs, to make associated third-party tracking domains look like they’re part of the first-party websites actually being visited. They point to the site https://mathon.fr/ as an example, noting that without CNAME uncloaking, Brave blocks six requests for tracking scripts served by ad companies like Google, Facebook, Criteo, Sirdan, and Trustpilot. But the page also makes four requests via a script hosted at a randomized path under the first-party subdomain 16ao.mathon.fr. When Brave 1.17 ships next month (currently available as a developer build), it will be able to uncloak the CNAME deception and block the Eulerian script.
Other browser vendors are planning related defenses. “Mozilla has been working on a fix in Firefox since last November,” notes The Register. “And in August, Apple’s Safari WebKit team proposed a way to prevent CNAME cloaking from being used to bypass the seven-day cookie lifetime imposed by WebKit’s Intelligent Tracking Protection system.”