Mozilla is adding new capabilities in the beta version of Firefox 36 to give users and website owners more control over what data, if any, is sent to other websites during browsing.
When a user navigates to a new website by clicking on a link or pulls an image or video file from another site, the browser typically sends the new site the address of the page that referred it. It's useful for webmasters to see where someone has come from but a privacy worry since other sensitive data can be revealed.
"As the Web got more complex, the amount of information in the referrer header ballooned, leading to bigger privacy problems," wrote Sid Stamm, principal security and privacy engineer at Mozilla.
Just earlier this week the U.S. government health insurance website HealthCare.gov was found to be sending personal data such as zip codes, income levels and whether users smoke or are pregnant to external sites via referral data sent to trackers in the website.
To help users keep control of such data, Mozilla has been working on changes to Firefox's Gecko rendering engine to make it easier for users or browser extensions to control referrer data.
And it has created a feature called "meta referrer" in the Firefox 36 beta that allows webmasters to include a tag in HTML documents specifying a referrer policy and what data can be sent.
For example, a policy can be set that strips the referrer header of a path, query string or fragment, Stamm wrote. Other policies can block all referrer information, he wrote.
The Electronic Frontier Foundation called the HealthCare.gov incident a major privacy concern since companies could use it to create profiles for targeted advertising.
But it's unclear if the referrer problem with HealthCare.gov will be fixed. Aaron Albright, director of the Media Relations Group for the Centers for Medicare and Medicaid Services, wrote via email there is no evidence that third-party companies have misused the information. It was unclear if the issue would be fixed.
"We will remain vigilant and will continue to focus on what more we can do to keep consumers' personal information secure," Albright wrote.
In 2010, Facebook made technical changes after was found sending a person's user ID in a referrer when a person clicked on an advertisement, potentially allowing a company to identify a specific user. Facebook labeled the issue an "unintentional oversight."
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