Search for “best headphones” on YouTube, and a video by Marques Brownlee, a popular tech YouTuber with nearly 9 million subscribers, will likely be one of the first to appear. Published last November, his video “My Favorite Wireless Headphones | 2018!” has over 2.7 million views. In it, Brownlee describes his favorite pairs of Bluetooth headphones and implores users to “hop on this wireless train before it leaves the station.” Below the video, he includes links to all of the products mentioned.

Paris Martineau covers platforms, online influence, and social media manipulation for WIRED.

What he fails to mention in either the description or video itself is that purchases made from those links may also trigger small payments to Brownlee himself. Princeton researchers say the links include referral codes that typically denote such payments.

Federal Trade Commission guidelines for social media endorsements require that influencers prominently disclose if they receive anything—cash, gifts, or something else—that could affect how users view their mention of a company or product. However, few do. Last year, an analysis of over 500,000 YouTube videos and more than 2.1 million Pinterest pins conducted by the Princeton researchers found that influencers rarely disclose their connections to such affiliate marketing links.

Even users savvy about influencer marketing can find it hard to identify affiliate marketing links. A new browser extension released by some of the same Princeton researchers makes them more obvious.

The extension, dubbed AdIntuition, displays a hot pink banner warning users that “This video contains affiliate links. If you click on highlighted links, the creator receives a commission.” The extension was released this week for the Chrome and Firefox browsers. The researchers say they’re interested in applying it to other browsers and platforms, like email lists or blogs, as well. Getting it inside apps—even YouTube’s app—poses additional challenges, researcher Arunesh Mathur said.

To the untrained eye, the links below Brownlee’s video seem ordinary. They begin with “amazon.to” followed by a backslash and some random characters, suggesting they’re truncated paths to one of Amazon’s many listings. But once clicked, they redirect the user to an Amazon product URL complete with an identifying tag that researchers say is used by online retailers to denote which affiliate marketing partner spurred the user’s purchase.

A screenshot of the AdInuition extension in action on one of the top YouTube search results for “best headphones.” Paris Martineau via YouTube

Brownlee did not respond to a request for comment. But he is far from the only YouTuber to fail to disclose a brand partnership. In last year’s study, the Princeton researchers found that only 10 percent of the YouTube videos they reviewed that featured affiliate links contained any written disclosure indicating compensation for an endorsement. Only a small number of those complied with FTC guidelines. The researchers said they have not updated the study since then. YouTube did not respond to a request for comment. (WIRED includes affiliate links in some articles.)

One of the researchers, Marshini Chetty, the director of the Princeton Human Computer Interaction Laboratory, said the team wanted to find a way to notify users when they watch videos that contain affiliate marketing. “The browser itself should be able to alert you to deceptive or misleading content such as [whether] the content that you're seeing is perhaps an ad,” she said. Chetty, Mathur, and Michael Swart created AdIntuition to do that.

The extension scans the description of YouTube videos a user visits to determine whether it likely contains signs of affiliate marketing partnerships. It checks all of the links (and their redirects) against a list of known affiliate marketing URLs and identifiers, and scans the text for words, phrases, or symbols commonly used to direct users to an influencer’s personalized coupon code, which also typically generates a commision when used to make a purchase. For example, if I wrote “Check out all the cool stories about influencer marketing at Wired.com and use WIRED1 for 1 percent off a subscription!” in the description of a YouTube video, the extension would flag the sentence as a possible influencer marketing because of the phrase “check out,” according to Swart, the primary builder of the extension.