It’s a fable for our times. Someone creates a tool that flags fake news on Facebook with big red warning signs. Someone else installs said tool, forgets about it, and then mistakenly assumes that the red warning signs are evidence of Facebook’s own efforts to tackle the problem. A reputable technology website then writes a story about Facebook’s new experiment without doing any fact checking, thus generating its own piece of fake news. The icing on the cake? The fake news detector fails to recognize it as fake news.

“I thought it was the most meta thing I’ve ever seen: a fake news article about a fake news detector,” said Daniel Sieradski, who created the BS Detector browser plug-in to alert users to unreliable news sources. He immediately contacted the TechCrunch authors to point out their mistake, and they updated the article.

Sieradski, a 37-year-old activist and independent journalist from Syracuse, created BS Detector as a reaction to Facebook’s failure to acknowledge any responsibility for the spread of misleading and false information on its platform.

The warning sign that appears on Facebook when users install BS Detector Photograph: Facebook

“The proliferation of misinformation has severely impacted people’s ability to make informed decisions when it comes to politics and other issues,” he said. He was irritated by Facebook CEO Mark Zuckerberg’s claims that fake news was a difficult problem to solve and so built BS Detector to prove that it’s possible.

“This is how anti-semitism surges in a public sphere instead of staying in the dark corners of the internet. These conspiracy theories move into mainstream discourse and become adopted as factual beliefs,” he added.

BS Detector, which Sieradski claims has been downloaded more than 25,000 times, works by cross-referencing news links with a database of questionable news sources. If there’s a match, it injects a red warning sign flagging that the website is not a reliable news source. There are different classifications for fake news, satire, extreme bias, conspiracy theory, junk science, state news or hate groups. Sieradski continuously updates the database and provides a way for websites to appeal the classification.

“Ultimately we could connect with media watchdog groups and work with them on an actual methodology to back our product,” he said.

Today, in a final twist in the tale, Facebook blocked people from linking to BS Detector citing security reasons. When the Guardian attempted to post a link to bsdetector.tech, a pop-up appeared stating “message failed”.

Facebook blocked BS Detector for security reasons Photograph: screengrab/Screengrab

“Obviously it’s not a security risk because there’s no security related behavior of the plug-in. It doesn’t affect the integrity of Facebook’s operations, it just inserts an HTML element into the page. It’s not doing anything mischievous. My suspicion is that they didn’t like the TechCrunch article.”

The Guardian contacted Facebook to find out why the link was blocked from the social network, but the company didn’t initially respond. Hours later the link could be posted once again – as predicted by Sieradski himself.

mark my words: within a few hours facebook will have unbanned https://t.co/Mf4ppZjZH7 claiming it was accidental or automated. — daniel sieradski (@selfagency) December 2, 2016

“This is how Facebook rolls. I expected nothing less than a quiet unblocking. It’s routine for them at this point. Drop a banhammer unjustly, get called out in the press, pretend it wasn’t on purpose,” he said.

A Facebook spokeswoman later issued this statement: “We maintain a set of systems to help us detect and block suspicious behavior on our site. We temporarily blocked people from sharing the domain bsdetector.tech because of other abuse we have seen from the .tech top-level domain. We have corrected the error.”