Amazon sellers are using hacked accounts to award themselves five-star ratings, with one hijacked user posting as many as 2,500 bogus reviews, a consumer watchdog has found, urging regulators to act.

Legitimate accounts are being hijacked to post fake reviews – sometimes thousands of them, UK consumer watchdog Which? found in an investigation into tricks sellers have been using to evade Amazon’s onsite review police and fraud detection algorithms. Some of the hacked users were told to delete the reviews themselves when they reported the problem to Amazon. Which? is demanding the e-commerce giant deal with the problem – and calling on regulators to make sure it does.

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“If they fail to put more rigorous systems in place, then the [Consumer Markets Authority] must intervene to ensure that fake reviews and other misleading tactics can be stamped out,” Natalie Hitchins, Which? head of home products and services, told the Telegraph.

Some sellers simply bribe users with cash and gift cards to post positive reviews – or to change negative ones. Others are exploiting Amazon’s marketing tools, creating fake “product variations” – like 40 colors of headphones – to multiply the number of positive reviews a user can give a product, or merging an old product’s positive reviews with an unrelated new product. These features are being “abused on a grand scale,” Which? found.

Facebook continues to host groups with tens of thousands of members set up to generate real-looking “verified purchase” reviews despite being warned by the CMA, and new groups spring up as quickly as the old ones are reported to the site and shut down, the investigation found, noting that five star reviews were being “sold” on eBay as well.

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