Analysis by AAP FactCheck has found ads for local travel, computer and technology firms in the replies to controversial posts on X, formerly known as Twitter.
The ads are placed by the platform's algorithms which directly target users without oversight from advertisers.
Advertisers could even be inadvertently funding the spreaders of misinformation, with X now rewarding content creators who attract platform users to their posts.
There is no suggestion any companies are aware their ads have been positioned near anti-Semitic content or misinformation.
One of the firms affected was Wallabies Travel, the official travel partner of the Wallabies and Rugby Australia.
An ad promoting hospitality packages for the 2025 British Lions tour appeared below a post that baselessly claimed the Hamas' attack on Israel on October 7 was a 'false flag' operation orchestrated by Israelis.
A Wallabies Travel ad appeared beneath an X post making baseless claims about the October 7 attacks. (William Summers)
A 'false flag' is a military action carried out with the intention of blaming it on an opponent. The term is often used by conspiracy theorists.
The post has racked up more than one million views.
Another Wallabies Travel ad was positioned beneath a viral post falsely claiming that COVID-19 and 9/11 were "Jewish psyop" (psychological operations).
Wallabies Travel declined to comment.
However, after being contacted by AAP, the company's ads no longer appeared beneath the posts in question.
Ads for the local arm of global computer giant Lenovo, Perth-based technology retailer PLE and several other Australian companies also appeared below anti-Semitic tweets or extreme misinformation.
Lenovo ANZ and PLE didn't respond when contacted for comment.
Ads for computer maker Lenovo's local arm have also been seen under controversial posts. (William Summers)
Advertisers may also be inadvertently funding some spreaders of misinformation and disinformation.
X's revenue-sharing initiative rewards eligible content creators who attract large audiences for advertisers with a share of ad revenue.
The platform says it's part of an effort to "help people earn a living directly on X."
An investigation by US media outfit NewsGuard found ads for dozens of global brands attached to anti-Semitic and misinformation content on X.
Dr Marian-Andrei Rizoiu, a data scientist at the University of Technology Sydney who studies how information spreads online, said social media algorithms aren't good at differentiating between misinformation and complex opinions.
"But also there is no monetary incentive to do so," Dr Rizoiu told AAP.
"Attraction brings attention, and attention is money. So any sort of curation is against their monetary interests."
Disney, Apple, IBM, and Warner Brothers in November pulled their ads from X after owner Elon Musk said he agreed with an anti-Semitic conspiracy theory posted by another user.
X did not respond to AAP's request for comment.
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