Oracle executive chairman Larry Ellison is going after Amazon and its Cloud Computing arm, once again.

During the database giant’s annual user conference in San Francisco Monday, the Oracle co-founder spent much of his keynote criticizing Amazon over perceived failings. It marks another instance of Ellison using his own company’s technology conference as a vehicle to slam rivals in a public setting. Some other companies Ellison has publicly called out during previous conferences over the past 4 years include Salesforce, SAP, and Microsoft.

At the event Monday, Ellison railed against the security measures of Amazon, as well as taking a few shots at other companies like Google and Facebook that have been criticized over recent data blunders.

Ellison’s criticism over Amazon Web Services and its security has to do with the nature of cloud computing, in which customers rent access to computing infrastructure in a pay-as-you-go model.

Cloud computing became popular in part due to the rise of virtualization technology, which allows companies like AWS and Microsoft to more efficiently partition their client’s corporate data across many servers, with some machines storing the data of multiple companies. One benefit of virtualization technology is that each company’s data remains separate from other company’s data while technically residing on the same computer. This has the effect of squeezing more performance out of each machine.

A flaw within AWS, Ellison contends, is that Amazon runs its “AWS cloud control code” on the same machines as it stores corporate data. Ellison then outlined an apocalyptic scenario in which bad actors could “change the Amazon code and hack the system,” thus gaining access to other company’s corporate data.

This problem with Amazon’s approach, Ellison said, is “a fundamental problem with the architecture of the cloud.” Oracle, he said, “will never put our cloud control code in the same computer that has customer code.”

Still, it should be noted that a doomsday scenario like the one Ellison described has yet to occur. Additionally, every cloud computing company is increasingly touting their own cyber security technologies as superior to their competitors in order to capitalize over the public dismay of recent high-profile hacks and data breaches.

Despite Ellison’s comments toward Amazon over the past few years, the online retail giant remains the leader in cloud computing, according to several technology analysts.

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Research firm Gartner said in August that Amazon held about 51% of the overall cloud computing market in 2017, followed by Microsoft with 13.3%, Alibaba with 4.6%, Google with 3.3% and IBM with 1.9%. Oracle was not mentioned in that Gartner report.

Fortune contacted Amazon for comment and will update this story if it responds.