This issue was addressed with improved iframe sandbox enforcement. This issue is fixed in macOS Big Sur 11.2, Security Update 2021-001 Catalina, Security Update 2021-001 Mojave, watchOS 7.3, tvOS 14.4, iOS 14.4 and iPadOS 14.4. Maliciously crafted web content may violate iframe sandboxing policy.
https://support.apple.com/en-us/HT212149
https://support.apple.com/en-us/HT212148
https://support.apple.com/en-us/HT212147
https://support.apple.com/en-us/HT212146