A memory corruption issue was addressed with improved validation. This issue is fixed in iOS 14.4.1 and iPadOS 14.4.1, Safari 14.0.3 (v. 14610.4.3.1.7 and 15610.4.3.1.7), watchOS 7.3.2, macOS Big Sur 11.2.3. Processing maliciously crafted web content may lead to arbitrary code execution.
https://www.debian.org/security/2021/dsa-4923
https://support.apple.com/kb/HT212323
https://support.apple.com/en-us/HT212223
https://support.apple.com/en-us/HT212222
https://support.apple.com/en-us/HT212221