An integer overflow was addressed with improved input validation. This issue is fixed in Security Update 2021-005 Catalina, iOS 14.8 and iPadOS 14.8, macOS Big Sur 11.6, watchOS 7.6.2. Processing a maliciously crafted PDF may lead to arbitrary code execution. Apple is aware of a report that this issue may have been actively exploited.
http://seclists.org/fulldisclosure/2021/Sep/25
http://seclists.org/fulldisclosure/2021/Sep/26
http://seclists.org/fulldisclosure/2021/Sep/27
http://seclists.org/fulldisclosure/2021/Sep/28
http://seclists.org/fulldisclosure/2021/Sep/38
http://seclists.org/fulldisclosure/2021/Sep/39
http://seclists.org/fulldisclosure/2021/Sep/40
http://seclists.org/fulldisclosure/2021/Sep/50
https://security.gentoo.org/glsa/202209-21
https://support.apple.com/en-us/HT212804
https://support.apple.com/en-us/HT212805
https://support.apple.com/en-us/HT212806
https://support.apple.com/en-us/HT212807