Security Update 2021-003 for macOS 10.15 is currently unavailable as a standalone DMG installer.
Normally manual packages are published 2-3 days after the SU releases.
Apparently Apple postponed the delivery of the standalone disk image after numerous reports about a bug breaking Kerberos functionality - a problem which also affects macOS 10.14 "Mojave".
There's probably a reason for which May 2021 security updates for Mojave and Catalina were postponed.
I think Apple pulled both the standalone DMGs and SU installers shortly after the Kerberos bug was reported.
Supposedly a fix is already in the works and I wouldn't be surprised if the bug affects both OSes.
Once the issue is fixed the Cupertino tech giant will release the manual packages (and SU installers) containing the revised code for Kerberos.
Apple now takes about a week to post the standalone updates.
Really poor service, and they don't do it at all for Big Sur and whatever comes next.
Catalina 2021-003 Security Update Not Available Yet in the Support Downloads page
