/usr/share/initramfs-tools/conf-hooks.d/dropbear is in dropbear-initramfs 2017.75-3build1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 | # Override variables from /etc/initramfs-tools/initramfs.conf, see
# initramfs-tools(8)
#
# Set the umask value of the generated initramfs file to avoid
# disclosing SSH host keys.
UMASK=0077
# Force use of busybox instead of klibc utilities
BUSYBOX=y
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