2021: notice malicious PHP files written in EVERY apache-writable directory in the filesystem.
Not very careful analysis:
Decode the php with https://www.unphp.net/
2021-02-28: Notice that these files all have different creation dates, from 2020-02 to 2021-02-13: the malware is able to set the dates
Delete all the index.phps except for the five that are supposed to be on the system. Find them with
find / -name index.php -print > /tmp/phps
find / -wholename /sys/kernel/slab -prune -o -regex ".*/\..*ico" -ls
2021-05-23: write ~daniel/scout/scout.pl to alarm on unregistered index.php files.