macOS High Sierra 10.13.1 insecure cron system

["Mark Wadham" <fd () rkw io>]

Recently I was working on an security issue in some other software that 
has yet
to be disclosed which created a rather interesting condition. As a 
non-root
user I was able to write to any file on the system that was not 
SIP-protected
but the resulting file would not be root-owned, even if it previously 
was.

This presented an interesting challenge for privilege escalation - how 
would you
exploit this to obtain root access? The obvious first attempt was the 
sudoers
file but sudo is smart enough not to process it if the file isn't 
root-owned so
that didn't work.

I then discovered (after a tip from a friend - thanks pndc!) that the 
cron
system in macOS does not care who the crontab files are owned by. 
Getting root
was a simple case of creating a crontab file at:

/var/at/tabs/root

with a 60-second cron line, eg:

* * * * * chown root:wheel /tmp/payload && chmod 4755 /tmp/payload

and then waiting for it to execute. It's not clear if this is a 
macOS-specific
issue or a hangover from the BSD-inherited cron system, I suspect the 
latter.

The issue has been reported to Apple so hopefully they will fix it.

Ref: 
https://m4.rkw.io/blog/macos-high-sierra-10131-insecure-cron-system.html

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