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What of cron?

What of cron?

Posted Jul 13, 2006 18:08 UTC (Thu) by hppnq (guest, #14462)
In reply to: What of cron? by cventers
Parent article: Denial of reality vulnerabilities

Cron gets the username from the filename, not the actual ownership of the file. (At least, on a default Dapper, this happens for files in /var/spool/cron/crontabs, where they end up if edited through crontab -e, for instance. Newer incantations seem to expect a username on the cronjob line.)

If the user "core" does not exist, a crontab -- at least, in /var/spool/cron/crontabs, haven't investigated /etc/cron.d and friends -- called "core" will be ignored by crond and an error message indicating the failure will be logged; otherwise, its jobs, if any, are run as the user core.

So if a user without root privileges can cause core files to be called "root", you're in trouble. On my default Dapper, this cannot be easily done -- but YMMV. ;-)

Oh, and yes, my Dapper also checks whether the file owner is actually the user indicated by the crontab filename. Phew. ;-)

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What of cron?

Posted Jul 13, 2006 18:11 UTC (Thu) by corbet (editor, #1) [Link]

/etc/cron.d is a very different place, it has nothing to do with per-user crontabs at all.

What of cron?

Posted Jul 13, 2006 18:26 UTC (Thu) by hppnq (guest, #14462) [Link]

Yes, that's what I meant. I just didn't investigate whether cron works as designed in that case. ;-)

(By the way, I did not mean to make the problem look any less serious than it is, though. Patch!)

What of cron?

Posted Jul 13, 2006 19:31 UTC (Thu) by hppnq (guest, #14462) [Link]

I just didn't investigate whether cron works as designed in that case.

Yup, it does. So also in the /etc/cron.d case, a cracker would at least need to be able to manipulate the core dump's filename as well. Which requires root privileges on my system.

Again, this bug is trivially exploitable. But not by just dumping core in /etc/cron.d.

What of cron?

Posted Jul 14, 2006 5:23 UTC (Fri) by hppnq (guest, #14462) [Link]

[Nice, my own thread.]

Well, investigating a bit more turns up that indeed, dumping core in /etc/cron.d is sufficient: cron really doesn't care at all what files are called in /etc/cron.d. OMG. OMG. OMG. Jon, you were right as always.

(But really, cron's security model is *unbelievably* stupid.)

What of cron?

Posted Jul 19, 2006 7:25 UTC (Wed) by hein.zelle (guest, #33324) [Link]

Has this behaviour of cron led to any separate security advisories / fixes yet?

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