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Date: Wed, 6 Jan 2016 23:06:20 +0000
From: halfdog <me@...fdog.net>
To: oss-security@...ts.openwall.com
Subject: Discuss: Daily/weekly cron jobs best practices

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Hello List,

Different Linux software packages use cron jobs for basic maintenance
activities, e.g.

* rotating/compressing/deleting logs (syslog, ntp)
* cleanup of caches (man, php)
* notifications (calender, SMART disk check)

Especially interesting are those cron job scripts run as user root but
processing files owned by dedicated service users. But shell scripts
often have their problems processing untrusted input:

* the tools invoked in shell scripts often have no or little
protection against file system modification races

* data processing with multiple separate processes, as usually
employed when using pipes in shell scripts, are inherently racy.

* the tools do not protect against symlink attacks

Due to that risks, what would be best practices, e.g. for a script
cleaning up log files for a daemon.

a) run shell script as daemon user. Ignore script security: a
malicious daemon user can already perform all those actions by
himself, not relying on insecure scripts. Pro: no need to perform
security audits on scripts. Con: below

b) run shell script as daemon user and also try to get it secured.
Pro: attackers without code execution possibilities but ability to
make daemon e.g. to create problematic files via the daemon are also
blocked.

c) try to make shell script secure, but still run as root. Pro: no
overhead due to uid-switch (pam/audit logging with su in scripts), no
risk to leak privileged resources, e.g. open FDs, to lower-priv daemon
context. Con: any script coding mistake or change in tool behaviour
(gzip, find, tar, ls, ....) might create privilege escalation hole.

d) do not use shell scripts for that kind of task, use OS-near
programming language, e.g. C, python, to write specialized helper.
Pro: Perfect protection possible (openat/fstat/O_NOFOLLOW...). Con:
higher maintenance effort.

Are there more variants, arguments? In my opinion, b) is a good
trade-off between maintainability and security.

Currently the cron scripts seem to be a weak point. I looked at the 8
daily scripts on my machine, 2 of them belonged to the "daemon"
example class from above and both were vulnerable to daemon to root
privilege escalation, see e.g. [1].

hd

[1]
http://www.halfdog.net/Security/2015/MandbSymlinkLocalRootPrivilegeEscalation/

- -- 
http://www.halfdog.net/
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