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Message-ID: <f28badcd-c805-3fa5-5a1f-cd65c4899885@orlitzky.com>
Date: Wed, 16 Aug 2017 12:10:09 -0400
From: Michael Orlitzky <michael@...itzky.com>
To: oss-security@...ts.openwall.com
Subject: CVE-2017-12847: nagios-core privilege escalation via PID file
manipulation
Product: Nagios Core
Versions-affected: 4.3.2 and earlier
Fixed-in: commits 1b19734 and 3baffa7, version 4.3.3
Bug-report: https://github.com/NagiosEnterprises/nagioscore/issues/404
Author: Michael Orlitzky
Acknowledgments: Bryan Heden (upstream) for his fast response and help
== Summary ==
The nagios daemon should create its PID file before dropping
privileges. This represents a minor security issue; additional factors
are needed to make it exploitable.
== Details ==
The purpose of the PID file is to hold the PID of the running daemon,
so that later it can be stopped, restarted, or otherwise signalled
(many daemons reload their configurations in response to a SIGHUP).
To fulfill that purpose, the contents of the PID file need to be
trustworthy. If the PID file is writable by a non-root user, then he
can replace its contents with the PID of a root process. Afterwards,
any attempt to signal the PID contained in the PID file will instead
signal a root process chosen by the non-root user (a vulnerability).
This is commonly exploitable by init scripts that are run as root and
which blindly trust the contents of their PID files. Nagios itself ships
such an init script (daemon-init.in), so the risk is not theoretical in
this case.
== Exploitation ==
An example scenario involving an init script would be,
1. I run "/etc/init.d/nagios start" to start the daemon.
2. nagios drops to the "nagios" user.
3. nagios writes its PID file, now owned by the "nagios" user.
4. Someone compromises the daemon, which sits on the network.
5. The attacker is generally limited in what he can do because the
daemon doesn't run as root. However, he can write "1" into the
PID file, and he does.
6. I run "/etc/init.d/nagios stop" to stop the daemon while I
investigate the weird behavior resulting from the hack.
7. The machine reboots, because I killed PID 1 (this is normally
restricted to root).
== Resolution ==
The problem is avoided by creating the PID file as root, before dropping
privileges.
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