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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