Date: Thu, 17 Aug 2017 12:48:39 -0400 From: Michael Orlitzky <michael@...itzky.com> To: oss-security@...ts.openwall.com Subject: CVE-2017-11746: tenshi privilege escalation via PID file manipulation Product: Tenshi (log monitoring tool) Vendor: Inverse Path (F-Secure) Versions-affected: 0.15 and earlier Fixed-in: commits 46b0148 and d0e7f28, version 0.16 Bug-report: https://github.com/inversepath/tenshi/issues/6 Author: Michael Orlitzky Acknowledgments: Andrea Barisani who fixed several other issues and got a new release out to help fix this one. == Summary == The tenshi 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 fulfil 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. Tenshi itself ships a few such init scripts: tenshi.debian-init, tenshi.suse-init, etc. == Exploitation == An example of a problematic scenario involving an init script would be, 1. I run "/etc/init.d/tenshi start" to start the daemon. 2. tenshi drops to the "tenshi" user. 3. tenshi writes its PID file, now owned by the "tenshi" user. 4. Someone compromises the daemon, which processes untrusted input. 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/tenshi 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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