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OpenSSH Local Privilege Escalation

OpenSSH Local Privilege Escalation
Posted Dec 23, 2016
Authored by Jann Horn, Google Security Research

OpenSSH can forward TCP sockets and UNIX domain sockets. If privilege separation is disabled, then on the server side, the forwarding is handled by a child of sshd that has root privileges. For TCP server sockets, sshd explicitly checks whether an attempt is made to bind to a low port (below IPPORT_RESERVED) and, if so, requires the client to authenticate as root. However, for UNIX domain sockets, no such security measures are implemented. This means that, using "ssh -L", an attacker who is permitted to log in as a normal user over SSH can effectively connect to non-abstract unix domain sockets with root privileges. On systems that run systemd, this can for example be exploited by asking systemd to add an LD_PRELOAD environment variable for all following daemon launches and then asking it to restart cron or so. The attached exploit demonstrates this - if it is executed on a system with systemd where the user is allowed to ssh to his own account and where privsep is disabled, it yields a root shell.

tags | exploit, shell, root, tcp
systems | unix
advisories | CVE-2016-10010
MD5 | b93e78906a304aa126934a6c44a6999b

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