rssh: root privilege escalation flaw

2005-12-30 / 2005-12-31
Credit: Derek Martin
Risk: High
Local: Yes
Remote: No

CVSS Base Score: 7.2/10
Impact Subscore: 10/10
Exploitability Subscore: 3.9/10
Exploit range: Local
Attack complexity: Low
Authentication: No required
Confidentiality impact: Complete
Integrity impact: Complete
Availability impact: Complete

Affected Software: rssh - all versions prior to 2.3.0 Vulnerability: local user privilege escalation Severity: *CRITICAL* Impact: local users can gain root access Solution: Please upgrade to v2.3.1 Summary ------- rssh is a restricted shell which allows a system administrator to limit users' access to a system via SSH to scp, sftp, rsync, rdist, and cvs. It also allows the system administrator the ability to chroot users to a configurable location. * PLEASE NOTE * This problem was fixed in 2.3.0, but there is another small bug (not security-related) in that version which prompted me to release 2.3.1 today. I will announce that separately in appropriate channels. Please upgrade to the 2.3.1 release, not the 2.3.0 release. Max Vozeler reported a flaw in the design of rssh_chroot_helper whereby it can be exploited to chroot to arbitrary directories and thereby gain root access. If rssh is installed on a system, and non-trusted users on that system have access which is not protected by rssh (i.e. they have full shell access), then they can use rssh_chroot_helper to chroot to arbitrary locations in the file system, and thereby gain root access. Workaround ---------- By careful configuration of file system mounts, it is possible to avoid this problem; but doing so requires a fair amount of contortion which will be difficult to re-engineer after an existing installation has already been configured. The exploit requires the user to be able to write executables in the directory they are chrooting to, and create hard links to SUID binaries within that directory structure, so by preventing either of these two things, the exploit will be foiled. System administrators can accomplish this by careful configuration of filesystem permissions, mount points, and mount options (such as no_exec, no_suid, etc.). I will not go into details since the far better solution is to upgrade. Fix --- The 2.3.0 release of rssh fixes this problem by forcing the chroot helper program to re-parse the config file instead of allowing the chroot home to be specified on the command line. Thus users not listed can not use it to chroot (or will chroot to the default location specified by the sysadmin), and users who are listed will be chrooted to the directories where they are supposed to go only. This version also fixes an unrelated bug which causes rssh_chroot_helper to crash on the ia64 architecture (and possibly others). Numerous people reported a problem with the way va_start/va_end was used in log.c, which causes a segfault on 64-bit Linux platforms. It is believed that this bug is not exploitable, since no code in this module is ever executed with root privileges. However this is also fixed in this release. Thanks -- Derek D. Martin GPG Key ID: 0x81CFE75D -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.2.1 (GNU/Linux) iD8DBQFDtWQwdjdlQoHP510RAuqGAKCWfpUza24U6HixK+51s/Y8R2pkewCfW5ml JozbIxiUbqB4P5fSyN5WmwM= =tbRo -----END PGP SIGNATURE-----

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