PIZZACODE SECURITY ALERT
program: rssh
risk: low[*]
problem: string format vulnerability in log.c
details:
rssh is a restricted shell for use with OpenSSH, allowing only scp
and/or sftp. For example, if you have a server which you only want to
allow users to copy files off of via scp, without providing shell
access, you can use rssh to do that. Additioanlly, running rsync,
rdist, and cvs are supported, and access can be configured on a
per-user basis using a simple text-based configuration file. The rssh
homepage is here:
http://www.pizzashack.org/rssh/
Florian Schilhabel has identified a format string bug which can allow
an attacker to run arbitrary code from an account configured to use
rssh. [*]In general the risk is low, as in most cases the user can
only compromise their own account. The risk is mittigated by the fact
that before this bug can be exploited, the user must log in
successfully through ssh. This means that either the user is known to
the system (and therefore the administrators), or that the system is
probably already compromised.
However, on some older systems with broken implementations of the
setuid() family of functions, a root compromise may be possible with
certain configurations of rssh. Specifically, if rssh is configured
to use a chroot jail, it will exec() rssh_chroot_helper, which must be
setuid root in order to call chroot(). Normally, rssh_chroot_helper
calls setuid(getuid()) and drops privileges before any of the logging
functions are called, making a root compromise impossible on most
systems. However, some older systems which handle saved UIDs
improperly may be vulnerable to a root compromise. Linux in
particular is not vulnerable to this, nor should modern
POSIX-compliant Unix variants be. POSIX defines that the setuid()
system call will set all UIDs (UID, saved UID, and effective UID) the
specified UID if it is called with root privileges. Therefore in
general, a root compromise is not possible, and I am not specifically
aware of any systems on which one is possible.
The 2.2.2 release of rssh fixes this string format vulnerability. I
have also gone over the code to make sure that no other such
vulnerabilities exist. In addition to fixing this problem, rssh
contains some new code to help identify certain problems for debugging
problems when rssh fails. Additional logging of error conditions is
performed.
--
Derek D. Martin
http://www.pizzashack.org/
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