NAME sshd-restricted-users-incorrect-configuration AUTHOR Marc SCHAEFER <schaeferat_private> Andreas Trottmann <andreas.trottmannat_private> THANKS OpenBSD security team VERSION $Id: sshd-restricted-users-incorrect-configuration,v 1.2 2000/01/25 10:27:56 schaefer Exp $ ABSTRACT In some cases where a system must be configured so that specific users only have access to POP or FTP (or a specific restricted shell, e.g. a BBS or lynx menu), the addition of the SSH protocol server (sshd) may create a security hole. The user, if they try to access the server per telnet succeed, but they are immediately thrown out (because their shell is /bin/false, e.g.), or a special restricted shell runs (e.g. they can change their passwd, etc). In that case, using sshd may create a subtle security hole allowing those users to, like normal users, use the SSH protocol to issue TCP connections coming from the attacked host. IMPACT Any remote user with an account on the machine, even without real shell access, may open a TCP connection which will: - appear to be open from root@localhost (in the IDENT identd protocol) - be able to connect to any services which are not firewalled on the loopback (even if they are firewalled or tcp_wrapper tcpd protected from the outside). - be able to connect to any remote machine from the attacked host, the connection appearing to come from the attacked host with a wrong IDENT (see above). IMMUNE CONFIGURATIONS You are immune to this problem if one (or more) of the following is true: - the group(s) where those users belong to is listed in /etc/ssh/sshd_config or equivalent configuration file as DenyGroups group1 group2 # etc (this is the recommended setup) - no user which has an account hasn't a shell (he will be able to do the above, except the root@ IDENT, anyway, if he has a shell) - your POP or FTP users do not authentify against the system password database (/etc/{passwd|shadow}), but against a private database and the user is locked in the system password database (passwd -l). - you only allow RSA authentification, and the users cannot modify their ~/.ssh from e.g. FTP. - you do not run sshd. Have TcpAllowForwarding to no in the configuration file doesn't seem to work, since it only denies -R style forwarding. OPERATING SYSTEMS UNIX FIX - There is no fix for the root@ IDENT bug for legitimate user. This is presumably a bug in ssh-1.2.27 and OpenSSH 1.2.1 and earlier releases: sshd should not do the forwarding as root but as the user. Note that it has not been investigated if this could create other problems. This bug is a long-standing known bug, and also is due to the fact IDENT information was never supposed to be trusted anyway. - Put all ftponly and poponly users in specially identified groups with DenyGroups ftponly poponly This will fix the open-port-from-no-shell-user - Or lock the user in the system password database and use a special database for FTP and POP. EXPLOIT Please do not request exploit from the listed authors. Requests for exploits will be ignored. A working exploit exists and has been tested on current Linux distributions. It is possible that an exploit be posted some time in the future (or that someone reads this and does it by himself ...). NOTES This advisory is for information only. No warranty either expressed or implied. Full disclosure and dissemination are allowed as long as this advisory is published in full. No responsability will be taken from abuse or lack of use of the information in this advisory.
This archive was generated by hypermail 2b30 : Fri Apr 13 2001 - 15:34:35 PDT