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Trustwave Action Response: Polkit Privilege Escalation Vulnerability - PwnKit (CVE-2021-4034)

Summary of Trustwave Actions (updated 1/26/2022):

Trustwave security and engineering teams became aware of the vulnerability in Polkit's pkexec component identified as CVE-2021-4034 (PwnKit) on January 25. We immediately investigated the vulnerability and potential exploits and continue to actively monitor the situation for our clients.

All major Linux distributions have made patches available, including Debian (https://security-tracker.debian.org/tracker/CVE-2021-4034), Ubuntu (https://ubuntu.com/security/CVE-2021-4034), and RedHat (https://access.redhat.com/security/vulnerabilities/RHSB-2022-001). Check your distribution for specific patches. As part of a strong patch management program, ensuring your infrastructure and applications are up to date is critical for mitigating cyber risk.

Administrators should be able to temporarily or alternatively mitigate exploitation by removing the SUID-bit from pkexec until a patch can be put in place.

For example: # chmod 0755 /usr/bin/pkexec

As a response tactic, we recommend clients utilize an Endpoint Detection and Response (EDR) solution capable of detecting this exploit. Most major EDR providers should have or will be releasing an emergency update to automatically detect the exploit. Solutions, where policy management is under Trustwave support, will receive those updates as they become available from vendors.

Trustwave is currently triaging this vulnerability and taking mitigation actions within our environments.

Trustwave MDR Advanced Clients:

For MDR Advanced clients, Trustwave is actively threat hunting attempts to exploit this vulnerability. 

The Trustwave Threat Hunting team has also published a practical guide to help the cybersecurity community address the vulnerability by identifying common behavior in exploitation.

Trustwave Product Updates:

Trustwave is actively working on checks for its scanner to detect this vulnerability. We will update the blog once the scanner update has been released. We will expedite the release of the update if the vulnerability and exploitation situation rapidly evolves.

What is CVE-2021-4034 (PwnKit)?

According to the Qualys Research Team who discovered this vulnerability, this is a “memory corruption vulnerability in polkit’s pkexec, a SUID-root program that is installed by default on every major Linux distribution. This easily exploited vulnerability allows any unprivileged user to gain full root privileges on a vulnerable host by exploiting this vulnerability in its default configuration.”

Quick Facts:

  • The vulnerability resides in "polkit" (formerly PolicyKit) and has existed in that code for over 12 years. This software is found by default in essentially all modern Linux distributions.
  • Specifically, it affects how "polkit" invokes "pkexec", a process that allows unprivileged users to interact with root level processes.
  • Qualys has exploited this vulnerability under Ubuntu, Debian, Fedora, and CentOS in their tests. It's assumed even minor distributions are also all affected.

The Potential Danger of CVE-2021-4034

A Privilege Escalation vulnerability allows an unprivileged local user to get full root privileges. The danger from any privilege escalation vulnerability such as CVE-2021-4034 is it can allow an unprivileged local user to get full root privileges. If a threat actor pairs the polkit escalation issue with a remote code execution vulnerability, the attacker can gain code execution as root with no limitations. With this widespread Privileged Escalation vulnerability, criminals may be looking for remote code execution (RCE) vulnerabilities that may be effective when paired with this vulnerability.


Qualys research blog: PwnKit: Local Privilege Escalation Vulnerability Discovered in polkit’s pkexec (CVE-2021-4034)

Dark Reading article with insights from Trustwave SpiderLabs: Experts Urge Firms to Patch Trivial-to-Exploit Flaw in Linux PolicyKit  

Threat Hunting guide for CVE-2021-4034 by Trustwave Threat Hunting Team: Trustwave Threat Hunting Guide: Identifying PwnKit (CVE-2021-4034) Exploitation