Apptainer blocked the SUID based exploit

Gregory Kurtzer tests the newly disclosed pkexec vulnerability in polkit against an Apptainer container to see whether the privilege escalation carries through the container runtime. On a Rocky Linux 8.4 host with a vulnerable polkit package, running the compiled exploit as a regular user yields a root shell almost immediately.

He then builds a Rocky Linux container from the default Docker Hub container image, deliberately installing the vulnerable polkit and polkit-libs packages, and opens a shell inside it with Apptainer. Running the same payload inside the container fails: pkexec reports that it must be installed setuid root, even though it is, because Apptainer blocks the privilege escalation path. The takeaway for HPC and shared-system administrators is that users running workloads through Apptainer or Singularity are not exposed to this exploit via the container runtime.

Key takeaways

• On a Rocky Linux 8.4 host with a vulnerable polkit, the compiled pkexec exploit gives a regular user a root shell in seconds.
• The test container is built from the default Rocky Linux Docker Hub image with vulnerable polkit and polkit-libs deliberately installed.
• Inside the apptainer shell the same payload fails, and pkexec only reports that it must be installed setuid root.
• apptainer builds single-file containers, so the whole container can be executed directly or entered with an interactive shell.
• Users running through Apptainer or Singularity are not exposed to this pkexec exploit via the container runtime.

Questions this video answers

Does the pkexec polkit exploit work inside an Apptainer container?

No. In this test the exploit gains root on the host but fails inside an Apptainer container that has the same vulnerable polkit packages installed. Apptainer blocks the setuid-based privilege escalation, so pkexec falls back to an error saying it must be installed setuid root, even though the binary is setuid root.

How was the apptainer test environment set up?

The host runs Rocky Linux 8.4 with a vulnerable polkit version. A Rocky Linux container is built from a recipe that pulls the default image from Docker Hub and installs polkit and polkit-libs at the vulnerable versions. Apptainer then opens a shell inside the single-file container as the same unprivileged user.

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