New Ubuntu User-Namespace Bypasses Let Local Attackers Expand Kernel Exploits

Main Takeaways

Ubuntu 24.04 LTS (and 23.10 when the feature is enabled) contains defense-in-depth gaps that let unprivileged users obtain powerful namespace capabilities. The techniques exploit default tools and permissive AppArmor profiles (including aa-exec, BusyBox, and LD_PRELOAD injection into trusted processes). Alone they don’t instantly fully compromise a system, but when chained with kernel vulnerabilities they become an effective escalation path. Administrators should apply hardening steps: enable stricter kernel AppArmor restrictions, disable overly broad profiles, and tighten sandbox configurations.

A security team demonstrated three realistic bypasses against Ubuntu’s user-namespace protections. The first method abuses the included aa-exec utility to switch into more permissive AppArmor profiles (for example, profiles used by certain desktop or sandboxed applications) and then runs unshare to create unrestricted namespaces. The second relies on BusyBox shells governed by permissive AppArmor rules, allowing an attacker to spawn a shell that can create namespaces. The third injects a malicious shared library via LD_PRELOAD into a trusted process (such as a file manager); that library launches a shell in the process context and enables privileged namespace creation. These techniques exploit policy and profile gaps rather than kernel bugs directly, but they significantly simplify privilege-escalation chains.

Impact

The bypasses mainly affect Ubuntu 24.04 LTS (where the relevant restrictions are enabled by default) and Ubuntu 23.10 when the feature is active. Because user namespaces are commonly used for containerization and sandboxing, these policy bypasses increase the attack surface for kernel exploits: an attacker able to create privileged namespaces can more readily trigger kernel flaws that otherwise require elevated capabilities. While Canonical describes these as weaknesses in defense-in-depth rather than standalone critical vulnerabilities, the practical risk is meaningful when combined with other bugs.

Recommended mitigations

Administrators should adopt layered hardening. Enable the kernel parameter that restricts unprivileged AppArmor actions to prevent aa-exec abuse. Disable or tighten overly broad AppArmor profiles that permit BusyBox or file-manager processes to create namespaces. Harden sandbox profiles (for example, bubblewrap/Flatpak rules) so applications cannot spawn unrestricted namespaces. Audit AppArmor with tools like aa-status, apply distribution updates as they become available, and consider automated enforcement (configuration management or endpoint agents) to roll out kernel parameters and profile changes across fleets.