Ubuntu snap-confine flaw can give attackers root access

Qualys has disclosed a local privilege escalation vulnerability in Ubuntu's snap-confine component, tracked as CVE-2026-8933.

The issue could allow an unprivileged local user to gain full root access on default installations of Ubuntu Desktop 24.04, 25.10 and 26.04. Qualys linked the flaw to a security-hardening change in snap-confine, which builds the execution environment for snap applications.

According to Qualys, recent Ubuntu releases moved snap-confine from a set-uid-root binary to a set-capabilities model intended to limit privilege use. Under that model, snap-confine runs with the effective user ID of the calling user while retaining near-root capabilities.

This design created a narrow window during sandbox setup in which temporary directories and files under /tmp were initially owned by the unprivileged user before ownership shifted to root. Qualys said the vulnerability arose from a race condition during that initialisation process.

How it works

According to the technical analysis, the exploit relies on two concurrent race conditions. In the first, an attacker mounts a FUSE filesystem over a temporary scratch directory after it is created, keeping the directory accessible outside the sandbox even after mount namespace isolation is applied.

In the second, the attacker creates a symlink to an arbitrary target file. When snap-confine then attempts to create a sandbox file, the underlying open() call can follow the symlink and write to the chosen target, Qualys said.

The analysis added that a further race lets an attacker widen file permissions to 0666 before snap-confine transfers ownership to root with fchown(). To bypass AppArmour confinement, the exploit targets the /run/udev/** path, where read-write access is permitted, according to Qualys.

By placing a malicious .rules file in /run/udev/rules.d/ and triggering a FUSE mount and unmount cycle, an attacker can force systemd-udevd to execute arbitrary commands as root, Qualys said.

Affected systems

Qualys identified Ubuntu Desktop 26.04, 25.10 and 24.04 systems updated to current snapd packages as affected by the vulnerable set-capabilities variant of snap-confine. The issue affects default installations of those desktop releases.

Following coordinated disclosure, Canonical has released patches through the Ubuntu Security Team, according to Qualys. Organisations running affected systems were urged to apply the latest snapd updates.

The vulnerability is notable because it affects a core part of the snap packaging and sandboxing system used widely across Ubuntu installations. Local privilege escalation flaws are often a particular concern for security teams because they can turn limited access on a machine into full administrative control.

Vendor response

Qualys credited Canonical staff Eduardo Barretto and Zygmunt Krynicki, along with the wider linux-distros community, for patch review and hardening measures around the snap ecosystem. The disclosure followed coordination with the Ubuntu Security Team.

Qualys also published a technical analysis covering source code references and proof-of-concept execution, alongside guidance for identifying affected systems. It is releasing detection identifiers for customers as they become available.

Security vendors often use such identifiers to help customers locate vulnerable assets across corporate environments. In this case, users can search for Ubuntu systems running snapd to identify machines that may require patching, Qualys said.

The bug also highlights the risks that can emerge when software changes are introduced to reduce privilege exposure. In this case, the move away from a set-uid-root binary was intended to narrow the attack surface, but Qualys said the resulting implementation left a timing gap that an attacker could exploit during sandbox initialisation.

"The issue stems from a security hardening change that inadvertently introduced a race condition during sandbox initialisation," said Saeed Abbasi, Head of Threat Research Unit, Director of Product, Qualys.