Incus WebUI Authentication Bypass Vulnerability (CVE-2026-33898)

Incus is a system container and virtual machine manager. Prior to version 6.23.0, the incus webui component contains an authentication bypass vulnerability (CVE-2026-33898). The vulnerability lies in the incorrect validation of authentication tokens by the web server spawned by incus webui. incus webui launches a local web server on a randomly assigned localhost port and issues a URL containing an authentication token for user access. While the Incus client correctly validates cookies, it fails to properly validate tokens passed directly in the URL. This flaw can be exploited by an attacker able to communicate with the temporary web server on localhost. Successful exploitation grants the attacker the same privileges as the user who initiated incus webui. This impacts system security as it may lead to privilege escalation or unauthorized access to Incus instances and potentially system resources. The vulnerability is resolved in Incus version 6.23.0.

Attack Chain

1. A user initiates the incus webui command, which spawns a temporary web server on a random localhost port (e.g., 127.0.0.1:8080).
2. Incus provides the user with a URL containing an authentication token (e.g., http://127.0.0.1:8080/?token=abcdef123456).
3. An attacker gains access to the network or system where the Incus webui is running, and identifies the localhost port being used.
4. The attacker crafts a malicious request to the incus webui web server, using a valid or even an invalid token in the URL. The vulnerability does not properly validate the token.
5. Due to the authentication bypass, the attacker is granted access to the Incus webui with the privileges of the user who started the incus webui instance.
6. If the attacker is a local user, they can leverage their newly acquired privileges to escalate their access to the system.
7. The attacker could manipulate Incus instances, access sensitive data, or potentially gain control of underlying system resources.
8. The attacker achieves privilege escalation or unauthorized access, depending on the attacker's initial location and goals.

Impact

Successful exploitation of CVE-2026-33898 allows an attacker to gain the privileges of the user running incus webui. If the attacker is a local user, this results in privilege escalation, allowing them to perform actions they were previously unauthorized to do. A remote attacker who can trick a local user into interacting with the Incus web UI server could gain unauthorized access to Incus instances and potentially sensitive system resources. The CVSS v3.1 base score for this vulnerability is 8.8, indicating a high severity.

Recommendation

• Upgrade Incus to version 6.23.0 or later to patch the vulnerability (CVE-2026-33898).
• Implement network segmentation to restrict access to the incus webui web server.
• Monitor network connections to the incus webui web server for suspicious activity (see the network connection Sigma rule below).
• Audit the use of incus webui in your environment and restrict its use to authorized personnel only.