CVSS 10.0 LXD, MicroCloud Flaw Lets Attackers Impersonate Trusted Clients Without a Private Key

The vulnerability allows an attacker to reach the HTTPS API and use a trusted client's public certificate to impersonate that client. Once inside, the attacker can act as the client, accessing the cluster and its virtual machines. If the impersonated account is privileged, the attacker could gain 'root access to the host machine,' Canonical said.

The flaw, tracked as CVE-2026-112816, is in how the server checks a client's certificate. The server compares every certificate the client sends against its trust store, but a TLS connection only proves the client owns the private key for the first certificate in the chain, the leaf. The rest of the chain is whatever the client chose to attach.

An attacker can take a trusted client's public certificate, which is not secret, and attach it to the chain behind their own. The server would then accept them as that trusted client, even though they do not hold the private key. This flaw was reported by the Incus project, which began as a community fork of LXD, and Canonical confirmed it affected LXD as well.

Using the flaw requires two things: network access to the HTTPS API and the public certificate of a client the server already trusts. A standalone LXD server does not expose that API by default, listening only on a local socket unless an administrator turns on the network API with the core.https_address setting.

MicroCloud, which ties several machines running LXD into one cluster, has a specific weak point. An attacker who can capture a member's public certificate as it joins the cluster can then log in as that member. The CVE record lists LXD 4.0.14, 5.0.10, 5.21.8 and 6.9 as affected, each before the fixed release that follows.

Canonical lists these fixed snap versions for the bug. Ubuntu's own archive packages of LXD are listed as not affected or not shipped, so the fixed releases are the snaps. There is no public sign the flaw has been used in an attack. The CVE record carries a U.S. CISA assessment, dated October 30, that the flaw is not known to have been exploited.

Users should update LXD and MicroCloud to the fixed versions. On a snap install, run sudo snap refresh lxd and sudo snap refresh microcloud. Canonical's advisory gives no temporary workaround for LXD. Because the attack requires network access to the API, limiting who can reach it reduces risk until you update.

The Incus project, reporting the same flaw, said there is no configuration fix and that the only step is to restrict network access to the listener. Incus users should move to 7.6.0, which fixed this flaw along with 17 other security issues. Canonical said the fixed LXD point releases were still rolling out from the candidate channel to stable at the time of its advisory.