Critical LXD and MicroCloud Vulnerability Allows Client Impersonation
A severe flaw in Canonical's LXD and MicroCloud, rated CVSS 10.0, enables attackers to impersonate trusted clients by exploiting the mutual TLS authentication process.
A critical vulnerability, tracked as CVE-2026-112816, has been discovered in Canonical's LXD and MicroCloud platforms, allowing attackers to impersonate trusted clients without possessing their private keys. This flaw, which carries a maximum CVSS score of 10.0, poses a significant threat to organizations relying on these container and cloud management solutions.
The vulnerability arises from a weakness in how LXD and MicroCloud handle mutual TLS (mTLS) authentication. In an mTLS setup, clients present a certificate to the server to prove their identity. A client is considered trusted if its certificate is present in the server's trust store. However, the flaw lies in the server's validation process: it only verifies the authenticity of the leaf certificate in the TLS chain, which the attacker can control. The rest of the certificate chain can be arbitrarily constructed by the attacker.
By obtaining the public certificate of a legitimate, trusted client—which is not a secret—an attacker can prepend this certificate to their own, effectively creating a forged certificate chain. When this forged chain is presented to the server, the server incorrectly trusts the attacker as the legitimate client, granting them unauthorized access to the cluster and its associated virtual machines. If the impersonated client holds privileged accounts, the attacker could potentially gain root access to the host machine.
The exploit requires two primary conditions: network access to the LXD/MicroCloud HTTPS API and the public certificate of a client already trusted by the server. While a standalone LXD server does not expose its API over the network by default, administrators can enable this feature using the core.https_address setting. MicroCloud, designed to cluster multiple LXD machines, presents a more specific risk. An attacker who can intercept a member's public certificate as it joins the cluster can then use it to impersonate that member.
Canonical has released updated versions to address the vulnerability. Affected LXD versions include 4.0.14, 5.0.10, 5.21.8, and 6.9. The fixed snap versions are LXD 6.10.1 (6/stable channel), 5.21.9 (5.21/stable), 5.0.11 (5.0/stable), and 4.0.15 (4.0/stable). For MicroCloud, the fixed versions are 3.4.1 (3/stable) and 2.1.5 (2/stable).
Interestingly, the vulnerability was initially reported by the Incus project, a community fork of LXD, which has also released a fix in version 7.6.0, alongside 17 other security issues. Ubuntu's own LXD packages are noted as not affected or not shipped, with the snap versions being the primary concern for users.
As of the advisory's release, there were no public reports of this vulnerability being exploited in the wild. The U.S. Cybersecurity and Infrastructure Security Agency (CISA) assessed that the flaw is not known to have been exploited and it has not been added to CISA's Known Exploited Vulnerabilities (KEV) list. No public exploit code has been released.
Users are strongly advised to update their LXD and MicroCloud installations to the latest fixed versions. For snap installations, this involves running sudo snap refresh lxd and sudo snap refresh microcloud. Canonical notes that fixed releases are still rolling out to stable channels, so users should verify their current version after the refresh. No temporary workaround is provided by Canonical, emphasizing the importance of immediate updates and restricting network access to the API until patching is complete.