LXD: Update of type field in restricted TLS certificate allows privilege escalation to cluster admin

QFinch assessment

A restricted TLS certificate user can escalate to cluster admin by changing their certificate type from client to server via PUT/PATCH to /1.0/certificates/{fingerprint}. The non-admin guard and reset block in doCertificateUpdate fail to validate or reset the Type field, allowing a caller-supplied value to persist to the database. The modified certificate is matched as a server certificate during TLS authentication, granting ProtocolCluster with full admin privileges.

Potential impact

Privilege escalation from restricted TLS certificate user (project-scoped) to cluster admin. Cluster admin can create privileged containers (security.privileged=true) or pass raw LXC config (raw.lxc), which provides root-level access to the host, leading to full host compromise. The attack requires a single PUT/PATCH request. The escalation is persistent and takes effect immediately after the identity cache refresh. The change in permissions is not logged. Affects any LXD deployment using legacy restricted TLS certificates (/1.0/certificates API). Suggested remediation 1. Add Type to the guard check at line 992: 2. Use the original type in the reset block at line 1008:

Technical context

doCertificateUpdate in lxd/certificates.go handles PUT/PATCH requests to /1.0/certificates/{fingerprint} for both privileged and unprivileged callers. The access handler is allowAuthenticated, so any trusted TLS user (including restricted) can reach this code. For unprivileged callers (restricted users who fail the EntitlementCanEdit check at line 975), two defenses are intended to prevent field tampering: 1. The guard block validates that Restricted, Name, and Projects match the original database record. Does not check Type. 2. The reset block rebuilds the dbCert struct using original values for Restricted, Name, and Certificate. Uses reqDBType (caller-supplied) for Type instead of the original dbInfo type. This allows the attacker to update the Type field of their own certificate from client to server, bypassing the authorization controls and escalating to cluster admin.