Mark Laing discovered that LXD's PKI mode, until version 5.21.2, could be bypassed if the client's certificate was present in the trust store.

Affected: Go: github.com/canonical/lxd

### Summary If a `server.ca` file is present in `LXD_DIR` at LXD start up, LXD is in "PKI mode". In this mode, only TLS clients that have a CA-signed certificate should be able to authenticate with LXD.

We have discovered that if a client that sends a non-CA signed certificate during the TLS handshake, that client is able to authenticate with LXD if their certificate is present in the trust store. - The LXD Go client (and by extension `lxc`) does not send non-CA signed certificates during the handshake. - A manual client (e.g. `cURL`) might send a non-CA signed certificate during the handshake.

#### Versions affected LXD 4.0 and above.

### Details When PKI mode was added to LXD it was intended that all client and server certificates *must* be signed by the certificate authority (see https://github.com/canonical/lxd/pull/2070/commits/84d917bdcca6fe1e3191ce47f1597c7d094e1909).

In PKI mode, the TLS listener configuration is altered to add the CA certificate but the `ClientAuth` field of `tls.Config` is not changed. The `ClientAuth` field is set to `tls.RequestClientCert`, which configures the TLS connection to request a certificate from the client, but not require one. This is necessary because untrusted requests are allowed for some endpoints.

If a client certificate is present in the trust store before PKI mode is enabled, calls to LXD using that certificate fail *when using the Go client for LXD*. I believe that what is happening is as follows: - During the TLS…

govulncheck-vulndb-0.0.20241209T183251-1.1 on GA media

Affected: Ubuntu:Pro:16.04:LTS: lxd, Ubuntu:Pro:18.04:LTS: lxd, Ubuntu:20.04:LTS: lxd

Mark Laing discovered that LXD's PKI mode, until version 5.21.2, could be bypassed if the client's certificate was present in the trust store.