VNC security — QEMU documentation

# VNC security

The VNC server capability provides access to the graphical console of the guest VM across the network. This has a number of security considerations depending on the deployment scenarios.

## Without passwords

The simplest VNC server setup does not include any form of authentication. For this setup it is recommended to restrict it to listen on a UNIX domain socket only. For example

    qemu-system-x86_64 [...OPTIONS...] -vnc unix:/home/joebloggs/.qemu-myvm-vnc

This ensures that only users on local box with read/write access to that path can access the VNC server. To securely access the VNC server from a remote machine, a combination of netcat+ssh can be used to provide a secure tunnel.

## With passwords

The VNC protocol has limited support for password based authentication. Since the protocol limits passwords to 8 characters it should not be considered to provide high security. The password can be fairly easily brute-forced by a client making repeat connections. For this reason, a VNC server using password authentication should be restricted to only listen on the loopback interface or UNIX domain sockets. Password authentication is not supported when operating in FIPS 140-2 compliance mode as it requires the use of the DES cipher. Password authentication is requested with the `password` option, and then once QEMU is running the password is set with the monitor. Until the monitor is used to set the password all clients will be rejected.

    qemu-system-x86_64 [...OPTIONS...] -vnc :1,password=on -monitor stdio
    (qemu) change vnc password
    Password: ****
    (qemu)

## With x509 certificates

The QEMU VNC server also implements the VeNCrypt extension allowing use of TLS for encryption of the session, and x509 certificates for authentication. The use of x509 certificates is strongly recommended, because TLS on its own is susceptible to man-in-the-middle attacks. Basic x509 certificate support provides a secure session, but no authentication. This allows any client to connect, and provides an encrypted session.

    qemu-system-x86_64 [...OPTIONS...]   -object tls-creds-x509,id=tls0,dir=/etc/pki/qemu,endpoint=server,verify-peer=off   -vnc :1,tls-creds=tls0 -monitor stdio

In the above example `/etc/pki/qemu` should contain at least three files, `ca-cert.pem`, `server-cert.pem` and `server-key.pem`. Unprivileged users will want to use a private directory, for example `$HOME/.pki/qemu`. NB the `server-key.pem` file should be protected with file mode 0600 to only be readable by the user owning it.

## With x509 certificates and client verification

Certificates can also provide a means to authenticate the client connecting. The server will request that the client provide a certificate, which it will then validate against the CA certificate. This is a good choice if deploying in an environment with a private internal certificate authority. It uses the same syntax as previously, but with `verify-peer` set to `on` instead.

    qemu-system-x86_64 [...OPTIONS...]   -object tls-creds-x509,id=tls0,dir=/etc/pki/qemu,endpoint=server,verify-peer=on   -vnc :1,tls-creds=tls0 -monitor stdio

## With x509 certificates, client verification and passwords

Finally, the previous method can be combined with VNC password authentication to provide two layers of authentication for clients.

    qemu-system-x86_64 [...OPTIONS...]   -object tls-creds-x509,id=tls0,dir=/etc/pki/qemu,endpoint=server,verify-peer=on   -vnc :1,tls-creds=tls0,password=on -monitor stdio
    (qemu) change vnc password
    Password: ****
    (qemu)

## With SASL authentication

The SASL authentication method is a VNC extension, that provides an easily extendable, pluggable authentication method. This allows for integration with a wide range of authentication mechanisms, such as PAM, GSSAPI/Kerberos, LDAP, SQL databases, one-time keys and more. The strength of the authentication depends on the exact mechanism configured. If the chosen mechanism also provides a SSF layer, then it will encrypt the datastream as well.

Refer to the later docs on how to choose the exact SASL mechanism used for authentication, but assuming use of one supporting SSF, then QEMU can be launched with:

    qemu-system-x86_64 [...OPTIONS...] -vnc :1,sasl=on -monitor stdio

## With x509 certificates and SASL authentication

If the desired SASL authentication mechanism does not supported SSF layers, then it is strongly advised to run it in combination with TLS and x509 certificates. This provides securely encrypted data stream, avoiding risk of compromising of the security credentials. This can be enabled, by combining the ‘sasl’ option with the aforementioned TLS + x509 options:

    qemu-system-x86_64 [...OPTIONS...]   -object tls-creds-x509,id=tls0,dir=/etc/pki/qemu,endpoint=server,verify-peer=on   -vnc :1,tls-creds=tls0,sasl=on -monitor stdio

## Configuring SASL mechanisms

The following documentation assumes use of the Cyrus SASL implementation on a Linux host, but the principles should apply to any other SASL implementation or host. When SASL is enabled, the mechanism configuration will be loaded from system default SASL service config /etc/sasl2/qemu.conf. If running QEMU as an unprivileged user, an environment variable SASL_CONF_PATH can be used to make it search alternate locations for the service config file.

If the TLS option is enabled for VNC, then it will provide session encryption, otherwise the SASL mechanism will have to provide encryption. In the latter case the list of possible plugins that can be used is drastically reduced. In fact only the GSSAPI SASL mechanism provides an acceptable level of security by modern standards. Previous versions of QEMU referred to the DIGEST-MD5 mechanism, however, it has multiple serious flaws described in detail in RFC 6331 and thus should never be used any more. The SCRAM-SHA-256 mechanism provides a simple username/password auth facility similar to DIGEST-MD5, but does not support session encryption, so can only be used in combination with TLS.

When not using TLS the recommended configuration is

    mech_list: gssapi
    keytab: /etc/qemu/krb5.tab

This says to use the ‘GSSAPI’ mechanism with the Kerberos v5 protocol, with the server principal stored in /etc/qemu/krb5.tab. For this to work the administrator of your KDC must generate a Kerberos principal for the server, with a name of `qemu/somehost.example.com@EXAMPLE.COM` replacing `somehost.example.com` with the fully qualified host name of the machine running QEMU, and `EXAMPLE.COM` with the Kerberos Realm.

When using TLS, if username+password authentication is desired, then a reasonable configuration is

    mech_list: scram-sha-256
    sasldb_path: /etc/qemu/passwd.db

The `saslpasswd2` program can be used to populate the `passwd.db` file with accounts. Note that the `passwd.db` file stores passwords in clear text.

Other SASL configurations will be left as an exercise for the reader. Note that all mechanisms, except GSSAPI, should be combined with use of TLS to ensure a secure data channel.