docs: recommend SCRAM-SHA-256 SASL mech instead of SHA-1 variant

The SHA-256 variant better meats modern security expectations.
Also warn that the password file is storing entries in clear
text.

Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>

docs/system/vnc-security.rst

@@ -168,7 +168,7 @@ 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-1 mechanism provides a simple
+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.
 
@@ -191,11 +191,12 @@ reasonable configuration is
 
 ::
 
-   mech_list: scram-sha-1
+   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.
+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

qemu.sasl

@@ -19,15 +19,15 @@ mech_list: gssapi
 
 # If using TLS with VNC, or a UNIX socket only, it is possible to
 # enable plugins which don't provide session encryption. The
-# 'scram-sha-1' plugin allows plain username/password authentication
+# 'scram-sha-256' plugin allows plain username/password authentication
 # to be performed
 #
-#mech_list: scram-sha-1
+#mech_list: scram-sha-256
 
 # You can also list many mechanisms at once, and the VNC server will
 # negotiate which to use by considering the list enabled on the VNC
 # client.
-#mech_list: scram-sha-1 gssapi
+#mech_list: scram-sha-256 gssapi
 
 # Some older builds of MIT kerberos on Linux ignore this option &
 # instead need KRB5_KTNAME env var.
@@ -38,7 +38,8 @@ mech_list: gssapi
 # mechanism this can be commented out.
 keytab: /etc/qemu/krb5.tab
 
-# If using scram-sha-1 for username/passwds, then this is the file
+# If using scram-sha-256 for username/passwds, then this is the file
 # containing the passwds. Use 'saslpasswd2 -a qemu [username]'
-# to add entries, and 'sasldblistusers2 -f [sasldb_path]' to browse it
+# to add entries, and 'sasldblistusers2 -f [sasldb_path]' to browse it.
+# Note that this file stores passwords in clear text.
 #sasldb_path: /etc/qemu/passwd.db