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Switch to GSSAPI (kerberos) instead of the insecure DIGEST-MD5
RFC 6331 documents a number of serious security weaknesses in the SASL DIGEST-MD5 mechanism. As such, libvirtd should not by using it as a default mechanism. GSSAPI is the only other viable SASL mechanism that can provide secure session encryption so enable that by defalt as the replacement. Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
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Daniel P. Berrange
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@ -1,31 +1,45 @@
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# If you want to use the non-TLS socket, then you *must* include
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# the GSSAPI or DIGEST-MD5 mechanisms, because they are the only
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# ones that can offer session encryption as well as authentication.
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# If you want to use the non-TLS socket, then you *must* pick a
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# mechanism which provides session encryption as well as
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# authentication.
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#
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# If you're only using TLS, then you can turn on any mechanisms
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# If you are only using TLS, then you can turn on any mechanisms
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# you like for authentication, because TLS provides the encryption
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#
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# Default to a simple username+password mechanism
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mech_list: digest-md5
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# If you are only using UNIX, sockets then encryption is not
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# required at all.
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#
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# Since SASL is the default for the libvirtd non-TLS socket, we
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# pick a strong mechanism by default.
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#
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# NB, previously DIGEST-MD5 was set as the default mechanism for
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# libvirt. Per RFC 6331 this is vulnerable to many serious security
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# flaws and should no longer be used. Thus GSSAPI is now the default.
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#
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# To use GSSAPI requires that a libvirtd service principal is
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# added to the Kerberos server for each host running libvirtd.
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# This principal needs to be exported to the keytab file listed below
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mech_list: gssapi
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# If using a TLS socket or UNIX socket only, it is possible to
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# enable plugins which don't provide session encryption. The
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# 'scram-sha-1' plugin allows plain username/password authentication
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# to be performed
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#
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#mech_list: scram-sha-1
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# Before you can use GSSAPI, you need a service principle on the
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# KDC server for libvirt, and that to be exported to the keytab
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# file listed below
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#mech_list: gssapi
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#
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# You can also list many mechanisms at once, then the user can choose
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# by adding '?auth=sasl.gssapi' to their libvirt URI, eg
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# qemu+tcp://hostname/system?auth=sasl.gssapi
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#mech_list: digest-md5 gssapi
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#mech_list: scram-sha-1 gssapi
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# Some older builds of MIT kerberos on Linux ignore this option &
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# instead need KRB5_KTNAME env var.
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# For modern Linux, and other OS, this should be sufficient
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#
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# There is no default value here, uncomment if you need this
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#keytab: /etc/libvirt/krb5.tab
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keytab: /etc/libvirt/krb5.tab
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# If using digest-md5 for username/passwds, then this is the file
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# If using scram-sha-1 for username/passwds, then this is the file
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# containing the passwds. Use 'saslpasswd2 -a libvirt [username]'
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# to add entries, and 'sasldblistusers2 -f [sasldb_path]' to browse it
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sasldb_path: /etc/libvirt/passwd.db
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#sasldb_path: /etc/libvirt/passwd.db
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@ -204,16 +204,72 @@ ResultActive=yes</pre>
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Further examples of PolicyKit setup can be found on the
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<a href="http://wiki.libvirt.org/page/SSHPolicyKitSetup">wiki page</a>.
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</p>
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<h2><a name="ACL_server_username">Username/password auth</a></h2>
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<h2><a name="ACL_server_sasl">SASL pluggable authentication</a></h2>
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<p>
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The plain TCP socket of the libvirt daemon defaults to using SASL for authentication.
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The SASL mechanism configured by default is DIGEST-MD5, which provides a basic
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username+password style authentication. It also provides for encryption of the data
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stream, so the security of the plain TCP socket is on a par with that of the TLS
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socket. If desired the UNIX socket and TLS socket can also have SASL enabled by
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setting the <code>auth_unix_ro</code>, <code>auth_unix_rw</code>, <code>auth_tls</code>
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config params in <code>libvirt.conf</code>.
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</p>
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Libvirt integrates with the cyrus-sasl library to provide a pluggable authentication
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system using the SASL protocol. SASL can be used in combination with libvirtd's TLS
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or TCP socket listeners. When used with the TCP listener, the SASL mechanism is
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rqeuired to provide session encryption in addition to authentication. Only a very
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few SASL mechanisms are able to do this, and of those that can do it, only the
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GSSAPI plugin is considered acceptably secure by modern standards:
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</p>
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<dl>
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<dt>GSSAPI</dt>
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<dd><strong>This is the current default mechanism to use with libvirtd</strong>.
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It uses the Kerberos v5 authentication protocol underneath, and assuming
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the Kerberos client/server are configured with modern ciphers (AES),
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it provides strong session encryption capabilities.</dd>
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<dt>DIGEST-MD5</dt>
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<dd>This was previously set as the default mechanism to use with libvirtd.
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It provides a simple username/password based authentication mechanism
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that includes session encryption.
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<a href="https://tools.ietf.org/html/rfc6331">RFC 6331</a>, however,
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documents a number of serious security flaws with DIGEST-MD5 and as a
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result marks it as <code>OBSOLETE</code>. Specific concerns are that
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it is vulnerable to MITM attacks and the MD5 hash can be brute-forced
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to reveal the password. A replacement is provided via the SCRAM mechanism,
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however, note that this does not provide encryption, so the SCRAM
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mechanism can only be used on the libvirtd TLS listener.
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</dd>
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<dt>PASSDSS-3DES-1</dt>
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<dd>This provides a simple username/password based authentication
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mechanism that includes session encryption. The current cyrus-sasl
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implementation does not provide a way to validate the server's
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public key identity, thus it is susceptible to a MITM attacker
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impersonating the server. It is also not enabled in many OS
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distros when building SASL libraries.</dd>
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<dt>KERBEROS_V4</dt>
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<dd>This uses the obsolete Kerberos v4 protocol to provide both authentication
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and session encryption. Kerberos v4 protocol has been obsolete since the
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early 1990's and has known security vulnerabilities so this will never be
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used in practice.</dd>
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</dl>
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<p>
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Other SASL mechanisms, not listed above, can only be used when the libvirtd
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TLS or UNIX socket listeners.
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</p>
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<h3><a name="ACL_server_username">Username/password auth</a></h3>
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<p>
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As noted above, the DIGEST-MD5 mechanism is considered obsolete and should
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not be used anymore. To provide a simple username/password auth scheme on
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the libvirt UNIX socket or TLS listeners, however, it is possible to use
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the SCRAM mechanism. The <code>auth_unix_ro</code>, <code>auth_unix_rw</code>,
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<code>auth_tls</code> config params in <code>libvirt.conf</code> can be used
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to turn on SASL auth in these listeners.
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</p>
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<p>
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Since the libvirt SASL config file defaults to using GSSAPI (Kerberos), a
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config change is rquired to enable plain password auth. This is done by
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editting <code>/etc/sasl2/libvirt.conf</code> to set the <code>mech_list</code>
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parameter to <code>scram-sha-1</code>.
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</p>
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<p>
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Out of the box, no user accounts are defined, so no clients will be able to authenticate
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on the TCP socket. Adding users and setting their passwords is done with the <code>saslpasswd2</code>
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@ -241,17 +297,13 @@ again:
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<pre>
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# saslpasswd2 -a libvirt -d fred
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</pre>
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<h2><a name="ACL_server_kerberos">Kerberos auth</a></h2>
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<h3><a name="ACL_server_kerberos">GSSAPI/Kerberos auth</a></h3>
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<p>
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The plain TCP socket of the libvirt daemon defaults to using SASL for authentication.
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The SASL mechanism configured by default is DIGEST-MD5, which provides a basic
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username+password style authentication. To enable Kerberos single-sign-on instead,
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the libvirt SASL configuration file must be changed. This is <code>/etc/sasl2/libvirt.conf</code>.
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The <code>mech_list</code> parameter must first be changed to <code>gssapi</code>
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instead of the default <code>digest-md5</code>, and keytab should be set to
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<code>/etc/libvirt/krb5.tab</code> . If SASL is enabled on the UNIX
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and/or TLS sockets, Kerberos will also be used for them. Like DIGEST-MD5, the Kerberos
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mechanism provides data encryption of the session.
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The plain TCP listener of the libvirt daemon defaults to using SASL for authentication.
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The libvirt SASL config also defaults to GSSAPI, so there is no need to edit the
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SASL config when using GSSAPI. If the libvirtd TLS or UNIX listeners are used,
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then the Kerberos session encryption will be disabled since it is not required
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in these scenarios - only the plain TCP listener needs encryption
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</p>
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<p>
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Some operating systems do not install the SASL kerberos plugin by default. It
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@ -979,9 +979,9 @@ Group: Development/Libraries
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# (client invokes 'nc' against the UNIX socket on the server)
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Requires: nc
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Requires: cyrus-sasl
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# Not technically required, but makes 'out-of-box' config
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# work correctly & doesn't have onerous dependencies
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Requires: cyrus-sasl-md5
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# Needed by default sasl.conf - no onerous extra deps, since
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# 100's of other things on a system already pull in krb5-libs
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Requires: cyrus-sasl-gssapi
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%description libs
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Shared libraries for accessing the libvirt daemon.
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