util.xml: Do not allow doctypes, comments or processing instructions
Yes. This is as bad as it sounds. CVE pending.
In Prosody itself, this only affects mod_websocket, which uses util.xml
to parse the <open/> frame, thus allowing unauthenticated remote DoS
using Billion Laughs. However, third-party modules using util.xml may
also be affected by this.
This commit installs handlers which disallow the use of doctype
declarations and processing instructions without any escape hatch. It,
by default, also introduces such a handler for comments, however, there
is a way to enable comments nontheless.
This is because util.xml is used to parse human-facing data, where
comments are generally a desirable feature, and also because comments
are generally harmless.
# MUC: Members-only rooms kick members who deregister
[Client] Romeo
jid: user@localhost
password: password
[Client] Juliet
jid: user2@localhost
password: password
-----
Romeo connects
Romeo sends:
<presence to="room@conference.localhost/Romeo">
<x xmlns="http://jabber.org/protocol/muc"/>
</presence>
Romeo receives:
<presence from='room@conference.localhost/Romeo'>
<x xmlns='http://jabber.org/protocol/muc#user'>
<status code='201'/>
<item jid="${Romeo's full JID}" affiliation='owner' role='moderator'/>
<status code='110'/>
</x>
</presence>
Romeo receives:
<message type='groupchat' from='room@conference.localhost'><subject/></message>
# Submit config form, set the room to members-only
Romeo sends:
<iq id='config1' to='room@conference.localhost' type='set'>
<query xmlns='http://jabber.org/protocol/muc#owner'>
<x xmlns='jabber:x:data' type='submit'>
<field var='FORM_TYPE'>
<value>http://jabber.org/protocol/muc#roomconfig</value>
</field>
<field var='muc#roomconfig_membersonly'>
<value>1</value>
</field>
</x>
</query>
</iq>
Romeo receives:
<iq id="config1" from="room@conference.localhost" type="result">
</iq>
# Romeo adds Juliet to the member list
Romeo sends:
<iq id='member1' to='room@conference.localhost' type='set'>
<query xmlns='http://jabber.org/protocol/muc#admin'>
<item affiliation='member' jid="${Juliet's JID}" />
</query>
</iq>
Romeo receives:
<message from='room@conference.localhost'>
<x xmlns='http://jabber.org/protocol/muc#user'>
<item jid="${Juliet's JID}" affiliation='member' />
</x>
</message>
Romeo receives:
<iq from='room@conference.localhost' id='member1' type='result'/>
# Juliet connects, and joins the room
Juliet connects
Juliet sends:
<presence to="room@conference.localhost/Juliet">
<x xmlns="http://jabber.org/protocol/muc"/>
</presence>
Juliet receives:
<presence from="room@conference.localhost/Romeo" />
Juliet receives:
<presence from="room@conference.localhost/Juliet" />
Juliet receives:
<message type='groupchat' from='room@conference.localhost'><subject/></message>
Romeo receives:
<presence from="room@conference.localhost/Juliet" />
# Tired of Romeo's company, Juliet unregisters from the room
Juliet sends:
<iq type='set' to='room@conference.localhost' id='unreg1'>
<query xmlns='jabber:iq:register'>
<remove/>
</query>
</iq>
Juliet receives:
<presence type='unavailable' from='room@conference.localhost/Juliet'>
<x xmlns='http://jabber.org/protocol/muc#user'>
<status code='321'/>
<item affiliation='none' role='none' jid="${Juliet's full JID}" />
<status code='110'/>
</x>
</presence>
Juliet receives:
<iq type='result' from='room@conference.localhost' id='unreg1'/>
Romeo receives:
<presence type='unavailable' from='room@conference.localhost/Juliet'>
<x xmlns='http://jabber.org/protocol/muc#user'>
<status code='321'/>
<item affiliation='none' role='none' jid="${Juliet's full JID}" />
</x>
</presence>
Romeo disconnects
Juliet disconnects