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: Mediated invites
[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="${Romeo's full JID}" affiliation='owner' role='moderator'/>
<status code='110'/>
</x>
</presence>
Romeo receives:
<message type='groupchat' from='room@conference.localhost'><subject/></message>
# Submit config form
Romeo sends:
<iq id='config1' to='room@conference.localhost' type='set'>
<query xmlns='http://jabber.org/protocol/muc#owner'>
<x xmlns='jabber:x:data' type='submit'>
<field var='FORM_TYPE'>
<value>http://jabber.org/protocol/muc#roomconfig</value>
</field>
</x>
</query>
</iq>
Romeo receives:
<iq id="config1" from="room@conference.localhost" type="result">
</iq>
# Juliet connects
Juliet connects
Juliet sends:
<presence/>
Juliet receives:
<presence/>
# Romeo invites Juliet to join the room
Romeo sends:
<message to="room@conference.localhost" id="invite1">
<x xmlns="http://jabber.org/protocol/muc#user">
<invite to="${Juliet's JID}" />
</x>
</message>
Juliet receives:
<message from="room@conference.localhost" id="invite1">
<x xmlns="http://jabber.org/protocol/muc#user">
<invite from="room@conference.localhost/Romeo">
<reason/>
</invite>
</x>
<body>room@conference.localhost/Romeo invited you to the room room@conference.localhost</body>
<x xmlns="jabber:x:conference" jid="room@conference.localhost"/>
</message>