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.
# XEP-0054 vCard-temp writable and readable by anyone
# mod_scansion_record on host 'localhost' recording started 2018-10-20T15:00:12Z
[Client] Romeo
jid: romeo@localhost
password: password
[Client] Juliet
jid: juliet@localhost
password: password
-----
Romeo connects
# Romeo sets his vCard
# FN and N are required by the schema and mod_vcard_legacy will always inject them
Romeo sends:
<iq id="lx3" type="set">
<vCard xmlns="vcard-temp">
<FN>Romeo Montague</FN>
<N>
<FAMILY>Montague</FAMILY>
<GIVEN>Romeo</GIVEN>
<MIDDLE/>
<PREFIX/>
<SUFFIX/>
</N>
</vCard>
</iq>
Romeo receives:
<iq type="result" id="lx3" to="${Romeo's full JID}"/>
Romeo sends:
<iq id="lx4" type="get">
<vCard xmlns="vcard-temp"/>
</iq>
Romeo receives:
<iq type="result" id="lx4" to="${Romeo's full JID}">
<vCard xmlns="vcard-temp">
<FN>Romeo Montague</FN>
<N>
<FAMILY>Montague</FAMILY>
<GIVEN>Romeo</GIVEN>
<MIDDLE/>
<PREFIX/>
<SUFFIX/>
</N>
</vCard>
</iq>
Romeo disconnects
Juliet connects
Juliet sends:
<iq type="get" id="lx3" to="romeo@localhost">
<vCard xmlns="vcard-temp"/>
</iq>
# Juliet can see Romeo's vCard since it's public
Juliet receives:
<iq type="result" from="romeo@localhost" id="lx3" to="${Juliet's full JID}">
<vCard xmlns="vcard-temp">
<FN>Romeo Montague</FN>
<N>
<FAMILY>Montague</FAMILY>
<GIVEN>Romeo</GIVEN>
<MIDDLE/>
<PREFIX/>
<SUFFIX/>
</N>
</vCard>
</iq>
Juliet disconnects
# recording ended on 2018-10-20T15:02:14Z