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182 lines
7.7 KiB
XML
182 lines
7.7 KiB
XML
<?xml version='1.0' encoding='UTF-8'?>
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<!DOCTYPE xep SYSTEM 'xep.dtd' [
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<!ENTITY rfc5056 "<span class='ref'><link url='http://tools.ietf.org/html/rfc5056'>RFC 5056</link></span> <note>RFC 5056: On the Use of Channel Bindings to Secure Channels <<link url='http://tools.ietf.org/html/rfc5056'>http://tools.ietf.org/html/rfc5056</link>>.</note>" >
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<!ENTITY iana-cb-types "<span class='ref'><link url='https://www.iana.org/assignments/channel-binding-types/channel-binding-types.xhtml'>IANA Channel-Binding Types Registry</link></span> <note>IANA Channel-Binding Types Registry <<link url='https://www.iana.org/assignments/channel-binding-types/channel-binding-types.xhtml'>https://www.iana.org/assignments/channel-binding-types/channel-binding-types.xhtml</link>>.</note>" >
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<!ENTITY % ents SYSTEM 'xep.ent'>
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%ents;
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]>
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<?xml-stylesheet type='text/xsl' href='xep.xsl'?>
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<xep>
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<header>
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<title>SASL Channel-Binding Type Capability</title>
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<abstract>This specification allows servers to annouce their supported SASL channel-binding types to clients.</abstract>
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&LEGALNOTICE;
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<number>0440</number>
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<status>Experimental</status>
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<type>Standards Track</type>
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<sig>Standards</sig>
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<approver>Council</approver>
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<dependencies>
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<spec>XMPP Core</spec>
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</dependencies>
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<supersedes/>
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<supersededby/>
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<shortname>sasl-cb-types</shortname>
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&flow;
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<revision>
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<version>0.2.0</version>
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<date>2020-08-04</date>
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<initials>fs</initials>
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<remark>
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Discuss interaction with SASL mechanism and add security considerations.
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Recommend implementation of tls-server-end-point.
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</remark>
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</revision>
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<revision>
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<version>0.1.0</version>
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<date>2020-06-14</date>
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<initials>XEP Editor (jsc)</initials>
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<remark>Accepted by vote of Council on 2020-05-27.</remark>
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</revision>
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<revision>
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<version>0.0.1</version>
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<date>2020-05-20</date>
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<initials>fs</initials>
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<remark><p>First draft.</p></remark>
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</revision>
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</header>
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<section1 topic='Introduction' anchor='intro'>
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<p>SASL channel-binding is a technique to increase the security of
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connections (&rfc5056;). Unfortunately, the SASL profile specified
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in &rfc6120; lacks a method for the server to announce its supported
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channel-binding types. This hinders the adoption of channel-binding,
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especially since the error protocol to execute after a client
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requested a channel-binding type unsupported by the server is
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basically unspecified.</p>
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<p>The extension defined herein fills the gap left by &rfc6120;, by
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allowing the server the announce its supported channel-binding
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types.</p>
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</section1>
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<section1 topic='Announcing the SASL Channel-Binding Type Capability' anchor='sasl-cb-type'>
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<p>This protocol consists of a single optional extension element
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named 'sasl-channel-binding' qualified by the 'urn:xmpp:sasl-cb:0'
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namespace. The 'sasl-channel-binding' element MUST contain one or
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more 'channel-binding' elements, of which each MUST have an
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attribute with the name 'type'. The value of the 'type' attribute
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SHOULD be the "Channel-binding unique prefix" of a channel-binding
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type which was registered with the &iana-cb-types;.</p>
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<p>A server declares that it supports particular channel-binding
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types by listing the supported types via the 'sasl-channel-binding'
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element defined herein. The 'sasl-channel-binding' element could
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appear as child element to the SASL <mechanisms/>
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stream-feature element, qualified by the
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'urn:ietf:params:xml:ns:xmpp-sasl' namespace, as specified in
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&rfc6120;. Another potential appearance of
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<sasl-channel-binding> is as child element of the
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<mechanisms/> stream-feature element as specified in the
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&xep0388;.</p>
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<example caption='Example <mechanisms/> stream feature with SASL Channel-Binding Type Capability.'><![CDATA[
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<stream:features>
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<mechanisms xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
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<mechanism>EXTERNAL</mechanism>
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<mechanism>SCRAM-SHA-1-PLUS</mechanism>
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<mechanism>PLAIN</mechanism>
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<sasl-channel-binding xmlns='urn:xmpp:sasl-cb:0'>
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<channel-binding type='tls-server-end-point'/>
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<channel-binding type='tls-exporter'/>
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</sasl-channel-binding>
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</mechanisms>
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</stream:features>]]></example>
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</section1>
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<section1 topic='Interaction with SASL mechanisms' anchor='sasl-mech-interaction'>
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<p>Some channel-binding enabled SASL mechanisms reflect the server's
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presumed channel-binding abilities back to the server. This prevents
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SASL-mechanism stripping attacks, where a Man in the Middle (MITM)
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removes certain SASL mechanisms in an attempt to downgrade the
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mechanism choosen for authentication to a non-channel-binding enabled
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one. An example of a SASL mechanism family with this feature is
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&rfc5802;. This standard specifies the gs2-cbind-flag. The flag has a
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tristate value of "I don't support channel-binding" (n), "I think you
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do not support channel-binding, but I do" (y), or, "Let us use
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channel-binding type X" (p).</p>
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<p>Clients using the information provided
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via <sasl-channel-binding/> MAY want to indicate to the server
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that they do not support channel-binding (even if they do) if no
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mutual supported channel-binding type was found. The only alternative
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is, that the client signals the server that he believes that the server
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does not support channel binding. But this may cause the server to
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terminate the connection, because it indicates a potential ongoing
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SASL-mechanism stripping attack.</p>
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</section1>
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<section1 topic='Security Considerations' anchor='security'>
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<p>If a client signals to the server that he does not support
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channel binding, because it found no mutual supported
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channel-binding types, another MITM attack
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vector is introduced. An active attacker could replace the
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<sasl-channel-binding;> list with channel bindings unlikely
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(or impossible) to be supported by the client. If the client is
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configured to use non-channel-binding SASL mechanisms as a fallback,
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this could be used to downgrade the connection security. Note that
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this attack is a different one than the SASL-mechanism stripping one:
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Here the attacker tempers with the announced channel-binding types,
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i.e., the values within <sasl-channel-binding;></p>
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<p>Depending on the application's security policy, clients may
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refrain from falling back to non-channel-binding SASL mechanisms
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if no mutual supported channel-binding type is available.
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Alternatively, they may try channel-binding with a supported type
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nevertheless. To mitigate the attack describe above, clients
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could "pin" the announced channel bindings types by a service. In that
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case, implementations may want to allow the set of pinned channel-binding
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types to be extended to stronger ones.</p>
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<p>As further mitigation, it is RECOMMENDED to implement the
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channel-binding type tls-server-end-point (&rfc5929;) to increase the
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probability of a mutual supported channel-binding type.</p>
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</section1>
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<section1 topic='IANA Considerations' anchor='iana'>
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<p>This document requires no interaction with &IANA;.</p>
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</section1>
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<section1 topic='XMPP Registrar Considerations' anchor='registrar'>
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<p>This document requires no interaction with the XMPP registrar.</p>
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</section1>
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<section1 topic='XML Schema' anchor='schema'>
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<p>TODO: Add if the XEP is scheduled for the state after 'experimental'.</p>
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</section1>
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<section1 topic='Acknowledgements' anchor='acknowledgements'>
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<p>Thanks to Sam Whited for the discussion about the underlying
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issue and incentivizing me to come up with this extension. Further
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thanks goes to Ruslan N. Marchenko for pointing out the possible
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MITM attack vector. Last but not least, Dave Cridland provided
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valuable feedback.</p>
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</section1>
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</xep>
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