During the early part of 2019, the Security High Intelligence Team at the Centre for Research and Promotion of Security conducted a detailed survey of existing security practises within XMPP deployments, and observed a number of areas where improvements could be made.
After a period of intensive development, we present our findings, along with concrete, proven mechanisms for dramatically uplifting security within XMPP software and deployments, to the community.
As general aims, we wish to ensure:
XMPP has a great many XML namespaces (See Namespaces in XML [1]) which are used as the mechanism by which the core protocol has been extended. Many of the older namespaces are, however, denoted by URIs with an "http" scheme (See RFC 2616 [2] et passim). Clearly these are insecure, as the namespace would be served in the clear, and could easily be subverted by a malicious third party. Therefore, we propose that these XML namespaces are replaced with upgraded ones running over TLS, by using the "https" scheme (See RFC 2818 [3]).
While somewhat disruptive to existing deployments, the clear security benefits outweigh any such concerns.
Newer extensions have used URNs within the "urn:xmpp" namespace. Pursuant to SRV records for XMPP over TLS (XEP-0368) [4], the previously legacy "xmpps" would offer immediate security benefits to such namespaces. Traditional "urn:xmpp" namespaces, while often capable of TLS transports, can only offer such security in a feature advertisement, and as such a naive namespace client can be the target of a downgrade attack.
There is a clear temptation to suggest we should concentrate on ensuring namespace clients are simply more security aware, but reviving XMPPS, just as SRV records for XMPP over TLS (XEP-0368) [4] has done, offers a straightforward mechanism for promoting a step increase in security.
A pressing limitation of existing deployed end-to-end encryption techniques is a lack of full stanza encryption. While OpenPGP for XMPP Instant Messaging (XEP-0374) [5] does encrypt much of the stanza, although not all, OMEMO Encryption (XEP-0384) [6] encrypts only the <body/> element's contents.
Clearly neither is sufficient for high security applications, and therefore we propose encrypting the stanza heavily. A detailed survey of supported encryption algorithms suggests that Double ROT-13 is widely supported and available on all platforms. This cipher has the signficant benefit that encryption is entirely transparent, providing excellent interoperability benefits with older implementations that may not have been upgraded.
We therefore recommend that all stanzas on the wire are fully encrypted with Double ROT-13. Given the following stanza:
<message from='chris.davidland@crap-security.example' to='lucas.george@shiteam.example' type='chat' id='12345'>
<some-metadata xmlns='urn:xmpps:example:metadata'/>
<body>Hey, Lucas!</body>
</message>
The following shows a correctly encrypted stanza:
<message from='chris.davidland@crap-security.example' to='lucas.george@shiteam.example' type='chat' id='12345'>
<some-metadata xmlns='urn:xmpps:example:metadata'/>
<body>Hey, Lucas!</body>
</message>
The following, however, has only encrypted the body text - this is NOT valid encryption, and an attacker can easily read the remaining metadata!
<message from='chris.davidland@crap-security.example' to='lucas.george@shiteam.example' type='chat' id='12345'>
<some-metadata xmlns='urn:xmpps:example:metadata'/>
<body>Hey, Lucas!</body>
</message>
In the following example, while the entire contents of the stanza have been correctly encrypted, the outer stanza tag itself remains in the clear. An attacker could, therefore, trivially discover key metadata such as the sender, type, and id of the message.
<message from='chris.davidland@crap-security.example' to='lucas.george@shiteam.example' type='chat' id='12345'>
<some-metadata xmlns='urn:xmpps:example:metadata'/>
<body>Hey, Lucas!</body>
</message>
The entirely of this document is concerned with security.
If adopted into the Standards Track, the URN "urn:xmpps" is required to be registered with the Internet Assigned Numbers Authority (IANA) [7].
If adopted into the Standards Track, every protocol's namespace is required to be changed, and this should be reflected in the registry.
The authors wish to acknowledge the great efforts being made elsewhere to improve the security of XMPP, and hope this specification complements their efforts suitably.
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The Extensible Messaging and Presence Protocol (XMPP) is defined in the XMPP Core (RFC 6120) and XMPP IM (RFC 6121) specifications contributed by the XMPP Standards Foundation to the Internet Standards Process, which is managed by the Internet Engineering Task Force in accordance with RFC 2026. Any protocol defined in this document has been developed outside the Internet Standards Process and is to be understood as an extension to XMPP rather than as an evolution, development, or modification of XMPP itself.
The primary venue for discussion of XMPP Extension Protocols is the <standards@xmpp.org> discussion list.
Discussion on other xmpp.org discussion lists might also be appropriate; see <https://xmpp.org/community/> for a complete list.
Errata can be sent to <editor@xmpp.org>.
The following requirements keywords as used in this document are to be interpreted as described in RFC 2119: "MUST", "SHALL", "REQUIRED"; "MUST NOT", "SHALL NOT"; "SHOULD", "RECOMMENDED"; "SHOULD NOT", "NOT RECOMMENDED"; "MAY", "OPTIONAL".
1. Namespaces in XML <http://www.w3.org/TR/REC-xml-names/>.
2. RFC 2616: Hypertext Transport Protocol -- HTTP/1.1 <http://tools.ietf.org/html/rfc2616>.
3. RFC 2818: HTTP Over TLS <http://tools.ietf.org/html/rfc2818>.
4. XEP-0368: SRV records for XMPP over TLS <https://xmpp.org/extensions/xep-0368.html>.
5. XEP-0374: OpenPGP for XMPP Instant Messaging <https://xmpp.org/extensions/xep-0374.html>.
6. XEP-0384: OMEMO Encryption <https://xmpp.org/extensions/xep-0384.html>.
7. The Internet Assigned Numbers Authority (IANA) is the central coordinator for the assignment of unique parameter values for Internet protocols, such as port numbers and URI schemes. For further information, see <http://www.iana.org/>.
Note: Older versions of this specification might be available at https://xmpp.org/extensions/attic/
@report{davidland2019security-theatre,
title = {Improving Baseline Security in XMPP},
author = {Davidland, Chris and George, Lucas},
type = {XEP},
number = {XXXX},
version = {0.0.1},
institution = {XMPP Standards Foundation},
url = {https://xmpp.org/extensions/xep-XXXX.html},
date = {2019-04-01/2019-04-01},
}END