IDN processing-related security considerations for draft-ietf-websec-strict-transport-sec
Jeff.Hodges at KingsMountain.com
Fri Sep 30 05:07:15 CEST 2011
In working towards completion of..
HTTP Strict Transport Security (HSTS)
The Web Origin Concept
..we are attempting to address the proper way to reference IDNA2008 and
IDNA2003 in terms of stipulating comparisons of domain names (that may or may
not be IDNs).
In discussions with our ADs and a few IDNA-literate folks, we've been informed
that the IDNA-specific language in the recently-released RFC6265 HTTP State
Management spec isn't quite up to the standards they would like to see at this
Thus I've performed some surgery on draft-ietf-websec-strict-transport-sec and
have included below the specific section portions that are IDNA specific (this
is from my working copy which isn't quite yet overall ready tonight to submit
The key context to keep in mind when reviewing the below is that the key
"processing" -- essentially a domain name comparison -- will occur deep within
the bowels of HTTP clients, well along the processing pipeline for URIs, and
presumably after IDNA-canonicalization and requisite validation/testing has
occurred. However, the guidance we've received is that given the complexities
and subtleties of IDNA processing and considerations, our specs really should
be more explicit about the foregoing assumption(s) and the downside risks if
the requisite validation/testing is not performed.
With that context in mind, thoughts on the below are solicited. Apologies for
just having these excerpts at this time, but I ought to have
-websec-strict-transport-sec-03 submitted in the next few days at most.
7. User Agent Processing Model
This section describes the HTTP Strict Transport Security processing
model for UAs. There are several facets to the model, enumerated by
the following subsections.
This processing model assumes that the UA implements IDNA2008
[RFC5890], or possibly IDNA2003 [RFC3490], as noted in Section 11
"Internationalized Domain Names for Applications (IDNA): Dependency
and Migration". It also assumes that all domain names manipulated in
this specification's context are already IDNA-canonicalized as
outlined in Section 8 "Domain Name IDNA-Canonicalization" prior to
the processing specified in this section.
The above assumptions mean that this processing model also
specifically assumes that appropriate IDNA and Unicode validations
and character list testing have occured on the domain names, in
conjunction with their IDNA-canonicalization, prior to the processing
specified in this section. See the IDNA-specific security
considerations in Section 13.2 "Internationalized Domain Names" for
rationale and further details.
8. Domain Name IDNA-Canonicalization
An IDNA-canonicalized domain name is the string generated by the
following algorithm, whose input must be a valid Unicode-encoded (in
NFC form [Unicode6]) string-serialized domain name:
1. Convert the domain name to a sequence of individual domain name
2. When implementing IDNA2008, convert each label that is not a Non-
Reserved LDH (NR-LDH) label, to an A-label. See Section 2.3.2 of
[RFC5890] for definitions of the former and latter, refer to
Sections 5.3 through 5.5 of [RFC5891] for the conversion
algorithm and requisite input validation and character list
Otherwise, when implementing IDNA2003, convert each label using
the "ToASCII" conversion in Section 4 of [RFC3490] (see also the
definition of "equivalence of labels" in Section 2 of the latter
3. Concatenate the resulting labels, separating each label from the
next with (".") a %x2E character.
See also Section 11 "Internationalized Domain Names for Applications
(IDNA): Dependency and Migration" and Section 13.2 "Internationalized
Domain Names" of this specification for further details and
11. Internationalized Domain Names for Applications (IDNA): Dependency
Textual domain names on the modern Internet may contain one or more
"internationalized" domain name labels. Such domain names are
referred to as "internationalized domain names" (IDNs). The
specification suites defining IDNs and the protocols for their use
are named "Internationalized Domain Names for Applications (IDNA)".
At this time, there are two such specification suites: IDNA2008
[RFC5890] and its predecessor IDNA2003 [RFC3490].
IDNA2008 obsoletes IDNA2003, but there are differences between the
two specifications, and thus there can be differences in processing
(e.g. converting) domain name labels that have been registered under
one from those registered under the other. There will be a
transition period of some time during which IDNA2003-based domain
name labels will exist in the wild. User agents SHOULD implement
IDNA2008 [RFC5890] and MAY implement [RFC5895] (see also Section 7 of
[RFC5894]) or [UTS46] in order to facilitate their IDNA transition.
If a user agent does not implement IDNA2008, the user agent MUST
13. Security Considerations
13.2. Internationalized Domain Names
Internet security relies in part on the DNS and the domain names it
hosts. Domain names are used by users to identify and connect to
Internet hosts and other network resources. For example, Internet
security is compromised if a user entering an internationalized
domain name (IDN) is connected to different hosts based on different
interpretations of the IDN.
The processing models specified in this specification assume that the
domain names they manipulate are IDNA-canonicalized, and that the
canonicalization process correctly performed all appropriate IDNA and
Unicode validations and character list testing per the requisite
specifications (e.g., as noted in Section 8 "Domain Name IDNA-
Canonicalization"). These steps are necessary in order to avoid
various potentially compromising situations.
In brief, some examples of issues that could stem from lack of
careful and consistent Unicode and IDNA validations are things such
as unexpected processing exceptions, truncation errors, and buffer
overflows, as well as false-positive and/or false-negative domain
name matching results. Any of the foregoing issues could possibly be
leveraged by attackers in various ways.
Additionally, IDNA2008 [RFC5890] differs from IDNA2003 [RFC3490] in
terms of disallowed characters and character mapping conventions.
This situation can also lead to false-positive and/or false-negative
domain name matching results, resulting in, for example, users
possibly communicating with unintended hosts, or not being able to
reach intended hosts.
For details, refer to the Security Considerations sections of
[RFC5890], [RFC5891], and [RFC3490], as well as the specifications
they normatively reference. Additionally, [RFC5894] provides
detailed background and rationale for IDNA2008 in particular, as well
as IDNA and its issues in general, and should be consulted in
conjunction with the former specifications.
13.3. Denial of Service (DoS)
More information about the Idna-update