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Subject: GCS Spoofing
The ABNF for XRI accommodates two types of authority: XRI authority and IRI authority. An XRI authority must begin with a GCS character: "!", "=", "@", "+" or "$". However, if the XRI reference begins with a character that is visually indistinguishable from one of the GCS characters, and the code point is allowed in the the "iunreserved" production, a machine processor would treat it as an IRI authority. This may be a cause for concern since it opens the door for spoofing. For example, an actual XRI may be: xri://@paypal*services/send-money and it could be spoofed (although not exactly) by using: xri://@paypal*services/send-money/sub.bad-domain.com/trustme.html And the "@" sign above is U+FE6B (small commercial at), "*" is U+FE61 (small asterisk), and the 4th and 5th "/" are U+2215 (division slash). In effect, the XRI would be interpreted as having an IRI authority (an IDN) of: "@paypal*services/send-money/sub.bad-domain.com". This is possible largely because IDNA allows the slash-like character in an IDN label, giving rise to the possibility of syntax spoofing. And because sub domains appear to right of their parent domains, the malicious domain can be created as a third (or higher) level sub domain so that it is outside of the registry's control. This is merely a subclass of syntax spoofing homograph attacks on IDN which applies to XRIs. I'm not sure where is the best place to address it, or if it would even be deemed a problem that the XRI TC would want to solve. If we do, there are at least ways to go about fixing it, at the syntax level (i.e. by mapping or limiting the use of GCS characters) or as a recommendation for XRI clients to disallow or warn the user of GCS-like characters appearing at the beginning of an XRI reference. wil.
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