[whatwg] [mimesniff] First pass at speccing the X-Content-Type-Options header

Gordon P. Hemsley gphemsley at gmail.com
Fri May 10 12:15:11 PDT 2013

I've made a first pass at speccing the X-Content-Type-Options header:


As it stands now:

* If any resource retrieved over HTTP has an X-Content-Type-Options
header with a value of "nosniff", sniffing is turned off. (That means
multiple headers, with possible other values, will not turn sniffing
back on.)

* Sniffing can only be turned off by the X-Content-Type-Options in
contexts that call the main MIME type sniffing algorithm. In general,
that means sniffing is only turned off in contexts that do not have a
type hint. Contexts such as <img> call context-sniffing algorithms
from the HTML spec, and so are subject to different algorithmic
restrictions. However, <script> does not have its own context-specific
algorithm; I don't know how the HTML spec handles that. I also don't
know how the HTML spec handles <iframe>, but I assume that it calls
the main MIME type sniffing algorithm as any primary browsing context

* If the X-Content-Type-Options header turns sniffing off, but the
supplied MIME type is unknown, non-scriptable types are still sniffed,
because we need to have some idea of what the resource is. (If no
known signatures are detected, the algorithm ends with either
"text/plain" or "application/octet-stream", depending on whether there
are any binary bytes in the resource header.)

Now, I have to admit up front that I didn't actually test whether this
behavior matches existing implementations. Instead, I used others'
reports [1][2] as a basis and made the least number of changes to the
existing spec as I probably could. I know Mozilla is looking for what
to implement and this, as now specced, seems like reasonable behavior
to me.

Let us now commence quibbling about the details.

[1] http://lists.whatwg.org/htdig.cgi/whatwg-whatwg.org/2012-November/037983.html
[2] https://bugzilla.mozilla.org/show_bug.cgi?id=471020

Gordon P. Hemsley
me at gphemsley.org

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