With security, its safer (and more accurate) to assume compatibility
breakage than not. Its not just syntax that can change but the rules
themselves. For example if we identify new vectors for code
injection, we might have to block additional APIs thus breaking sites
that would otherwise effectively support CSP without any change in
syntax.
Even something as relatively simple to reason about like HTTP itself
is versioned, that is why each transaction starts with the HTTP
version for the request. CSP is a complex security model; I would say
backwards breakage in the future is inevitable regardless of how much
we churn on it now, given it will have to evolve hand in hand with our
understanding of the threat models. It cannot anticipate attack
vectors we don't yet know about.
Lucas.
On Jul 8, 2009, at 9:21 AM, Gervase Markham wrote:
So the versioning in the UA is to guard against a policy syntax
change. But the syntax is so simple a list of (key/value pairs) that
it's very, very hard to imagine a requirement which would mean we
*had* to break the syntax. And yet, every request the browser ever
sends acquires another six bytes, until the end of time. (This is
not a UA token which changes over time as browsers change, like OS,
it's one which has to be present for ever.)
I don't think the risk of needing a breaking syntax change is worth
it. In that very unlikely event, we should instead plan to deploy a
new header, as you say below. There's more downstream bandwidth than
upstream, and there's more every year.
The other approach is to version the response, a few extra bytes only
when a server supports CSP. Yay, bandwidth win! But then what do we
do?
How does the server know which version to send? Should it send every
version it knows about, and the client process the highest version it
knows how to process? That means if we ever have a CSP-2 either
clients
are sending two complete headers (or three, or more) or they're
sending
their preferred version and users of clients which only support CSP-1
get zero protection rather than the 99% they actually support.
But this scary scenario fails to take into account the frankly tiny
chance that we'll need to make one breaking syntax change, let alone
two. Even with the spec as it is. Careful design can reduce the
chances even further.
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