AW: [CSP] different perspective on Report-Only

But, to stress my main point:

The spec must make it clear to users and browser implementors that Content-Security-Policy headers have always precedence over Content-Security-Policy-Report-Only headers,

so that an attacker cannot deactivate a policy that is in place by adding Content-Security-Policy-Report-Only headers.

Kind Regards,

Von: Mike West []
Gesendet: Montag, 29. Dezember 2014 16:59
An: Martin Thomson
Cc: Ludwig, Sven;
Betreff: Re: [CSP] different perspective on Report-Only

We currently intentionally allow multiple reporting endpoints (even in a single directive), and I believe folks are using at least the ability to specify one "prod" endpoint for the `Content-Security-Policy` header, and one "testing" endpoint for the `Content-Security-Policy-Report-Only` header. I've had conversations with folks interested in testing multiple levels of report-only (e.g. "Here's what we think we can do next, and here's what we'd like to be doing."), but I don't know if anyone has put that into practice. We could certainly add metrics to Chrome and Firefox to see how widespread that is. I suspect not very. :)

Given that report-only functions only when delivered as a response header, it seems that an attacker capable of injecting headers could do significantly better for herself than CSP-RO, especially given the limitations of cross-origin reporting (no paths, no cookies, etc).


Mike West <<>>, @mikewest

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On Sun, Dec 28, 2014 at 5:42 AM, Martin Thomson <<>> wrote:
Perhaps the right interpretation is "report to no-one but X".  That
would cause two conflicting directives to turn into "don't report to

(If you think of each directive as narrowing scope, then logically the
absence of a -report-only directive is to report to everyone, but I
think we can handle that.)

On 26 December 2014 at 17:40, Ludwig, Sven <<>> wrote:
> Hi,
> following the excellent talk by
> Adam Barth, CSP supports setting multiple policies (i.e. multiple
> Content-Security-Policy headers) in a response, which then all must be
> fulfilled. One reason for this principle as mentioned in the talk is that an
> attacker might somehow be able to add his own CSP header to the response,
> however without replacing existing headers coming from the server. In this
> case the attack does not open up security, because additional
> Content-Security-Policy headers can only introduce more restrictions.
> Having said that, the header Content-Security-Policy-Report-Only could be
> considered by an attacker to add, to open up security.
> Right now I am not sure if this could be an issue.
> If any Content-Security-Policy headers have precedence over any
> Content-Security-Policy-Report-Only headers, the attacker would still not be
> able to open up security in the above mentioned way. Actually, I expect it
> to work like that. This should be mentioned in the section

> Kind Regards,
> Sven
> Sven Ludwig
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Received on Monday, 29 December 2014 22:35:53 UTC