[CSP] 'allow-meta': proposal for combining header and <meta> policies

Many of you have complained that we don't allow <meta>-specified
security policies if there is already a header-specified policy, and
about the restriction on multiple meta-specified policies. Certainly
there are cases where allowing a meta policy to refine a more general
site policy would be useful if we can do so without enabling new attacks
via the very mechanism we're building.

I propose an 'allow-meta;' directive. When specified in header policies
(would have to be in all policies if there are multiple headers) this
would signal the browser that a meta-specified policy refinement is
expected and is not an injection. The first meta-specified security
policy would then be enforced. Similarly, as long as meta-specified
policies contain the "allow-meta" directive the UA could expect and
process additional policies in the document.

Taking a stab at updating the steps in "3.1.3 HTML meta Element":

  Content security policy (http-equiv="content-security-policy")

     1. If the user agent is already enforcing a policy for
        the document and the 'allow-meta' flag has not
        been set, abort these steps.

And for language for the directive itself:

  3.2.5.? allow-meta

  The allow-meta directive enables the processing of an additional
  policy specified through a <meta> element. The syntax for the
  name and value of the directive are described by the following
  ABNF grammar:

    directive-name     = "allow-meta"

  When the allow-meta directive is encountered the user agent
  MUST set the allow-meta flag which governs whether subsequent
  <meta> elements containing security policies will be processed.

I chose "allow-meta" because it generally matches our whitelisting
approach. It is easy enough to add if you need multiple meta-specified
policies but by default authors who don't think about it can't have
their pages damaged by unexpected injected <meta> elements. However,
since most of you don't seem worried about that I could also live with
flipping the sense and having a "no-meta-policy" directive instead. That
way paranoid folks like myself could protect against injected meta
policies (if we remember to do so).

-Dan Veditz

Received on Friday, 14 February 2014 17:34:09 UTC