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Consider recording the "duplicate-attribute" error state. #3257

mikewest opened this Issue Nov 27, 2017 · 5 comments


6 participants

mikewest commented Nov 27, 2017

CSP attempts to defend against some kinds of dangling-markup attacks by preventing the execution of scripts that include "<script" in an attribute name or value. Discarding attributes during parsing makes it possible to trivially work around this defense, as described in As dropping repeated attributes is also the root cause of the dangling-markup risk in the first place, it might be reasonable to dig into it a little more deeply to see if something more fundamental can be done.

One idea we (briefly) discussed at TPAC is to prevent nonced script execution for <script> elements which had duplicate-attribute parse errors. An approach to this that I'm exploring in does more or less the following:

  1. Records duplicate-attribute parse errors on the tag during tokenization.
  2. Persists that error bit onto the Node generated during tree-building.
  3. Reads that error bit during "Is element nonceable?".


/cc @whatwg/security (@ckerschb, @dbates-wk, @patrickkettner in particular as folks working on CSP)

mikewest added a commit to w3c/webappsec-csp that referenced this issue Nov 27, 2017


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annevk commented Nov 27, 2017


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bzbarsky commented Nov 28, 2017


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andypaicu commented Nov 9, 2018

Unfortunately this is still a pretty big hole in nonce based CSP policies because it opens very simple avenues for injection points to become dangling-markup nonce-stealing attacks.

Does the above approach detailed by Mike seem reasonable. What are peoples thoughts?


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gsnedders commented Nov 9, 2018

To me this seems like a good idea given the potential threats, though it definitely adds some complexity (given the need to propagate the error). That said, my opinion probably shouldn't for a huge amount here. 🙂


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hsivonen commented Nov 12, 2018

Propagating a "this script is questionable enough that it should not run" bit to the DOM is doable from the parser perspective. Gecko already does this for scripts that were truncated by early EOF.

Let's not overdo it, though. Let's make it just one bit that says the there was at least one duplicate attribute without trying to retain knowledge of which attribute was duplicated and how all the way through the stack. Or even just one bit saying the script isn't eligible to run (in case we want to add other reasons later or combine it right away with the truncation-by-early-EOF case).

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