HTML API: Prevent setting double escape script content

[sirreal]

The WP_Tag_Processor::set_modifiable_text() method should treat <script the same way as it treats </script. Either of these sequences may affect the script elements close.

Trac ticket: https://core.trac.wordpress.org/ticket/63738

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[sirreal]

This could allow some more safe inputs with a regular expression based check. There may also be ways to escape the script tag contents.

Before exploring those improvements, I'd like to address this issue where some potentially dangerous script tag contents are allowed.

[sirreal]

The dangerous thing here is to enter the double escaped state. I wrote extensively about that here.

This check could be for both <!-- and <script, which are necessary to enter the double escaped state, but I think the <script check is sufficient for now.

Strictly speaking, the double escaped state is something like /<!---*(?!>).*<script[ \t\n\r\f\/>]/i and the dangerous closer /<\/script[ \t\n\r\f\/>]/i.

[dmsnell]

with better understanding of this now it still makes me wonder about allowing vs. not allowing it. we can detect if we enter and exit the double-escaped state, so technically we could allow entering it before rejecting the update.

but assuming we can allow this, it carries the risk downstream to unaware code. not allowing it at all means simpler code doesn’t have to know about these nuances. but then again, we can’t always go out of our way to ensure that broken code doesn’t break.

no insight, just questions

[sirreal]

I've been wondering about this as well. I'm confident in the logic around processing script tag contents. It would require some refactoring, but we could determine whether script tag contents are dangerous (with regards to HTML structure) in spec-compliant ways instead of these very basic tests.

• 👍 This check ensures unsafe scripts are not allowed
• 👎 This check disallows some safe scripts that appear dangerous.

A good example is that <!--<script></script> is perfectly fine. It includes both of the substrings that are dangerous, but when combined in the correct way they become safe. The HTML API is smart enough to recognize and allow this, but it's very likely to confuse other simpler processing such as regular expression based approaches.

My feeling, at least at this time, is that it's preferable to be more restrictive now and fail on some inputs that are not safe but look dangerous. In the future it should be easy to make these checks more accurate and allow more safe but dangerous-seeming input.

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[dmsnell]

earmarking for follow-up: we could probably create a new unit test that better spells out what we’re wanting to enshrine here

/**
 * @param 'early-exit'|'prevents-exit' $violation Whether the violating contents close the SCRIPT element early or prevent its normal closure.
 */
public function rejects_script_contents_which_escape_the_script_element_boundaries( string $violating_script_contents, string $violation ) {
	…
}

[dmsnell]

This is good. Adding it to the Updates to the HTML API in 6.9 post!

[sirreal]

Merged in [60706].