I'm trying to identify the difference between encodeForHTMLAttribute and encodeForJavaScript. Still, I couldn't find a scenario where untrusted data is used as javascript data values, which broke the code when escaped with encodeForHTMLAttribute, but works securely after escaped using encodeForJavaScript.
I know that for all javascript, its recommended to use encodeForJavaScript. But I like to see the difference.
The answer to your question is boring: The fact is that passing an HTML Entity encoded string to JavaScript is largely an unspecified operation.* JavaScript expects data passed to it to be escaped for JavaScript, with the exception of some API methods, it has no idea what you're sending it.
HTML and JavaScript are different languages. If you'll note, they both have different reserved characters--some the same, meaning they each have reserved characters that make up the language that have to be treated specially when using them in that language.
The correct way to ensure that javascript will always treat incoming code as data, is to escape for JavaScript. Passing it an HTMLEntity encoded String, might work, but we have to say might because that behavior is unspecified. One reason that the question I linked at the beginning partially answers your question is that it is common for JavaScript frameworks to DO that kind of processing on your input... so you better be sure that if that happens, the data is appropriately escaped for JavaScript. Otherwise it will unwrap your HTMLEntity encoding, render and execute script code.
I saved the damning part for last: You can write legal JavaScript with only six characters. And none of these characters are commonly escaped by HTML encoders. Here's an entire guide on using JavaScript escaping like this to evade XSS filters. And for proof that it works, look here.
Or run it yourself in an HTML file on your computer:
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However, if you escape what's between the <script> tags for Javascript... it will be treated purely as data and will not be executed.
*Except for those JavaScript functions that are designed to take HTML-encoded strings as input.