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 remended to use encodeForJavaScript. But I like to see the difference.

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 remended to use encodeForJavaScript. But I like to see the difference.

• javascript
• security
• escaping
• xss
• esapi

• This is basically covered at OWASP. See Rule 1 and Rule 3. – Obsidian Age Commented Jul 10, 2017 at 2:29
• I already went through it, I'm trying to see why encodeForHTMLAttribute does not work for javascript code, so far I tried few codes, but all of them worked properly even with encodeForHTMLAttribute, without using javascript encoding. – Chamila Wijayarathna Commented Jul 10, 2017 at 2:34
• I see the difference is that encodeForHTMLAttribute encode with &#xHH; format while encodeForJavaScript encode with \xHH format. But what I don't get is how something gets secured by encoding with \xHH does not get secured when encoded with &#xHH; – Chamila Wijayarathna Commented Jul 10, 2017 at 2:54

1 Answer 1

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 ining 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 mon 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 monly 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 puter:

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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.