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POCombo box populates a text field - JavaScript and user input
 

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  1. POCombo box populates a text field - JavaScript and user input
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    <h2>Setup</h2> <p>What I need in the end: a combo box and a text field. Selecting an item from the combo box populates a value in the text field. Items in the combo box come from user input.</p> <p>This is my current code that works nice:</p> <pre><code>function fill() { var select = document.getElementById("select"); var value = select.options[select.selectedIndex].value; var textfield = document.getElementById("textfield"); switch (value) { case "1": textfield.value = "some user-provided text"; break; case "2": textfield.value = "containing possibly &lt; &gt; &amp; etc."; break; } } &lt;select id="select" onchange="fill()"&gt; &lt;option&gt;Select something...&lt;/option&gt; &lt;option value="1"&gt;Select option 1&lt;/option&gt; &lt;option value="2"&gt;Select option 2&lt;/option&gt; &lt;/select&gt; &lt;input type="text" id="textfield"/&gt; </code></pre> <h2>Problem</h2> <p>I am not sure what to do if the user-provided text to populate contains dangerous characters. Usually I would HTML-encode it before writing it in the HTML code, but now I am writing it into JavaScript code.</p> <p>All of the JavaScript is generated by a Java backend. I can't really create it like this:</p> <pre><code>String value = getValueFromUser(); write("textfield.value = \"" + value + "\";"); </code></pre> <p>Because then someone would enter <code>"; somethingBad(); //</code> and some other user would on selecting this option get <code>somethingBad()</code> executed.</p> <p>If I HTML-encode the value:</p> <pre><code>String value = htmlEncode(getValueFromUser()); write("textfield.value = \"" + value + "\";"); </code></pre> <p>Then (security questions aside) after selecting a value in the combo box the text field will be populated with HTML-encoded text.</p> <h2>Question</h2> <p>How do I encode the user-provided values so that:</p> <ul> <li>They cannot be misused for JavaScript code injection, and</li> <li>They look the same after being populated into the text field?</li> </ul>
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    1. COIf your server-side code needs to include user-entered data within what will become a JS string, rather than `htmlEncode()` you can do a string replace to change any `"` character to `\"`. If the user-entered data might include carriage returns or line feeds you should replace them too. Then what the browser sees will just be a valid JS string.
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    2. COSo how would you describe this transformation? Getting the user input to be a valid JS string?...
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    3. COWould it be overkill to just send JSON-encoded plain text and JSON-encoded html entities altogether? This would move all encoding to the server, thus avoiding any potential security issues. Home-grown encoding schemes can and usually will fail. Keep it simple...
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