I have a question that has been on my mind for some time now. I believe the answer is a straightforward "No", but I haven't been able to figure out if simply replacing < and > with < and > effectively prevents reflected and persistent XSS attacks.
To clarify, I'm not referring to CSRF vulnerabilities.
If this method does not provide sufficient protection against XSS attacks, could you please offer an example of how one could bypass this defense mechanism?
Not all cross-site scripting (XSS) attacks involve the characters < or >, as it depends on where the data is inserted.
To prevent injection of malicious scripts when using a string from an untrusted source within an attribute (enclosed in "), it is important to escape the character " as ".
If not escaped, there is a risk of JavaScript injection. For instance, using <a href="{{str}}"> where str contains something like " onmouseover='something-evil'".
Avoiding XSS attacks requires more than just escaping characters like <, >, ', ", and &. Here are a few scenarios where simple escaping is not sufficient:
Illustration 1:
<img src="{{myImage}}">
Potential XSS exploit:
myImage = "x.jpg\" onerror=\"alert('XSS')\""
Illustration 2:
<script>var value = {{myValue}};</script>
Potential XSS exploit:
myValue = "10;alert('XSS')"
Refer to https://www.owasp.org/index.php/XSS_(Cross_Site_Scripting)_Prevention_Cheat_Sheet for comprehensive measures to prevent these vulnerabilities.
No, simply checking for untrusted data in HTML is not enough to protect against XSS attacks. It's important to also consider potential vulnerabilities in JavaScript and CSS. For example, even a seemingly harmless code like "var myVar = [input];" can be exploited to execute malicious scripts without using angle brackets. To learn more about different types of XSS attacks, refer to the XSS cheat sheet available at
Since ASP.NET was mentioned in the tag, it is recommended to utilize the AntiXSS library for encoding output securely. Functions such as Encoder.CssEncode(), Encoder.HtmlEncode(), Encoder.HtmlAttributeEncode(), and Encoder.JavaScriptEncode() should be used to prevent XSS vulnerabilities.
It is unnecessary to implement custom character substitution techniques in .NET when tools like the AntiXSS library are readily available.
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