Data URLs

XSS Attack

Data URIs allow HTML tags to be created with inline content, rather than reaching out to and making an additional request to the server. For example, an inline image might look like:

<img src="data:image/png;base64,iVBORw0KGgoAAAA..."/>

So, if we want to run JS we can actually inject either of these into a URL:

data:text/html,<script>alert('hi');</script>
data:text/javascript,alert(1)

Definition

Data URLs are composed of four parts: a prefix (data:), a MIME type indicating the type of data, an optional base64 token if non-textual, and the data itself:

data:[<mediatype>][;base64],<data>

The mediatype is a MIME type string, such as 'image/jpeg' for a JPEG image file. If omitted, defaults to text/plain;charset=US-ASCII

If the data is textual, you can simply embed the text (using the appropriate entities or escapes based on the enclosing document's type). Otherwise, you can specify base64 to embed base64-encoded binary data. You can find more info on MIME types here and here.

A few examples:

• data:,Hello%2C%20World!

  • Simple text/plain data. Note the use of percent-encoding (URL-encoding) for the quote and space characters. Also, for CSV data (MIME type "text/csv"), percent-encoding is needed to preserve the line endings that delimit rows of the spreadsheet.

• data:text/plain;base64,SGVsbG8sIFdvcmxkIQ==

  • base64-encoded version of the above

• data:text/html,%3Ch1%3EHello%2C%20World!%3C%2Fh1%3E

  • An HTML document with <h1>Hello, World!</h1>

• data:text/html,<script>alert('hi');</script>

  • An HTML document that executes a JavaScript alert. Note that the closing script tag is required.

https://www.nccgroup.com/us/about-us/newsroom-and-events/blog/2019/april/a-novel-csp-bypass-using-data-uri/www.nccgroup.com

Data URLs - HTTP | MDN