> For the complete documentation index, see [llms.txt](https://wiki.zacheller.dev/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://wiki.zacheller.dev/web-app-pentest/xss/data-urls.md).

# Data URLs

## XSS Attack

[Data URIs](https://developer.mozilla.org/en-US/docs/Web/HTTP/Basics_of_HTTP/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](https://developer.mozilla.org/en-US/docs/Web/HTTP/Basics_of_HTTP/MIME_types) 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](https://developer.mozilla.org/en-US/docs/Web/HTTP/Basics_of_HTTP/MIME_types) 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](https://developer.mozilla.org/en-US/docs/Web/HTTP/Basics_of_HTTP/MIME_types) and [here](https://developer.mozilla.org/en-US/docs/Web/HTTP/Basics_of_HTTP/MIME_types/Complete_list_of_MIME_types).

A few examples:

* `data:,Hello%2C%20World!`
  * Simple text/plain data. Note the use of [percent-encoding](https://developer.mozilla.org/en-US/docs/Glossary/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.

{% embed url="<https://www.nccgroup.com/us/about-us/newsroom-and-events/blog/2019/april/a-novel-csp-bypass-using-data-uri/>" %}

{% embed url="<https://developer.mozilla.org/en-US/docs/Web/HTTP/Basics_of_HTTP/Data_URIs>" %}


---

# Agent Instructions
This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com.

## Querying This Documentation
If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter, and the optional `goal` query parameter:

```
GET https://wiki.zacheller.dev/web-app-pentest/xss/data-urls.md?ask=<question>&goal=<endgoal>
```

`ask` is the immediate question: it should be specific, self-contained, and written in natural language.
`goal` is optional and describes the broader end goal you are ultimately trying to accomplish on behalf of the user. GitBook uses it to tailor the answer towards what is most useful for that goal.

The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.