data: URLs

Syntax

data:[<media-type>][;base64],<data>

data:

The scheme of the URL.

<media-type> Optional

The MIME type indicating the type of data, such as image/jpeg for a JPEG image file. If omitted, defaults to text/plain;charset=US-ASCII. You can find a full dissection of MIME type structure and a table of common MIME types on the web.

;base64 Optional

Indicates that the data should be base64-decoded; see encoding data into base64 format.

<data>

The data itself. If the data contains characters defined in RFC 3986 as reserved characters, or contains space characters, newline characters, or other non-printing characters, those characters must be percent-encoded. If the data is textual, you can 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.

A few examples:

data:,Hello%2C%20World%21

The text/plain data Hello, World!. Note how the comma is percent-encoded as %2C, and the space character as %20.

data:text/plain;base64,SGVsbG8sIFdvcmxkIQ==

base64-encoded version of the above

data:text/html,%3Ch1%3EHello%2C%20World%21%3C%2Fh1%3E

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

data:text/html,%3Cscript%3Ealert%28%27hi%27%29%3B%3C%2Fscript%3E

An HTML document with <script>alert('hi');</script> that executes a JavaScript alert. Note that the closing script tag is required.

Encoding data into base64 format

Base64 is a group of binary-to-text encoding schemes that represent binary data in an ASCII string format by translating it into a radix-64 representation. By consisting only of characters permitted by the URL syntax ("URL safe"), we can safely encode binary data in data URLs. Base64 uses the characters + and /, which may have special meanings in URLs. Because Data URLs have no URL path segments or query parameters, this encoding is safe in this context.

Encoding in JavaScript

The Web APIs have native methods to encode or decode to base64: Base64.

Encoding on a Unix system

Base64 encoding of a file or string on Linux and macOS systems can be achieved using the command-line base64 (or, as an alternative, the uuencode utility with -m argument).

echo -n hello|base64
# outputs to console: aGVsbG8=

echo -n hello>a.txt
base64 a.txt
# outputs to console: aGVsbG8=

base64 a.txt>b.txt
# outputs to file b.txt: aGVsbG8=

Encoding on Microsoft Windows

On Windows, Convert.ToBase64String from PowerShell can be used to perform the Base64 encoding:

[convert]::ToBase64String([Text.Encoding]::UTF8.GetBytes("hello"))
# outputs to console: aGVsbG8=

Alternatively, a GNU/Linux shell (such as WSL) provides the utility base64:

bash$ echo -n hello | base64
# outputs to console: aGVsbG8=

Common problems

This section describes problems that commonly occur when creating and using data URLs.

data:text/html,lots of text…<p><span class%3D"bottom">bottom</span>?arg=val</p>

This represents an HTML resource whose contents are:

lots of text…
<p><span class="bottom">bottom</span>?arg=val</p>

Syntax

The format for data URLs is very simple, but it's easy to forget to put a comma before the "data" segment, or to incorrectly encode the data into base64 format.

Formatting in HTML

A data URL provides a file within a file, which can potentially be very wide relative to the width of the enclosing document. As a URL, the data should be formattable with whitespace (linefeed, tab, or spaces), but there are practical issues that arise when using base64 encoding.

Length limitations

Browsers are not required to support any particular maximum length of data. Chromium and Firefox limit data URLs to 512MB, and Safari (WebKit) limits them to 2048MB. Note that Firefox 97 increased the limit from 256KB to 32MB, and Firefox 136 increased it to 512MB.

Lack of error handling

Invalid parameters in media, or typos when specifying 'base64', are ignored, but no error is provided.

No support for query strings, etc.

The data portion of a data URL is opaque, so an attempt to use a query string (page-specific parameters, with the syntax <url>?parameter-data) with a data URL will just include the query string in the data the URL represents.

Security issues

A number of security issues (for example, phishing) have been associated with data URLs, and navigating to them in the browser's top level. To mitigate such issues, top-level navigation to data: URLs is blocked in all modern browsers. See this blog post from the Mozilla Security Team for more details.

Specifications

Browser compatibility

See also

• Base64
• Percent-encoding
• atob()
• btoa()
• CSS <url>
• URI