Data URLs, URLs prefixed with the
data: scheme, allow content creators to embed small files inline in documents. They were formerly known as "data URIs" until that name was retired by the WHATWG.
Note: Data URLs are treated as unique opaque origins by modern browsers, rather than inheriting the origin of the settings object responsible for the navigation.
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:
mediatype is a MIME type string, such as
'image/jpeg' for a JPEG image file. If omitted, defaults to
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. You can find more info on MIME types here and here.
A few examples:
- 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.
- base64-encoded version of the above
- An HTML document with
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 in ASCII characters, base64 strings are generally url-safe, and that's why they can be used to encode data in Data URLs.
The Web APIs have native methods to encode or decode to base64: Base64 encoding and decoding.
Base64 encoding of a file or string on Linux and Mac OS X systems can be achieved using the command-line
base64 (or, as an alternative, the
uuencode utility with
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=
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
bash$ echo -n hello | base64 # outputs to console: aGVsbG8=
This section describes problems that commonly occur when creating and using
data:text/html,lots of text...<p><a name%3D"bottom">bottom</a>?arg=val
This represents an HTML resource whose contents are:
lots of text...<p><a name="bottom">bottom</a>?arg=val
- The format for
dataURLs 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
dataURL provides a file within a file, which can potentially be very wide relative to the width of the enclosing document. As a URL, the
datashould be formatable with whitespace (linefeed, tab, or spaces), but there are practical issues that arise when using base64 encoding.
- Length limitations
- Although Firefox supports
dataURLs of essentially unlimited length, browsers are not required to support any particular maximum length of data. For example, the Opera 11 browser limited URLs to 65535 characters long which limits
dataURLs to 65529 characters (65529 characters being the length of the encoded data, not the source, if you use the plain
data:, without specifying a MIME type).
- 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 (e.g. 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 has been blocked in Firefox 59+ (release version, Nightly/Beta from 58), and we hope to see other browsers follow suit soon. See Blocking Top-Level Navigations to data URLs for Firefox 58 for more details.
|RFC 2397||The "data" URL scheme|
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