Greasemonkey Hacks

Hack #31. Identify Password Fields

Decorate password fields with a special background pattern.

This hack improves the usability of forms by highlighting password fields with a special background. This makes it faster to fill out forms, because you don't need to worry about accidentally typing your password in the wrong box in clear text.

The script makes password fields less legible, but in practice, this doesn't matter much, because what you type is displayed only as asterisks or dots anyway.

The Code

This user script runs on all pages. It inserts a CSS rule that decorates any input field with class GM_PasswordField. The CSS rule sets a background image, encoded as a data: URL. The image is a 4 x 4 image, which is then tiled to fill the password input box. The resulting pattern alternates black, white, and transparent lines, so that it is recognizable on any background color. Finally, we use an XPath query to find all password fields and set their class attributes to link them to the special CSS rule.

Save the following user script as identify-password-fields.user.js:

	// ==UserScript==
	// @name		 Identify Password Fields
	// @namespace	 http://blog.monstuff.com/archives/cat_greasemonkey.html
	// @description	 Decorates password fields with a background pattern
	// @include		 *
	// ==/UserScript==
	 
	// based on code by Julien Couvreur
	// and included here with his gracious permission

	// add a CSS rule
	var rule = "input.GM_PasswordField { background-image: url(data:image/gif,"+
		"GIF89a%04%00%04%00%B3%00%00%FF%FF%FF%FF%FF%00%FF%00%FF%FF%00%00%00%FF"+
		"%FF%00%FF%00%00%00%FF%00%00%00%CC%CC%CC%FF%FF%FF%00%00%00%00%00%00%00"+
		"%00%00%00%00%00%00%00%00%00%00%00!%F9%04%01%00%00%09%00%2C%00%00%00%0"+
		"0%04%00%04%00%00%04%070I4k%A22%02%00%3B) }";

	 var styleNode = document.createElement("style");
	 styleNode.type = "text/css";
	 styleNode.innerHTML = rule;
	 document.getElementsByTagName('head')[0].appendChild(styleNode);

	 // find all password fields and mark them with a class
	 var xpath = "//input[translate(@type,'PASSWORD','password')='password']";
	 var res = document.evaluate(xpath, document, null,
		  XPathResult.UNORDERED_NODE_SNAPSHOT_TYPE, null);
	 for (var inputIndex = 0; inputIndex < res.snapshotLength; inputIndex++) {
		  passwordInput = res.snapshotItem(inputIndex);
		  passwordInput.className += " GM_PasswordField";
	 }

Running the Hack

After installing the user script (Tools → Install This User Script), go to http://login.passport.net, or any site with an account registration or login form. The password field in the login form will be rendered with a stripped background pattern, as shown in Figure 4-3.

Hacking the Hack

It turns out that CSS already supports a powerful way of doing field selection. And because Firefox supports web standards such as CSS so well, it is possible to avoid the XPath query altogether.

The revised user script in this section doesn't need to enumerate the password fields. Instead, it uses a single CSS selector, input[type='password'], to set the background image on all password fields.

Tip

Learn more about CSS selectors at http://www.xml.com/lpt/a/2003/06/18/css3-selectors.html.

Figure 4-3. Highlighted password field

Save the following user script as identify-password-fields2.user.js:

	// ==UserScript==
	// @name			Identify Password Fields
	// @namespace		http://blog.monstuff.com/archives/cat_greasemonkey.html
	// @description		Decorates password fields with a background pattern
	// @include			*
	// ==/UserScript==
	// based on code by Julien Couvreur
	// and included here with his gracious permission
	var rule = "input[type='password'] { background-image: "	+
	  "url(data:image/gif,GIF89a%04%00%04%00%B3%00%00%FF%FF%FF" +
	  "%FF%FF%00%FF%00%FF%FF%00%00%00%FF%FF%00%FF%00%00%00%FF"	+
	  "%00%00%00%CC%CC%CC%FF%FF%FF%00%00%00%00%00%00%00%00%00"	+
	  "%00%00%00%00%00%00%00%00%00!%F9%04%01%00%00%09%00%2C%00" +
	  "%00%00%00%04%00%04%00%00%04%070I4k%A22%02%00%3B) }";

	 var styleNode = document.createElement("style");
	 styleNode.type = "text/css";
	 styleNode.innerHTML = rule;
	 document.getElementsByTagName('head')[0].appendChild(styleNode);

Uninstall the first script, install this one, and refresh http://login.passport.net. The effect is the same: the password field is highlighted. Hooray for CSS!

—Julien Couvreur