Index: src/wp-admin/css/common.css
===================================================================
--- src/wp-admin/css/common.css	(revision 41336)
+++ src/wp-admin/css/common.css	(working copy)
@@ -2864,33 +2864,12 @@
 	}
 }
 
-/* Thickbox for Plugin Install screen */
-body.about-php #TB_window,
-body.plugin-install-php #TB_window,
-body.import-php #TB_window,
-body.plugins-php #TB_window,
-body.update-core-php #TB_window,
-body.index-php #TB_window {
+/* Thickbox for the Plugin details modal. */
+#TB_window.plugin-details-modal {
 	background: #fcfcfc;
 }
 
-/* IE 8 needs a change in the pseudo element content */
-.ie8 body.about-php #TB_window:before,
-.ie8 body.plugin-install-php #TB_window:before,
-.ie8 body.import-php #TB_window:before,
-.ie8 body.plugins-php #TB_window:before,
-.ie8 body.update-core-php #TB_window:before,
-.ie8 body.index-php #TB_window:before {
-	content: " ";
-	background: none;
-}
-
-body.about-php #TB_window.thickbox-loading:before,
-body.plugin-install-php #TB_window.thickbox-loading:before,
-body.import-php #TB_window.thickbox-loading:before,
-body.plugins-php #TB_window.thickbox-loading:before,
-body.update-core-php #TB_window.thickbox-loading:before,
-body.index-php #TB_window.thickbox-loading:before {
+#TB_window.plugin-details-modal.thickbox-loading:before {
 	content: "";
 	display: block;
 	width: 20px;
@@ -2910,80 +2889,38 @@
 	(-webkit-min-device-pixel-ratio: 1.25),
 	(min-resolution: 120dpi) {
 
-	body.about-php #TB_window.thickbox-loading:before,
-	body.plugin-install-php #TB_window.thickbox-loading:before,
-	body.import-php #TB_window.thickbox-loading:before,
-	body.plugins-php #TB_window.thickbox-loading:before,
-	body.update-core-php #TB_window.thickbox-loading:before,
-	body.index-php #TB_window.thickbox-loading:before {
+	#TB_window.plugin-details-modal.thickbox-loading:before {
 		background-image: url(../images/spinner-2x.gif);
 	}
 }
 
-body.about-php #TB_title,
-body.plugin-install-php #TB_title,
-body.import-php #TB_title,
-body.plugins-php #TB_title,
-body.update-core-php #TB_title,
-body.index-php #TB_title {
+.plugin-details-modal #TB_title {
 	float: left;
 	height: 1px;
 }
 
-body.about-php #TB_ajaxWindowTitle,
-body.plugin-install-php #TB_ajaxWindowTitle,
-body.import-php #TB_ajaxWindowTitle,
-body.plugins-php #TB_ajaxWindowTitle,
-body.update-core-php #TB_ajaxWindowTitle,
-body.index-php #TB_ajaxWindowTitle {
+.plugin-details-modal #TB_ajaxWindowTitle {
 	display: none;
 }
 
-/* only on these screens */
-.about-php #TB_closeWindowButton,
-.plugin-install-php #TB_closeWindowButton,
-.import-php #TB_closeWindowButton,
-.plugins-php #TB_closeWindowButton,
-.update-core-php #TB_closeWindowButton,
-.index-php #TB_closeWindowButton {
+.plugin-details-modal #TB_closeWindowButton {
 	left: auto;
 	right: -30px;
 	color: #eee;
 }
 
-
-body.about-php #TB_closeWindowButton:hover,
-body.about-php #TB_closeWindowButton:focus,
-body.plugin-install-php #TB_closeWindowButton:hover,
-body.plugin-install-php #TB_closeWindowButton:focus,
-body.import-php #TB_closeWindowButton:hover,
-body.import-php #TB_closeWindowButton:focus,
-body.plugins-php #TB_closeWindowButton:hover,
-body.plugins-php #TB_closeWindowButton:focus,
-body.update-core-php #TB_closeWindowButton:hover,
-body.update-core-php #TB_closeWindowButton:focus,
-body.index-php #TB_closeWindowButton:hover,
-body.index-php #TB_closeWindowButton:focus {
+.plugin-details-modal #TB_closeWindowButton:hover,
+.plugin-details-modal #TB_closeWindowButton:focus {
 	color: #00a0d2;
 	outline: none;
 	box-shadow: none;
 }
 
-body.about-php .tb-close-icon,
-body.plugin-install-php .tb-close-icon,
-body.import-php .tb-close-icon,
-body.plugins-php .tb-close-icon,
-body.update-core-php .tb-close-icon,
-body.index-php .tb-close-icon {
+.plugin-details-modal .tb-close-icon {
 	display: none;
 }
 
-body.about-php #TB_closeWindowButton:after,
-body.plugin-install-php #TB_closeWindowButton:after,
-body.import-php #TB_closeWindowButton:after,
-body.plugins-php #TB_closeWindowButton:after,
-body.update-core-php #TB_closeWindowButton:after,
-body.index-php #TB_closeWindowButton:after {
+.plugin-details-modal #TB_closeWindowButton:after {
 	content: "\f335";
 	font: normal 32px/29px 'dashicons';
 	speak: none;
@@ -2993,12 +2930,7 @@
 
 /* move plugin install close icon to top on narrow screens */
 @media screen and ( max-width: 830px ) {
-	body.about-php #TB_closeWindowButton,
-	body.plugin-install-php #TB_closeWindowButton,
-	body.import-php #TB_closeWindowButton,
-	body.plugins-php #TB_closeWindowButton,
-	body.update-core-php #TB_closeWindowButton,
-	body.index-php #TB_closeWindowButton {
+	.plugin-details-modal #TB_closeWindowButton {
 		right: 0;
 		top: -30px;
 	}
Index: src/wp-admin/js/plugin-install.js
===================================================================
--- src/wp-admin/js/plugin-install.js	(revision 41336)
+++ src/wp-admin/js/plugin-install.js	(working copy)
@@ -7,11 +7,11 @@
 jQuery( document ).ready( function( $ ) {
 
 	var tbWindow,
-		$focusedBefore,
 		$iframeBody,
 		$tabbables,
 		$firstTabbable,
 		$lastTabbable,
+		$focusedBefore = $(),
 		$uploadViewToggle = $( '.upload-view-toggle' ),
 		$wrap = $ ( '.wrap' ),
 		$body = $( document.body );
@@ -58,6 +58,16 @@
 	 */
 	$body
 		.on( 'thickbox:iframe:loaded', tbWindow, function() {
+			/*
+			 * Return if it's not the modal with the plugin details iframe. Other
+			 * thickbox instances might want to load an iframe with content from
+			 * an external domain. Avoid to access the iframe contents when we're
+			 * not sure the iframe loads from the same domain.
+			 */
+			if ( ! tbWindow.hasClass( 'plugin-details-modal' ) ) {
+				return;
+			}
+
 			iframeLoaded();
 		})
 		.on( 'thickbox:removed', function() {
@@ -149,11 +159,13 @@
 
 		tb_click.call(this);
 
-		// Set ARIA role and ARIA label.
-		tbWindow.attr({
-			'role': 'dialog',
-			'aria-label': plugininstallL10n.plugin_modal_label
-		});
+		// Set ARIA role, ARIA label, and add a CSS class.
+		tbWindow
+			.attr({
+				'role': 'dialog',
+				'aria-label': plugininstallL10n.plugin_modal_label
+			})
+			.addClass( 'plugin-details-modal' );
 
 		// Set title attribute on the iframe.
 		tbWindow.find( '#TB_iframeContent' ).attr( 'title', title );
Index: src/wp-includes/js/thickbox/thickbox.css
===================================================================
--- src/wp-includes/js/thickbox/thickbox.css	(revision 41336)
+++ src/wp-includes/js/thickbox/thickbox.css	(working copy)
@@ -94,7 +94,7 @@
 #TB_load {
 	position: fixed;
 	display: none;
-	z-index: 103;
+	z-index: 100050;
 	top: 50%;
 	left: 50%;
 	background-color: #E8E8E8;