diff --git a/CHANGES.md b/CHANGES.md
index 0acf7cd6302..52dba53d7bf 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -10,11 +10,12 @@ Fixed Issues:
 * [#12885](http://dev.ckeditor.com/ticket/12885): Added missing getData parameter documentation.
 * [#11982](http://dev.ckeditor.com/ticket/11982): Bullet added in wrong position after *enter* key pressed in nested list.
 * [#13027](http://dev.ckeditor.com/ticket/13027): Fixed: Keyboard Navigation in dialogs with multiple tabs not following CI 162 instructions or [ARIA Authoring practices](http://www.w3.org/TR/2013/WD-wai-aria-practices-20130307/#tabpanel).
+* [#12256](http://dev.ckeditor.com/ticket/12256): Fixed: Basic styles' classes are lost when pasting from MS Word if [basics styles](http://docs.ckeditor.com/#!/api/CKEDITOR.config-cfg-coreStyles_bold) were configured to use classes.
 
 Other Changes:
 
 * [#12844](http://dev.ckeditor.com/ticket/12844): Upgraded the [testing environment](http://docs.ckeditor.com/#!/guide/dev_tests) to [Bender.js](https://github.com/benderjs/benderjs) `0.2.*`.
-* [#12930](http://dev.ckeditor.com/ticket/12930): Because of licenses, removed truncated-mathjax from the test directory, now `bender.config.mathJaxLibPath` need to be configured manually.
+* [#12930](http://dev.ckeditor.com/ticket/12930): Because of licenses, removed `truncated-mathjax/` from the `tests/` directory. Now `bender.config.mathJaxLibPath` must be configured manually in order to run MathJax plugin's tests.
 
 ## CKEditor 4.4.7
 
diff --git a/plugins/pastefromword/filter/default.js b/plugins/pastefromword/filter/default.js
index 0b84da34e3f..5d2037c1a02 100644
--- a/plugins/pastefromword/filter/default.js
+++ b/plugins/pastefromword/filter/default.js
@@ -596,6 +596,10 @@
 					element.name = styleDef.element;
 					CKEDITOR.tools.extend( element.attributes, CKEDITOR.tools.clone( styleDef.attributes ) );
 					element.addStyle( CKEDITOR.style.getStyleText( styleDef ) );
+					// Mark style classes as allowed so they will not be filtered out (#12256).
+					if ( styleDef.attributes && styleDef.attributes[ 'class' ] ) {
+						element.classWhiteList = ' ' + styleDef.attributes[ 'class' ] + ' ';
+					}
 				} : function() {};
 			},
 
@@ -1058,7 +1062,13 @@
 					// Only Firefox carry style sheet from MS-Word, which
 					// will be applied by us manually. For other browsers
 					// the css className is useless.
-					'class': falsyFilter,
+					// We need to keep classes added as a style (#12256).
+					'class': function( value, element ) {
+						if ( element.classWhiteList && element.classWhiteList.indexOf( ' ' + value + ' ' ) != -1 ) {
+							return value;
+						}
+						return false;
+					},
 
 					// MS-Word always generate 'background-color' along with 'bgcolor',
 					// simply drop the deprecated attributes.
diff --git a/tests/plugins/pastefromword/manual/_assets/removedclasses.css b/tests/plugins/pastefromword/manual/_assets/removedclasses.css
new file mode 100644
index 00000000000..2c8d18fdbab
--- /dev/null
+++ b/tests/plugins/pastefromword/manual/_assets/removedclasses.css
@@ -0,0 +1,4 @@
+.myboldclass {
+	font-weight: bold;
+	color: red;
+}
\ No newline at end of file
diff --git a/tests/plugins/pastefromword/manual/removedclasses.html b/tests/plugins/pastefromword/manual/removedclasses.html
new file mode 100644
index 00000000000..90acf74e06e
--- /dev/null
+++ b/tests/plugins/pastefromword/manual/removedclasses.html
@@ -0,0 +1,18 @@
+<textarea cols="80" id="editor1" name="editor1" rows="10">
+	&lt;p&gt;&lt;b&gt;Apollo 11&lt;/b&gt; was the spaceflight that landed the first humans, Americans &lt;a href=&quot;http://en.wikipedia.org/wiki/Neil_Armstrong&quot; title=&quot;Neil Armstrong&quot;&gt;Neil Armstrong&lt;/a&gt; and &lt;a href=&quot;http://en.wikipedia.org/wiki/Buzz_Aldrin&quot; title=&quot;Buzz Aldrin&quot;&gt;Buzz Aldrin&lt;/a&gt;, on the Moon on July 20, 1969, at 20:18 UTC. Armstrong became the first to step onto the lunar surface 6 hours later on July 21 at 02:56 UTC.&lt;/p&gt;
+</textarea>
+<script>
+	var editor = CKEDITOR.replace( 'editor1', {
+		coreStyles_bold: {
+			element: 'span',
+			attributes: { 'class': 'myboldclass' },
+			overrides: [ 'strong', 'b' ]
+		},
+		contentsCss: '_assets/removedclasses.css',
+		allowedContent: true
+	} );
+
+	editor.on( 'paste', function( evt ) {
+		console.log( evt.data.dataValue );
+	} );
+</script>
\ No newline at end of file
diff --git a/tests/plugins/pastefromword/manual/removedclasses.md b/tests/plugins/pastefromword/manual/removedclasses.md
new file mode 100644
index 00000000000..d433691071b
--- /dev/null
+++ b/tests/plugins/pastefromword/manual/removedclasses.md
@@ -0,0 +1,6 @@
+@bender-ui: collapsed
+@bender-ckeditor-plugins: wysiwygarea, toolbar, undo, basicstyles, pastefromword, sourcearea, elementspath
+
+1. Paste bold text from word.
+
+Expected result: bold text should be bold and red.
\ No newline at end of file
diff --git a/tests/plugins/pastefromword/pastefromword.js b/tests/plugins/pastefromword/pastefromword.js
index f07e157e552..b94f946718b 100644
--- a/tests/plugins/pastefromword/pastefromword.js
+++ b/tests/plugins/pastefromword/pastefromword.js
@@ -23,6 +23,36 @@
 			} );
 
 			wait();
+		},
+
+		'test keep custom class': function() {
+			bender.editorBot.create( {
+				name: 'keep_custom_class',
+				config: {
+					coreStyles_bold: {
+						element: 'span',
+						attributes: { 'class': 'customboldclass' },
+						overrides: [ 'strong', 'b' ]
+					},
+					allowedContent: true
+				}
+			}, function( bot ) {
+				var editor = bot.editor;
+
+				editor.once( 'paste', function( evt ) {
+					var dataValue = evt.data.dataValue;
+					resume( function() {
+						assert.areSame( '<p>Foo <span class="customboldclass">bar</span> bom</p>', dataValue );
+					} );
+				}, null, null, 5 ); // Test PFW only.
+
+				editor.fire( 'paste', {
+					type: 'auto',
+					dataValue: '<p style="margin: 0cm 0cm 8pt;"><font face="Calibri">Foo <b style="mso-bidi-font-weight: normal;">bar</b> bom</font></p>'
+				} );
+
+				wait();
+			} );
 		}
 	} );