diff --git a/core/lib/Drupal/Component/Utility/Xss.php b/core/lib/Drupal/Component/Utility/Xss.php
index 7a06e124e6c4..9081fae3fe21 100644
--- a/core/lib/Drupal/Component/Utility/Xss.php
+++ b/core/lib/Drupal/Component/Utility/Xss.php
@@ -192,6 +192,7 @@ class Xss {
     $mode = 0;
     $attribute_name = '';
     $skip = FALSE;
+    $skip_protocol_filtering = FALSE;
 
     while (strlen($attributes) != 0) {
       // Was the last operation successful?
@@ -203,6 +204,20 @@ class Xss {
           if (preg_match('/^([-a-zA-Z]+)/', $attributes, $match)) {
             $attribute_name = strtolower($match[1]);
             $skip = ($attribute_name == 'style' || substr($attribute_name, 0, 2) == 'on');
+
+            // Values for attributes of type URI should be filtered for
+            // potentially malicious protocols (for example, an href-attribute
+            // starting with "javascript:"). However, for some non-URI
+            // attributes performing this filtering causes valid and safe data
+            // to be mangled. We prevent this by skipping protocol filtering on
+            // such attributes.
+            // @see \Drupal\Component\Utility\UrlHelper::filterBadProtocol()
+            // @see http://www.w3.org/TR/html4/index/attributes.html
+            $skip_protocol_filtering = substr($attribute_name, 0, 5) === 'data-' || in_array($attribute_name, array(
+              'title',
+              'alt',
+            ));
+
             $working = $mode = 1;
             $attributes = preg_replace('/^[-a-zA-Z]+/', '', $attributes);
           }
@@ -228,7 +243,7 @@ class Xss {
         case 2:
           // Attribute value, a URL after href= for instance.
           if (preg_match('/^"([^"]*)"(\s+|$)/', $attributes, $match)) {
-            $thisval = UrlHelper::filterBadProtocol($match[1]);
+            $thisval = $skip_protocol_filtering ? $match[1] : UrlHelper::filterBadProtocol($match[1]);
 
             if (!$skip) {
               $attributes_array[] = "$attribute_name=\"$thisval\"";
@@ -240,7 +255,7 @@ class Xss {
           }
 
           if (preg_match("/^'([^']*)'(\s+|$)/", $attributes, $match)) {
-            $thisval = UrlHelper::filterBadProtocol($match[1]);
+            $thisval = $skip_protocol_filtering ? $match[1] : UrlHelper::filterBadProtocol($match[1]);
 
             if (!$skip) {
               $attributes_array[] = "$attribute_name='$thisval'";
@@ -251,7 +266,7 @@ class Xss {
           }
 
           if (preg_match("%^([^\s\"']+)(\s+|$)%", $attributes, $match)) {
-            $thisval = UrlHelper::filterBadProtocol($match[1]);
+            $thisval = $skip_protocol_filtering ? $match[1] : UrlHelper::filterBadProtocol($match[1]);
 
             if (!$skip) {
               $attributes_array[] = "$attribute_name=\"$thisval\"";
diff --git a/core/modules/editor/src/EditorXssFilter/Standard.php b/core/modules/editor/src/EditorXssFilter/Standard.php
index b34e3496bafa..43ef797684ac 100644
--- a/core/modules/editor/src/EditorXssFilter/Standard.php
+++ b/core/modules/editor/src/EditorXssFilter/Standard.php
@@ -7,7 +7,9 @@
 
 namespace Drupal\editor\EditorXssFilter;
 
+use Drupal\Component\Utility\Html;
 use Drupal\Component\Utility\Xss;
+use Drupal\Component\Utility\SafeMarkup;
 use Drupal\filter\FilterFormatInterface;
 use Drupal\editor\EditorXssFilterInterface;
 
@@ -85,7 +87,39 @@ class Standard extends Xss implements EditorXssFilterInterface {
     // Also blacklist tags that are explicitly forbidden in either text format.
     $blacklisted_tags = array_merge($blacklisted_tags, $forbidden_tags);
 
-    return static::filter($html, $blacklisted_tags);
+    $output = static::filter($html, $blacklisted_tags);
+
+    // Since data-attributes can contain encoded HTML markup that could be
+    // decoded and interpreted by editors, we need to apply XSS filtering to
+    // their contents.
+    return static::filterXssDataAttributes($output);
+  }
+
+  /**
+   * Applies a very permissive XSS/HTML filter to data-attributes.
+   *
+   * @param string $html
+   *   The string to apply the data-attributes filtering to.
+   *
+   * @return string
+   *   The filtered string.
+   */
+  protected static function filterXssDataAttributes($html) {
+    if (stristr($html, 'data-') !== FALSE) {
+      $dom = Html::load($html);
+      $xpath = new \DOMXPath($dom);
+      foreach ($xpath->query('//@*[starts-with(name(.), "data-")]') as $node) {
+        // The data-attributes contain an HTML-encoded value, so we need to
+        // decode the value, apply XSS filtering and then re-save as encoded
+        // value. There is no need to explicitly decode $node->value, since the
+        // DOMAttr::value getter returns the decoded value.
+        $value = Xss::filterAdmin($node->value);
+        $node->value = SafeMarkup::checkPlain($value);
+      }
+      $html = Html::serialize($dom);
+    }
+
+    return $html;
   }
 
 
diff --git a/core/modules/editor/tests/src/Unit/EditorXssFilter/StandardTest.php b/core/modules/editor/tests/src/Unit/EditorXssFilter/StandardTest.php
index ee25b00636e3..13c831129d81 100644
--- a/core/modules/editor/tests/src/Unit/EditorXssFilter/StandardTest.php
+++ b/core/modules/editor/tests/src/Unit/EditorXssFilter/StandardTest.php
@@ -512,6 +512,17 @@ xss:ex/*XSS*//*/*/pression(alert("XSS"))\'>', 'exp/*<A>');
     // sites, it only forbids linking to any protocols other than those that are
     // whitelisted.
 
+    // Test XSS filtering on data-attributes.
+    // @see \Drupal\editor\EditorXssFilter::filterXssDataAttributes()
+
+    // The following two test cases verify that XSS attack vectors are filtered.
+    $data[] = array('<img src="butterfly.jpg" data-caption="&lt;script&gt;alert();&lt;/script&gt;" />', '<img src="butterfly.jpg" data-caption="alert();" />');
+    $data[] = array('<img src="butterfly.jpg" data-caption="&lt;EMBED SRC=&quot;http://ha.ckers.org/xss.swf&quot; AllowScriptAccess=&quot;always&quot;&gt;&lt;/EMBED&gt;" />', '<img src="butterfly.jpg" data-caption="" />');
+
+    // When including HTML-tags as visible content, they are double-escaped.
+    // This test case ensures that we leave that content unchanged.
+    $data[] = array('<img src="butterfly.jpg" data-caption="&amp;lt;script&amp;gt;alert();&amp;lt;/script&amp;gt;" />', '<img src="butterfly.jpg" data-caption="&amp;lt;script&amp;gt;alert();&amp;lt;/script&amp;gt;" />');
+
     return $data;
   }
 
diff --git a/core/modules/filter/src/Tests/FilterUnitTest.php b/core/modules/filter/src/Tests/FilterUnitTest.php
index 87eb773e3bee..d2b515b95789 100644
--- a/core/modules/filter/src/Tests/FilterUnitTest.php
+++ b/core/modules/filter/src/Tests/FilterUnitTest.php
@@ -9,6 +9,8 @@ namespace Drupal\filter\Tests;
 
 use Drupal\Component\Utility\Html;
 use Drupal\Component\Utility\SafeMarkup;
+use Drupal\editor\EditorXssFilter\Standard;
+use Drupal\filter\Entity\FilterFormat;
 use Drupal\filter\FilterPluginCollection;
 use Drupal\simpletest\KernelTestBase;
 
@@ -176,6 +178,74 @@ class FilterUnitTest extends KernelTestBase {
     $output = $test($input);
     $this->assertIdentical($expected, $output->getProcessedText());
     $this->assertIdentical($attached_library, $output->getAssets());
+
+    // So far we've tested that the caption filter works correctly. But we also
+    // want to make sure that it works well in tandem with the "Limit allowed
+    // HTML tags" filter, which it is typically used with.
+    $html_filter = $this->filters['filter_html'];
+    $html_filter->setConfiguration(array(
+      'settings' => array(
+        'allowed_html' => '<img>',
+        'filter_html_help' => 1,
+        'filter_html_nofollow' => 0,
+      )
+    ));
+    $test_with_html_filter = function ($input) use ($filter, $html_filter) {
+      // 1. Apply HTML filter's processing step.
+      $output = $html_filter->process($input, 'und');
+      // 2. Apply caption filter's processing step.
+      $output = $filter->process($output, 'und');
+      return $output->getProcessedText();
+    };
+    // Editor XSS filter.
+    $test_editor_xss_filter = function ($input) {
+      $dummy_filter_format = FilterFormat::create();
+      return Standard::filterXss($input, $dummy_filter_format);
+    };
+
+    // All the tricky cases encountered at https://drupal.org/node/2105841.
+    // A plain URL preceded by text.
+    $input = '<img data-caption="See http://drupal.org" src="llama.jpg" />';
+    $expected = '<figure><img src="llama.jpg" /><figcaption>See http://drupal.org</figcaption></figure>';
+    $this->assertIdentical($expected, $test_with_html_filter($input));
+    $this->assertIdentical($input, $test_editor_xss_filter($input));
+
+    // An anchor.
+    $input = '<img data-caption="This is a &lt;a href=&quot;http://drupal.org&quot;&gt;quick&lt;/a&gt; test…" src="llama.jpg" />';
+    $expected = '<figure><img src="llama.jpg" /><figcaption>This is a <a href="http://drupal.org">quick</a> test…</figcaption></figure>';
+    $this->assertIdentical($expected, $test_with_html_filter($input));
+    $this->assertIdentical($input, $test_editor_xss_filter($input));
+
+    // A plain URL surrounded by parentheses.
+    $input = '<img data-caption="(http://drupal.org)" src="llama.jpg" />';
+    $expected = '<figure><img src="llama.jpg" /><figcaption>(http://drupal.org)</figcaption></figure>';
+    $this->assertIdentical($expected, $test_with_html_filter($input));
+    $this->assertIdentical($input, $test_editor_xss_filter($input));
+
+    // A source being credited.
+    $input = '<img data-caption="Source: Wikipedia" src="llama.jpg" />';
+    $expected = '<figure><img src="llama.jpg" /><figcaption>Source: Wikipedia</figcaption></figure>';
+    $this->assertIdentical($expected, $test_with_html_filter($input));
+    $this->assertIdentical($input, $test_editor_xss_filter($input));
+
+    // A source being credited, without a space after the colon.
+    $input = '<img data-caption="Source:Wikipedia" src="llama.jpg" />';
+    $expected = '<figure><img src="llama.jpg" /><figcaption>Source:Wikipedia</figcaption></figure>';
+    $this->assertIdentical($expected, $test_with_html_filter($input));
+    $this->assertIdentical($input, $test_editor_xss_filter($input));
+
+    // A pretty crazy edge case where we have two colons.
+    $input = '<img data-caption="Interesting (Scope resolution operator ::)" src="llama.jpg" />';
+    $expected = '<figure><img src="llama.jpg" /><figcaption>Interesting (Scope resolution operator ::)</figcaption></figure>';
+    $this->assertIdentical($expected, $test_with_html_filter($input));
+    $this->assertIdentical($input, $test_editor_xss_filter($input));
+
+    // An evil anchor (to ensure XSS filtering is applied to the caption also).
+    $input = '<img data-caption="This is an &lt;a href=&quot;javascript:alert();&quot;&gt;evil&lt;/a&gt; test…" src="llama.jpg" />';
+    $expected = '<figure><img src="llama.jpg" /><figcaption>This is an <a href="alert();">evil</a> test…</figcaption></figure>';
+    $this->assertIdentical($expected, $test_with_html_filter($input));
+    $expected_xss_filtered = '<img data-caption="This is an &lt;a href=&quot;alert();&quot;&gt;evil&lt;/a&gt; test…" src="llama.jpg" />';
+    $this->assertIdentical($expected_xss_filtered, $test_editor_xss_filter($input));
   }
 
   /**
diff --git a/core/tests/Drupal/Tests/Component/Utility/XssTest.php b/core/tests/Drupal/Tests/Component/Utility/XssTest.php
index 1ffdfe0de6ea..e704538bf0b4 100644
--- a/core/tests/Drupal/Tests/Component/Utility/XssTest.php
+++ b/core/tests/Drupal/Tests/Component/Utility/XssTest.php
@@ -488,6 +488,37 @@ class XssTest extends UnitTestCase {
     $this->assertTrue(stripos($value, '<?xml') === FALSE, 'HTML tag stripping evasion -- starting with a question sign (processing instructions).');
   }
 
+  /**
+   * Check that strings in HTML attributes are are correctly processed.
+   *
+   * @covers ::attributes
+   * @dataProvider providerTestAttributes
+   */
+  public function testAttribute($value, $expected, $message, $allowed_tags = NULL) {
+    $value = Xss::filter($value, $allowed_tags);
+    $this->assertEquals($expected, $value, $message);
+  }
+
+  /**
+   * Data provider for testFilterXssAdminNotNormalized().
+   */
+  public function providerTestAttributes() {
+    return array(
+      array(
+        '<img src="http://example.com/foo.jpg" title="Example: title" alt="Example: alt">',
+        '<img src="http://example.com/foo.jpg" title="Example: title" alt="Example: alt">',
+        'Image tag with alt and title attribute',
+        array('img')
+      ),
+      array(
+        '<img src="http://example.com/foo.jpg" data-caption="Drupal 8: The best release ever.">',
+        '<img src="http://example.com/foo.jpg" data-caption="Drupal 8: The best release ever.">',
+        'Image tag with data attribute',
+        array('img')
+      ),
+    );
+  }
+
   /**
    * Checks that \Drupal\Component\Utility\Xss::filterAdmin() correctly strips unallowed tags.
    */