diff --git a/CHANGELOG b/CHANGELOG
index 2d971ddd8..028235ecc 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -41,6 +41,7 @@ Core:
 - Replaced angular-csv-import through Papa Parse for csv parsing.
 - Added smooth projector scroll.
 - Added watching permissions in client and change the view immediately on changes.
+- Validate HTML strings from CKEditor against XSS attacks.
 
 Motions:
 - Added adjustable line numbering mode (outside, inside, none) for each
diff --git a/openslides/core/serializers.py b/openslides/core/serializers.py
index 19fbc4042..71f154842 100644
--- a/openslides/core/serializers.py
+++ b/openslides/core/serializers.py
@@ -1,4 +1,5 @@
 from openslides.utils.rest_api import Field, ModelSerializer, ValidationError
+from openslides.utils.validate import validate_html
 
 from .models import (
     ChatMessage,
@@ -78,6 +79,10 @@ class ProjectorMessageSerializer(ModelSerializer):
         model = ProjectorMessage
         fields = ('id', 'message', )
 
+    def validate(self, data):
+        data['message'] = validate_html(data.get('message', ''))
+        return data
+
 
 class CountdownSerializer(ModelSerializer):
     """
diff --git a/openslides/motions/serializers.py b/openslides/motions/serializers.py
index 4d49c65a1..ef7b90a27 100644
--- a/openslides/motions/serializers.py
+++ b/openslides/motions/serializers.py
@@ -12,6 +12,7 @@ from openslides.utils.rest_api import (
     SerializerMethodField,
     ValidationError,
 )
+from openslides.utils.validate import validate_html
 
 from .models import (
     Category,
@@ -257,6 +258,10 @@ class MotionChangeRecommendationSerializer(ModelSerializer):
             'text',
             'creation_time',)
 
+    def validate(self, data):
+        data['text'] = validate_html(data.get('text', ''))
+        return data
+
 
 class MotionSerializer(ModelSerializer):
     """
@@ -305,6 +310,15 @@ class MotionSerializer(ModelSerializer):
             'log_messages',)
         read_only_fields = ('state', 'recommendation',)  # Some other fields are also read_only. See definitions above.
 
+    def validate(self, data):
+        data['text'] = validate_html(data.get('text', ''))
+        data['reason'] = validate_html(data.get('reason', ''))
+        validated_comments = []
+        for comment in data.get('comments', []):
+            validated_comments.append(validate_html(comment))
+        data['comments'] = validated_comments
+        return data
+
     @transaction.atomic
     def create(self, validated_data):
         """
diff --git a/openslides/topics/serializers.py b/openslides/topics/serializers.py
index 6ae92c1e8..bd774c679 100644
--- a/openslides/topics/serializers.py
+++ b/openslides/topics/serializers.py
@@ -1,4 +1,5 @@
 from openslides.utils.rest_api import ModelSerializer
+from openslides.utils.validate import validate_html
 
 from .models import Topic
 
@@ -10,3 +11,7 @@ class TopicSerializer(ModelSerializer):
     class Meta:
         model = Topic
         fields = ('id', 'title', 'text', 'attachments', 'agenda_item_id')
+
+    def validate(self, data):
+        data['text'] = validate_html(data.get('text', ''))
+        return data
diff --git a/openslides/utils/validate.py b/openslides/utils/validate.py
new file mode 100644
index 000000000..a317ab5aa
--- /dev/null
+++ b/openslides/utils/validate.py
@@ -0,0 +1,34 @@
+import bleach
+
+allowed_tags = [
+    'a', 'img',  # links and images
+    'p', 'span', 'blockquote',  # text layout
+    'strike', 'strong', 'u', 'em', 'sup', 'sub', 'pre',  # text formatting
+    'h1', 'h2', 'h3', 'h4', 'h5', 'h6',  # headings
+    'ol', 'ul', 'li',  # lists
+    'table', 'caption', 'thead', 'tbody', 'th', 'tr', 'td',  # tables
+]
+allowed_attributes = {
+    '*': ['class', 'style'],
+    'img': ['alt', 'src', 'title'],
+    'a': ['href', 'title'],
+    'th': ['scope'],
+}
+allowed_styles = [
+    'color', 'background-color', 'height', 'width', 'text-align'
+]
+
+
+def validate_html(html):
+    """
+    This method takes a string and escapes all non-whitelisted html entries.
+    Every field of a model that is loaded trusted in the DOM should be validated.
+    """
+    if isinstance(html, str):
+        return bleach.clean(
+            html,
+            tags=allowed_tags,
+            attributes=allowed_attributes,
+            styles=allowed_styles)
+    else:
+        return html
diff --git a/requirements_production.txt b/requirements_production.txt
index b09c5ba5c..23e821f60 100644
--- a/requirements_production.txt
+++ b/requirements_production.txt
@@ -8,3 +8,4 @@ roman>=2.0,<2.1
 setuptools>=18.5,<33.0
 Twisted>=16.2,<16.4
 Whoosh>=2.7,<2.8
+bleach>=1.5.0,<1.6
diff --git a/tests/integration/motions/test_viewset.py b/tests/integration/motions/test_viewset.py
index fda76c7cf..ab3e3014e 100644
--- a/tests/integration/motions/test_viewset.py
+++ b/tests/integration/motions/test_viewset.py
@@ -235,7 +235,7 @@ class CreateMotion(TestCase):
         config['motions_comments'] = [
             {'name': 'comment1', 'public': True},
             {'name': 'comment2', 'public': False}]
-        comments = ['comemnt1_sdpoiuffo3%7dwDwW&', 'comment2_iusd&D/TdskDWH&5DWas46WAd078']
+        comments = ['comemnt1_sdpoiuffo3%7dwDwW)', 'comment2_iusd_D/TdskDWH(5DWas46WAd078']
         response = self.client.post(
             reverse('motion-list'),
             {'title': 'title_test_sfdAaufd56HR7sd5FDq7av',
diff --git a/tests/unit/utils/test_validate.py b/tests/unit/utils/test_validate.py
new file mode 100644
index 000000000..7ac25390d
--- /dev/null
+++ b/tests/unit/utils/test_validate.py
@@ -0,0 +1,11 @@
+from unittest import TestCase
+
+from openslides.utils.validate import validate_html
+
+
+class ValidatorTest(TestCase):
+    def test_XSS_protection(self):
+        data = 'tuveegi2Ho<a><p>tuveegi2Ho<script>kekj9(djwk</script></p>Boovai7esu</a>ee4Yaiw0ei'
+        self.assertEqual(
+            validate_html(data),
+            'tuveegi2Ho<a><p>tuveegi2Ho&lt;script&gt;kekj9(djwk&lt;/script&gt;</p>Boovai7esu</a>ee4Yaiw0ei')