Sanitize Html message content

core/lib/core.dart

@@ -6,6 +6,7 @@ export 'presentation/extensions/url_extension.dart';
 export 'presentation/extensions/capitalize_extension.dart';
 export 'presentation/extensions/list_extensions.dart';
 export 'domain/extensions/datetime_extension.dart';
+export 'presentation/extensions/html_extension.dart';
 
 // Utils
 export 'presentation/utils/theme_utils.dart';
@@ -44,3 +45,6 @@ export 'data/network/dio_client.dart';
 export 'presentation/state/success.dart';
 export 'presentation/state/failure.dart';
 export 'presentation/state/app_state.dart';
+
+// Validator
+export 'presentation/validator/html_message_purifier.dart';

core/lib/presentation/extensions/html_extension.dart

@@ -0,0 +1,15 @@
+
+extension HtmlExtension on String {
+
+  String removeFontSizeZeroPixel() => replaceAll(RegExp('font-size:0px;'), '');
+
+  String removeHeightZeroPixel() => replaceAll(RegExp('height:0px;'), '');
+
+  String removeWidthZeroPixel() => replaceAll(RegExp('width:0px;'), '');
+
+  String removeMaxWidthZeroPixel() => replaceAll(RegExp('max-width:0px;'), '');
+
+  String removeMaxHeightZeroPixel() => replaceAll(RegExp('max-height:0px;'), '');
+
+  String changeStyleBackground() => replaceAll(RegExp('background:'), 'background-color:');
+}

core/lib/presentation/validator/html_message_purifier.dart

@@ -0,0 +1,42 @@
+import 'sane_html_validator.dart' show SaneHtmlValidator;
+import 'package:core/core.dart';
+
+class HtmlMessagePurifier {
+  String purifyHtmlMessage(
+    String html,
+    {
+      Set<String>? allowElementIds,
+      Set<String>? allowClassNames,
+      Set<String>? allowAttributes
+    }
+  ) {
+    return sanitizeHtml(
+        html,
+        allowElementId: (elementId) => allowElementIds != null ? allowElementIds.contains(elementId) : false,
+        allowClassName: (className) => allowClassNames != null ? allowClassNames.contains(className) : false,
+        allowAttributes: (attribute) => allowAttributes != null ? allowAttributes.contains(attribute) : false)
+      .changeStyleBackground()
+      .removeFontSizeZeroPixel()
+      .removeMaxHeightZeroPixel()
+      .removeMaxWidthZeroPixel()
+      .removeHeightZeroPixel()
+      .removeWidthZeroPixel();
+  }
+
+  String sanitizeHtml(
+    String htmlString,
+    {
+      bool Function(String)? allowElementId,
+      bool Function(String)? allowClassName,
+      bool Function(String)? allowAttributes,
+      Iterable<String>? Function(String)? addLinkRel
+    }
+  ) {
+    return SaneHtmlValidator(
+      allowElementId: allowElementId,
+      allowClassName: allowClassName,
+      allowAttributes: allowAttributes,
+      addLinkRel: addLinkRel,
+    ).sanitize(htmlString);
+  }
+}

core/lib/presentation/validator/sane_html_validator.dart

@@ -0,0 +1,274 @@
+
+import 'package:html/dom.dart';
+import 'package:html/parser.dart' as html_parser;
+
+final _allowedElements = <String>{
+  'H1',
+  'H2',
+  'H3',
+  'H4',
+  'H5',
+  'H6',
+  'H7',
+  'H8',
+  'BR',
+  'B',
+  'I',
+  'STRONG',
+  'EM',
+  'A',
+  'PRE',
+  'CODE',
+  'IMG',
+  'TT',
+  'DIV',
+  'INS',
+  'DEL',
+  'SUP',
+  'SUB',
+  'P',
+  'OL',
+  'UL',
+  'TABLE',
+  'THEAD',
+  'TBODY',
+  'TFOOT',
+  'BLOCKQUOTE',
+  'DL',
+  'DT',
+  'DD',
+  'KBD',
+  'Q',
+  'SAMP',
+  'VAR',
+  'HR',
+  'RUBY',
+  'RT',
+  'RP',
+  'LI',
+  'TR',
+  'TD',
+  'TH',
+  'S',
+  'STRIKE',
+  'SUMMARY',
+  'DETAILS',
+  'CAPTION',
+  'FIGURE',
+  'FIGCAPTION',
+  'ABBR',
+  'BDO',
+  'CITE',
+  'DFN',
+  'MARK',
+  'SMALL',
+  'SPAN',
+  'TIME',
+  'WBR',
+};
+
+final _alwaysAllowedAttributes = <String>{
+  'abbr',
+  'accept',
+  'accept-charset',
+  'accesskey',
+  'action',
+  'align',
+  'alt',
+  'aria-describedby',
+  'aria-hidden',
+  'aria-label',
+  'aria-labelledby',
+  'axis',
+  'border',
+  'cellpadding',
+  'cellspacing',
+  'char',
+  'charoff',
+  'charset',
+  'checked',
+  'clear',
+  'cols',
+  'colspan',
+  'color',
+  'compact',
+  'coords',
+  'datetime',
+  'dir',
+  'disabled',
+  'enctype',
+  'for',
+  'frame',
+  'headers',
+  'height',
+  'hreflang',
+  'hspace',
+  'ismap',
+  'label',
+  'lang',
+  'maxlength',
+  'media',
+  'method',
+  'multiple',
+  'name',
+  'nohref',
+  'noshade',
+  'nowrap',
+  'open',
+  'prompt',
+  'readonly',
+  'rel',
+  'rev',
+  'rows',
+  'rowspan',
+  'rules',
+  'scope',
+  'selected',
+  'shape',
+  'size',
+  'span',
+  'start',
+  'summary',
+  'tabindex',
+  'target',
+  'title',
+  'type',
+  'usemap',
+  'valign',
+  'value',
+  'vspace',
+  'width',
+  'itemprop',
+};
+
+bool _alwaysAllowed(String _) => true;
+
+bool _validLink(String url) {
+  try {
+    final uri = Uri.parse(url);
+    return uri.isScheme('https') ||
+        uri.isScheme('http') ||
+        uri.isScheme('mailto') ||
+        !uri.hasScheme;
+  } on FormatException {
+    return false;
+  }
+}
+
+bool _validUrl(String url) {
+  try {
+    final uri = Uri.parse(url);
+    return uri.isScheme('https') || uri.isScheme('http') || !uri.hasScheme;
+  } on FormatException {
+    return false;
+  }
+}
+
+final _citeAttributeValidator = <String, bool Function(String)>{
+  'cite': _validUrl,
+};
+
+final _elementAttributeValidators =
+  <String, Map<String, bool Function(String)>>{
+    'A': {
+      'href': _validLink,
+    },
+    'IMG': {
+      'src': _alwaysAllowed,
+      'longdesc': _validUrl,
+    },
+    'DIV': {
+      'itemscope': _alwaysAllowed,
+      'itemtype': _alwaysAllowed,
+    },
+    'BLOCKQUOTE': _citeAttributeValidator,
+    'DEL': _citeAttributeValidator,
+    'INS': _citeAttributeValidator,
+    'Q': _citeAttributeValidator,
+  };
+
+/// An implementation of [html.NodeValidator] that only allows sane HTML tags
+/// and attributes protecting against XSS.
+///
+/// Modeled after the [rules employed by Github][1] when sanitizing GFM (Github
+/// Flavored Markdown). Notably this excludes CSS styles and other tags that
+/// easily interferes with the rest of the page.
+///
+/// [1]: https://github.com/jch/html-pipeline/blob/master/lib/html/pipeline/sanitization_filter.rb
+class SaneHtmlValidator {
+  final bool Function(String)? allowElementId;
+  final bool Function(String)? allowClassName;
+  final bool Function(String)? allowAttributes;
+  final Iterable<String>? Function(String)? addLinkRel;
+
+  SaneHtmlValidator({
+    required this.allowElementId,
+    required this.allowClassName,
+    required this.allowAttributes,
+    required this.addLinkRel,
+  });
+
+  String sanitize(String htmlString) {
+    final root = html_parser.parseFragment(htmlString);
+    _sanitize(root);
+    return root.outerHtml;
+  }
+
+  void _sanitize(Node node) {
+    if (node is Element) {
+      final tagName = node.localName!.toUpperCase();
+      if (!_allowedElements.contains(tagName)) {
+        node.remove();
+        return;
+      }
+      node.attributes.removeWhere((k, v) {
+        final attrName = k.toString();
+        if (attrName == 'id') {
+          return allowElementId == null || !allowElementId!(v);
+        }
+        if (attrName == 'class') {
+          if (allowClassName == null) return true;
+          node.classes.removeWhere((cn) => !allowClassName!(cn));
+          return node.classes.isEmpty;
+        }
+
+        return !_isAttributeAllowed(tagName, attrName, v);
+      });
+      if (tagName == 'A') {
+        final href = node.attributes['href'];
+        if (href != null && addLinkRel != null) {
+          final rels = addLinkRel!(href);
+          if (rels != null && rels.isNotEmpty) {
+            node.attributes['rel'] = rels.join(' ');
+          }
+        }
+      }
+    }
+    if (node.hasChildNodes()) {
+      // doing it in reverse order, because we could otherwise skip one, when a
+      // node is removed...
+      for (var i = node.nodes.length - 1; i >= 0; i--) {
+        _sanitize(node.nodes[i]);
+      }
+    }
+  }
+
+  bool _isAttributeAllowed(String tagName, String attrName, String value) {
+    if (_alwaysAllowedAttributes.contains(attrName)) return true;
+
+    if (allowAttributes != null && allowAttributes!(attrName)) return true;
+
+    // Special validators for special attributes on special tags (href/src/cite)
+    final attributeValidators = _elementAttributeValidators[tagName];
+    if (attributeValidators == null) {
+      return false;
+    }
+
+    final validator = attributeValidators[attrName];
+    if (validator == null) {
+      return false;
+    }
+
+    return validator(value);
+  }
+}

core/pubspec.yaml

@@ -42,6 +42,8 @@ dependencies:
 
   built_collection: 5.1.0
 
+  html: 0.15.0
+
 dev_dependencies:
   flutter_test:
     sdk: flutter