Added option to allow custom CSS properties (variables) that begin wi…

…th '--' characters. Defaults to 'false' so custom properties will be removed. Added protected function to allow for custom evaluation of the custom property name prior to marking it for removal.

src/HtmlSanitizer/HtmlSanitizer.cs

@@ -204,6 +204,11 @@ public HtmlSanitizer(HtmlSanitizerOptions options)
         /// </value>
         public ISet<string> AllowedCssProperties { get; private set; }
 
+        /// <summary>
+        /// Allow all custom CSS properties (variables) prefixed with <c>--</c>.
+        /// </summary>
+        public bool AllowCssCustomProperties { get; set; }
+
         /// <summary>
         /// Gets or sets a regex that must not match for legal CSS property values.
         /// </summary>
@@ -726,6 +731,19 @@ protected void SanitizeStyle(IElement element, string baseUrl)
             SanitizeStyleDeclaration(element, styles, baseUrl);
         }
 
+        /// <summary>
+        /// Verify if the given CSS property name is allowed. By default this will
+        /// check if the property is in the <see cref="AllowedCssProperties"/> set,
+        /// or if the property is a custom property and <see cref="AllowCssCustomProperties"/> is true.
+        /// </summary>
+        /// <param name="propertyName">The name of the CSS property.</param>
+        /// <returns>True if the property is allowed or not.</returns>
+        protected virtual bool IsAllowedCssProperty(string propertyName)
+        {
+            return AllowedCssProperties.Contains(propertyName)
+                || AllowCssCustomProperties && propertyName != null && propertyName.StartsWith("--");
+        }
+
         private void SanitizeStyleDeclaration(IElement element, ICssStyleDeclaration styles, string baseUrl)
         {
             var removeStyles = new List<Tuple<ICssProperty, RemoveReason>>();
@@ -736,7 +754,7 @@ private void SanitizeStyleDeclaration(IElement element, ICssStyleDeclaration sty
                 var key = DecodeCss(style.Name);
                 var val = DecodeCss(style.Value);
 
-                if (!AllowedCssProperties.Contains(key))
+                if (!IsAllowedCssProperty(key))
                 {
                     removeStyles.Add(new Tuple<ICssProperty, RemoveReason>(style, RemoveReason.NotAllowedStyle));
                     continue;

test/HtmlSanitizer.Tests/Tests.cs

@@ -2903,7 +2903,7 @@ public void ThreadTest()
             var waiting = numThreads;
             var methods = typeof(HtmlSanitizerTests).GetTypeInfo().GetMethods()
                 .Where(m => m.GetCustomAttributes(typeof(Xunit.FactAttribute), false).Cast<Xunit.FactAttribute>().Any(f => f.Skip == null))
-                .Where(m => m.Name != "ThreadTest" && m.Name != "HexColorTest");
+                .Where(m => m.Name != nameof(ThreadTest) && m.Name != nameof(HexColorTest));
             var threads = Shuffle(methods, random)
                 .Take(numThreads)
                 .Select(m => new Thread(() =>
@@ -3589,4 +3589,44 @@ public void KeepChildNodesTextTest()
         var sanitized = sanitizer.Sanitize(input);
         Assert.Equal("&lt;img&gt;&amp;lt;img&amp;gt;", sanitized);
     }
+
+    [Fact]
+    public void AllowStyleAttributeCssCustomPropertiesTest()
+    {
+        var input = "<div style=\"--my-var: 1px\"></div>";
+        var sanitizer = new HtmlSanitizer { AllowCssCustomProperties = true };
+        sanitizer.AllowedTags.Remove("iframe");
+        var sanitized = sanitizer.Sanitize(input);
+        Assert.Equal("<div style=\"--my-var: 1px\"></div>", sanitized);
+    }
+
+    [Fact]
+    public void AllowStyleTagCssCustomPropertiesTest()
+    {
+        var input = "<style>:root { --my-var: 1px }</style>";
+        var sanitizer = new HtmlSanitizer { AllowCssCustomProperties = true };
+        sanitizer.AllowedTags.Add("style");
+        var sanitized = sanitizer.Sanitize(input);
+        Assert.Equal("<style>:root { --my-var: 1px }</style>", sanitized);
+    }
+
+    [Fact]
+    public void DisallowStyleAttributeCssCustomPropertiesTest()
+    {
+        var input = "<div style=\"--my-var: 1px\"></div>";
+        var sanitizer = new HtmlSanitizer();
+        sanitizer.AllowedTags.Remove("iframe");
+        var sanitized = sanitizer.Sanitize(input);
+        Assert.Equal("<div></div>", sanitized);
+    }
+
+    [Fact]
+    public void DisallowStyleTagCssCustomPropertiesTest()
+    {
+        var input = "<style>:root { --my-var: 1px }</style>";
+        var sanitizer = new HtmlSanitizer();
+        sanitizer.AllowedTags.Add("style");
+        var sanitized = sanitizer.Sanitize(input);
+        Assert.Equal("<style>:root { }</style>", sanitized);
+    }
 }