Fix #469

Author: mganss

src/HtmlSanitizer/HtmlSanitizer.cs

@@ -721,7 +721,7 @@ private void SanitizeStyleDeclaration(IElement element, ICssStyleDeclaration sty
 
                         foreach (var url in urls)
                         {
-                            sb.Append(val, ix, url.Match.Index);
+                            sb.Append(val, ix, url.Match.Index - ix);
                             sb.Append("url(");
                             sb.Append(url.Match.Groups[1].Value);
                             sb.Append(url.Url);

test/HtmlSanitizer.Tests/Tests.cs

@@ -3487,4 +3487,14 @@ public void VarUrlTest()
         var sanitized = sanitizer.Sanitize(html);
         Assert.Equal(html, sanitized);
     }
+
+    [Fact]
+    public void Number469Test()
+    {
+        // see https://github.com/mganss/HtmlSanitizer/issues/469
+        var html = @"<div style=""height: 0; background-image: url('https://example.com/1.jpg'), url('https://example.com/2.jpg'), url('https://example.com/3.jpg'); display: none;""/>";
+        var sanitizer = new HtmlSanitizer();
+        var sanitized = sanitizer.Sanitize(html);
+        Assert.Equal(@"<div style=""height: 0; background-image: url(&quot;https://example.com/1.jpg&quot;), url(&quot;https://example.com/2.jpg&quot;), url(&quot;https://example.com/3.jpg&quot;); display: none""></div>", sanitized);
+    }
 }