Optimize dkim signature parser logic

Author: tinohager

src/Nager.EmailAuthentication/FragmentParsers/DkimSignatureDataFragmentParserV1.cs

@@ -516,9 +516,8 @@ private static ParsingResult[] ValidateSignedHeaderFields(ValidateRequest valida
 
             var parts = validateRequest.Value.Split(':');
 
-            //TODO: check that headers are signed at most twice (only oversigning)
             //https://security.stackexchange.com/questions/265408/how-many-times-need-e-mail-headers-be-signed-with-dkim-to-mitigate-dkim-header-i#:~:text=If%20the%20e%2Dmail%20uses,field%20of%20the%20DKIM%20signature.
-            var groupedHeaders = parts.GroupBy(o => o).Select(g => new { g.Key, Count = g.Count() });
+            var groupedHeaders = parts.GroupBy(o => o).Select(g => new { Key = g.Key.ToLower(), Count = g.Count() });
             foreach (var groupedHeader in groupedHeaders)
             {
                 if (groupedHeader.Count == 2)
@@ -530,6 +529,15 @@ private static ParsingResult[] ValidateSignedHeaderFields(ValidateRequest valida
                         Message = $"{groupedHeader.Key} oversigning detected"
                     });
                 }
+                else if (groupedHeader.Count > 2)
+                {
+                    parsingResults.Add(new ParsingResult
+                    {
+                        Status = ParsingStatus.Warning,
+                        Field = validateRequest.Field,
+                        Message = $"{groupedHeader.Key} multiple occurrences of the same header detected"
+                    });
+                }
             }
 
             var recommendedHeaders = new string[] { "from", "to", "subject", "reply-to", "date", "cc", "content-type" };

src/Nager.EmailAuthentication/Nager.EmailAuthentication.csproj

@@ -20,7 +20,7 @@
 
     <TargetFrameworks>net8.0;net9.0</TargetFrameworks>
 
-    <Version>2.0.2</Version>
+    <Version>2.0.3</Version>
   </PropertyGroup>
 
   <ItemGroup>