diff --git a/src/Microsoft.AspNetCore.Http.Abstractions/HostString.cs b/src/Microsoft.AspNetCore.Http.Abstractions/HostString.cs
index e8c6ac3b35..4b4b24f7e5 100644
--- a/src/Microsoft.AspNetCore.Http.Abstractions/HostString.cs
+++ b/src/Microsoft.AspNetCore.Http.Abstractions/HostString.cs
@@ -4,6 +4,7 @@
 using System;
 using System.Globalization;
 using Microsoft.AspNetCore.Http.Abstractions;
+using Microsoft.AspNetCore.Http.Internal;
 
 namespace Microsoft.AspNetCore.Http
 {
@@ -123,20 +124,29 @@ namespace Microsoft.AspNetCore.Http
                 return string.Empty;
             }
 
-            string host, port;
-
-            GetParts(out host, out port);
-
-            if (host.IndexOf('[') == -1)
-            { 
-                var mapping = new IdnMapping();
-                host = mapping.GetAscii(host);
+            int i;
+            for (i = 0; i < _value.Length; ++i)
+            {
+                if (!HostStringHelper.IsSafeHostStringChar(_value[i]))
+                {
+                    break;
+                }
             }
 
-            return string.IsNullOrEmpty(port)
-                ? host
-                : string.Concat(host, ":", port);
-            
+            if (i != _value.Length)
+            {
+                string host, port;
+                GetParts(out host, out port);
+
+                var mapping = new IdnMapping();
+                host = mapping.GetAscii(host);
+
+                return string.IsNullOrEmpty(port)
+                    ? host
+                    : string.Concat(host, ":", port);
+            }
+
+            return _value;
         }
 
         /// <summary>
diff --git a/src/Microsoft.AspNetCore.Http.Abstractions/Internal/HostStringHelper.cs b/src/Microsoft.AspNetCore.Http.Abstractions/Internal/HostStringHelper.cs
new file mode 100644
index 0000000000..f4cfac52af
--- /dev/null
+++ b/src/Microsoft.AspNetCore.Http.Abstractions/Internal/HostStringHelper.cs
@@ -0,0 +1,36 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+namespace Microsoft.AspNetCore.Http.Internal
+{
+    internal class HostStringHelper
+    {
+        // Allowed Characters:
+        // A-Z, a-z, 0-9, ., 
+        // -, %, [, ], : 
+        // Above for IPV6
+        private static bool[] SafeHostStringChars = {
+            false, false, false, false, false, false, false, false,     // 0x00 - 0x07
+            false, false, false, false, false, false, false, false,     // 0x08 - 0x0F
+            false, false, false, false, false, false, false, false,     // 0x10 - 0x17
+            false, false, false, false, false, false, false, false,     // 0x18 - 0x1F
+            false, false, false, false, false, true,  false, false,     // 0x20 - 0x27
+            false, false, false, false, false, true,  true,  false,     // 0x28 - 0x2F
+            true,  true,  true,  true,  true,  true,  true,  true,      // 0x30 - 0x37
+            true,  true,  true,  false, false, false, false, false,     // 0x38 - 0x3F
+            false, true,  true,  true,  true,  true,  true,  true,      // 0x40 - 0x47
+            true,  true,  true,  true,  true,  true,  true,  true,      // 0x48 - 0x4F
+            true,  true,  true,  true,  true,  true,  true,  true,      // 0x50 - 0x57
+            true,  true,  true,  true,  false, true,  false, false,     // 0x58 - 0x5F
+            false, true,  true,  true,  true,  true,  true,  true,      // 0x60 - 0x67
+            true,  true,  true,  true,  true,  true,  true,  true,      // 0x68 - 0x6F
+            true,  true,  true,  true,  true,  true,  true,  true,      // 0x70 - 0x77
+            true,  true,  true,  false, false, false, false, false,     // 0x78 - 0x7F
+        };
+
+        public static bool IsSafeHostStringChar(char c)
+        {
+            return c < SafeHostStringChars.Length && SafeHostStringChars[c];
+        }
+    }
+}