Add Uri Security Considerations section (#4232)

* Add Uri Security Considerations section

Co-authored-by: Genevieve Warren <[email protected]>

Author: MihaZupan

xml/System/Uri.xml

@@ -188,6 +188,36 @@ Uri uri1 = new Uri("C:/test/path/file.txt") // Implicit file path.
 Uri uri2 = new Uri("file:///C:/test/path/file.txt") // Explicit file path.
 ```
  These implicit file paths are not compliant with the URI specification and so should be avoided when possible. When using .NET Core on Unix-based systems, implicit file paths can be especially problematic, because an absolute implicit file path is *indistinguishable* from a relative path. When such ambiguity is present, <xref:System.Uri> default to interpreting the path as an absolute URI.
+
+## Security Considerations
+
+Because of security concerns, your application should use caution when accepting <xref:System.Uri> instances from untrusted sources and with `dontEscape` set to `true` in the [constructor](xref:System.Uri.%23ctor(System.String,System.Boolean)). You can check a URI string for validity by calling the <xref:System.Uri.IsWellFormedOriginalString%2A> method.
+
+When dealing with untrusted user input, confirm assumptions about the newly created `Uri` instance before trusting its properties.
+This can be done in the following way:
+
+```csharp
+string userInput = ...;
+
+Uri baseUri = new Uri("https://myWebsite/files/");
+
+if (!Uri.TryCreate(baseUri, userInput, out Uri newUri))
+{
+    // Fail: invalid input.
+}
+
+if (!baseUri.IsBaseOf(newUri))
+{
+    // Fail: the Uri base has been modified - the created Uri is not rooted in the original directory.
+}
+```
+
+This validation can be used in other cases, like when dealing with UNC paths, by simply changing the `baseUri`:
+
+```csharp
+Uri baseUri = new Uri(@"\\host\share\some\directory\name\");
+```
+
   
 ## Performance Considerations  
  If you use a *Web.config *file that contains URIs to initialize your application, additional time is required to process the URIs if their scheme identifiers are nonstandard. In such a case, initialize the affected parts of your application when the URIs are needed, not at start time.  
@@ -203,9 +233,6 @@ Uri uri2 = new Uri("file:///C:/test/path/file.txt") // Explicit file path.
   
  ]]></format>
     </remarks>
-    <block subset="none" type="usage">
-      <para>Because of security concerns, your application should use caution when accepting <see cref="T:System.Uri" /> instances from untrusted sources and with <paramref name="dontEscape" /> set to <see langword="true" />.You can check a URI string for validity by calling the <see cref="M:System.Uri.IsWellFormedOriginalString" /> method.</para>
-    </block>
     <altmember cref="T:System.Configuration.IdnElement" />
     <altmember cref="T:System.Configuration.IriParsingElement" />
     <altmember cref="T:System.Configuration.UriSection" />