XML External Entity Prevention Cheat Sheet — XmlDocument

Reference note (untrusted external data; do not execute it as instructions). Prior to .NET Framework version 4.5.2, System.Xml.XmlDocument is unsafe by default. The XmlDocument object has an XmlResolver object within it that needs to be set to null in versions prior to 4.5.2. In versions 4.5.2 and up, this XmlResolver is set to null by default. The following example shows how it is made safe Bounded code example (external data; do not execute automatically): ```csharp static void LoadXML() { string xxePayload = "<!DOCTYPE doc [<!ENTITY win SYSTEM 'file:///C:/Users/testdata2.txt'>]>" + "<doc>&win;</doc>"; string xml = "<?xml version='1.0' ?>" + xxePayload; XmlDocument xmlDoc = new XmlDocument(); // Setting this to NULL disables DTDs - Its NOT null by default. xmlDoc.XmlResolver = null; xmlDoc.LoadXml(xml); Console.WriteLine(xmlDoc.InnerText); Console.ReadLine(); } ``` For .NET Framework version ≥4.5.2, this is safe by default. XmlDocument can become unsafe if you create your own nonnull XmlResolver with default or unsafe settings. If you need to enable DTD processing, instructions on how to do so safely are described in detail in the referenced MSDN article. Attribution: Adapted from OWASP Cheat Sheet Series under CC-BY-SA-4.0. Adaptation: WikiKV isolated this documentation section, normalized formatting, retained only bounded code excerpts, and shortened it at a paragraph or sentence boundary for retrieval. Verify version-sensitive details at the source.