System.Xml.XmlSecureResolver class

Article
01/04/2024

This article provides supplementary remarks to the reference documentation for this API.

The XmlUrlResolver class is the default resolver for all classes in the System.Xml namespace. It's used to load XML documents and to resolve external resources such as entities, DTDs or schemas, and import or include directives.

You can override this default by specifying the XmlResolver object to use. For example, if you want to restrict the resources that the underlying XmlResolver can access, you can use an XmlSecureResolver object.

XmlSecureResolver wraps around a concrete implementation of XmlResolver and restricts the resources that the underlying XmlResolver has access to. For example, XmlSecureResolver has the ability to prohibit cross-domain redirection, which occurs from an embedded Uniform Resource Identifier (URI) reference.

When you construct an XmlSecureResolver object, you provide a valid XmlResolver implementation along with a URL, an instance of an evidence object, or a permission set, which is used by the XmlSecureResolver to determine security. Either a System.Security.PermissionSet is generated or the existing one is used and PermissionSet.PermitOnly is called on it to help secure the underlying XmlResolver.

Important

XmlSecureResolver objects can contain sensitive information such as user credentials. Be careful when caching XmlSecureResolver objects and should not pass the XmlSecureResolver object to an untrusted component.

Important

There are differences in the security infrastructure for code running on the .NET common language runtime (CLR) and for code running on the CLR that is integrated within Microsoft SQL Server 2005. This can lead to cases where code developed for the .NET CLR operates differently when used on the SQL Server integrated CLR. One of these differences affects the XmlSecureResolver class when you have evidence that is based on a URL (that is, when you use the CreateEvidenceForUrl(String) method or the XmlSecureResolver constructor). The policy resolution mechanism of the SQL Server integrated CLR does not use the Url or Zone information. Instead, it grants permissions based on the GUID that the server adds when assemblies are loaded. When you use the XmlSecureResolver in the SQL Server integrated CLR, provide any required evidence directly by using a specified PermissionSet.

To use a secure resolver

Create an XmlSecureResolver with the correct permission set.

Create an XmlReaderSettings object that uses the XmlSecureResolver object.

XmlReaderSettings settings = new XmlReaderSettings();
settings.XmlResolver = myResolver;

Dim settings As New XmlReaderSettings()
settings.XmlResolver = myResolver

Pass the XmlReaderSettings object to the Create method when you're creating the XmlReader object.

XmlReader reader = XmlReader.Create("books.xml", settings);

Dim reader As XmlReader = XmlReader.Create("books.xml", settings)

To restrict access by using a URL

Use the XmlSecureResolver(XmlResolver, String) constructor to create an XmlSecureResolver object that is allowed to access your local intranet site only.

XmlSecureResolver myResolver = new XmlSecureResolver(new XmlUrlResolver(), "http://myLocalSite/");

Dim myResolver As New XmlSecureResolver(New XmlUrlResolver(), "http://myLocalSite/")

To restrict access by using a permission set

Create a WebPermission object.

WebPermission myWebPermission = new WebPermission(PermissionState.None);

Dim myWebPermission As New WebPermission(PermissionState.None)

Specify the URLs that you want to allow access to.

myWebPermission.AddPermission(NetworkAccess.Connect, "http://www.contoso.com/");
myWebPermission.AddPermission(NetworkAccess.Connect, "http://litwareinc.com/data/");

myWebPermission.AddPermission(NetworkAccess.Connect, "http://www.contoso.com/")
myWebPermission.AddPermission(NetworkAccess.Connect, "http://litwareinc.com/data/")

Add the web permissions to the PermissionSet object.

PermissionSet myPermissions = new PermissionSet(PermissionState.None);
myPermissions.AddPermission(myWebPermission);

Dim myPermissions As New PermissionSet(PermissionState.None)
myPermissions.AddPermission(myWebPermission)

Use the XmlSecureResolver(XmlResolver, PermissionSet) constructor to create an XmlSecureResolver object by using the permission set.
```
XmlSecureResolver myResolver = new XmlSecureResolver(new XmlUrlResolver(), myPermissions);
```
```
Dim myResolver As New XmlSecureResolver(New XmlUrlResolver(), myPermissions)
```
See the XmlSecureResolver reference page for another example.

To restrict access by using evidence

You can restrict access by using the XmlSecureResolver(XmlResolver, Evidence) constructor and specifying Evidence. The Evidence is used to create the PermissionSet that is applied to the underlying XmlResolver. The XmlSecureResolver calls PermitOnly on the created PermissionSet before opening any resources.

Here are some common scenarios and the type of evidence to provide for each:

If you are working in a fully trusted environment, use your assembly to create the evidence:

Evidence myEvidence = this.GetType().Assembly.Evidence;
XmlSecureResolver myResolver;
myResolver = new XmlSecureResolver(new XmlUrlResolver(), myEvidence);

Dim myEvidence As Evidence = Me.GetType().Assembly.Evidence
Dim myResolver As XmlSecureResolver
myResolver = New XmlSecureResolver(New XmlUrlResolver(), myEvidence)

If you are working in a semi-trusted environment, you have code or data coming from an outside source, and you know the origin of the outside source and have a verifiable URI, use the URI to create the evidence:


Evidence myEvidence = XmlSecureResolver.CreateEvidenceForUrl(sourceURI);
XmlSecureResolver myResolver = new XmlSecureResolver(new XmlUrlResolver(), myEvidence);

Dim myEvidence As Evidence = XmlSecureResolver.CreateEvidenceForUrl(sourceURI)
Dim myResolver As New XmlSecureResolver(New XmlUrlResolver(), myEvidence)

If you are working in a semi-trusted environment and you have code or data coming from an outside source, but you don't know the origin of the outside source, either:

Set the evidence parameter to null. This allows no access to resources.

-or-

If your application requires some access to resources, request evidence from the caller.

To use the secure resolver to load an XSLT style sheet

Create an XmlSecureResolver with the correct permission set.

Pass the XmlSecureResolver to the Load method.

XslCompiledTransform xslt = new XslCompiledTransform();
xslt.Load("http://serverName/data/xsl/sort.xsl", null, myResolver);

Dim xslt As New XslCompiledTransform()
xslt.Load("http://serverName/data/xsl/sort.xsl", Nothing, myResolver)

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