.NET Security Cheat Sheet

Introduction
This page intends to provide quick basic .NET security tips for developers.

The .NET Framework
The .NET Framework is Microsoft's principal platform for line of business development. It is the supporting API for ASP.NET, Windows Desktop applications, Windows Communication Foundation services, SharePoint, Visual Studio Tools for Office and other technologies.

Updating the Framework
The .NET Framework is kept up-to-date by Microsoft with the Windows Update service. Developers do not normally need to run seperate updates to the Framework. Windows update can be accessed at http://windowsupdate.microsoft.com/ or from the Windows Update program on a Windows computer.

Individual frameworks can be kept up to date using NuGet. As Visual Studio prompts for updates, build it into your lifecycle.

.NET Framework Guidance
The .NET Framework is the set of APIs that support an advanced type system, data, graphics, network, file handling and most of the rest of what is needed to write line of business apps in the Microsoft ecosystem. It is a nearly ubiquitous library that is strong named and versioned at the assembly level.

Data Access

 * Use Parameterized SQL commands for all data access, without exception.
 * Do not use SqlCommand with a string parameter made up of a concatenated SQL String.
 * Whitelist allowable values coming from the user. Use enums, TryParse or lookup values to assure that the data coming from the user is as expected.
 * Apply the principle of least privilege when setting up the Database User in your database of choice. The database user should only be able to access items that make sense for the use case.
 * Use an indirect reference map. Don't allow users to see the primary key of a database row.

Encryption

 * Never, ever write your own encryption.
 * When using a hashing function, use a salt value added to the original value before hashing.
 * Use a strong hash algorithm.
 * In .NET 4.5 the strongest hashing algorithm is System.Security.Cryptography.SHA512.
 * Make sure your application can support a change in algorithm.

General

 * Always check the MD5 hashes of the .NET Framework assemblies to prevent the possibility of rootkits in the framework. Altered assemblies are possible and simple to produce. Checking the MD5 hashes will prevent using altered assemblies on a server or client machine. See [[File:Presentation - .NET Framework Rootkits - Backdoors Inside Your Framework.ppt]]
 * Lock down the config file.
 * Remove all aspects of configuration that are not in use.
 * Encrypt sensitive parts of the web.config using aspnet_regiis -pe

ASP.NET Web Forms Guidance
ASP.NET Web Forms is the original browser-based application development API for the .NET framework, and is still the most common line of business platform the web application development.


 * Always use HTTPS.

HTTP validation and encoding

 * Do not disable validateRequest in the web.config or the page setup. This value enables the AntiXSS library in ASP.NET and should be left intact to present cross site scripting.
 * Whitelist allowable values anytime user input is accepted. The regex namespace is particularly useful for checking to make sure an email address or URI is as expected.
 * Always encode output to the browser using Server.HtmlEncode.
 * Validate the URI format using Uri.IsWellFormedUriString.

Authorization and authentication

 * Use cookies for persistence when possible. Cookieless Auth will default to UseDeviceProfile.
 * Don't trust the URI of the request for persistence of the session or authorization. It can be easily faked.
 * Reduce the session time from the default of 20 minutes. If slidingExpiration is used this timeout starts after the last request, so active users won't be affected.
 * If you don't require SSL, disable slidingExpiration.
 * Always implement proper access controls.
 * Compare user provided username with User.Identity.Name.
 * Check roles against User.Identity.IsInRole.
 * Use the native membership provider.
 * Explicitly authorize resource requests, using User.Identity.Name.
 * Leverage role based authorization using User.Identity.IsInRole.

ASP.NET MVC Guidance
ASP.NET MVC is the new hotness from Microsoft, employing the popular MVC pattern used by frameworks like Ruby on Rails.


 * Always use HTTPS.
 * Use the Syncronizer token pattern. In Web Forms, this is handled by ViewState, but in MVC you need to use ValidateAntiForgeryToken.
 * Decorate controller methods using PrincipalPermission to prevent unrestricted URL access.
 * Make use of IsLocalUrl in logon methods.

if (MembershipService.ValidateUser(model.UserName, model.Password)) {        FormsService.SignIn(model.UserName, model.RememberMe); if (IsLocalUrl(returnUrl)) {            return Redirect(returnUrl); }        else {            return RedirectToAction("Index", "Home"); }    }


 * Use the AntiForgeryToken on every form post to prevent CSRF attacks. In the HTML:

<% using(Html.Form(“Form", "Update")) { %>       <%= Html.AntiForgeryToken %>    <% } %>

and on the controller method:

[ValidateAntiForgeryToken] public ViewResult Update {       // gimmee da codez }

Windows Forms Guidance

 * Use partial trust when possible. Partially trusted Windows applications make the attack surface of an application much smaller. Manage a list of what permissions your app must use, and what it may use, and then make the request for those permissions declaratively at run time.
 * Use ClickOnce deployment. For enhance3d permissions, use permission elevation at runtime or trusted application deployment at install time.

WCF Guidance

 * Always use HTTPS.

Authors and Primary Editors
Troy Hunt - troyhunt [at] hotmail.com Bill Sempf - bill [at] pointweb.net User:Bill Sempf Patrick Leclerc - patrick.leclerc [at] owasp.org Jerry Hoff - jerry.hoff [at] owasp.org