OWASP Top 10 .NET Protection: A Guide (Part 1)

1. Injection

1. Using an object-relational mapper (ORM) or stored procedures is the most effective way of countering the SQL Injection vulnerability.
2. Use parameterized queries where a direct SQL query must be used.
3. Practise Least Privilege - Connect to the database using an account with a minimum set of permissions required to do it's job i.e. not the same account

var sql = @"Update [User] SET FirstName = @FirstName WHERE Id = @Id";
context.Database.ExecuteSqlCommand(
    sql,
    new SqlParameter("@FirstName", firstname),
    new SqlParameter("@Id", id));

string strQry = "SELECT * FROM Users WHERE UserName='" + txtUser.Text + "' AND Password='"
                + txtPassword.Text + "'";
EXEC strQry // SQL Injection vulnerability!

1. Use System.Diagnostics.Process.Start to call underlying OS functions.

System.Diagnostics.Process process = new System.Diagnostics.Process();
System.Diagnostics.ProcessStartInfo startInfo = new System.Diagnostics.ProcessStartInfo();
startInfo.FileName = "validatedCommand";
startInfo.Arguments = "validatedArg1 validatedArg2 validatedArg3";
process.StartInfo = startInfo;
process.Start();

2. Use whitelist validation on all user-supplied input. Input validation prevents improperly formed data from entering an information system.

//User input
string ipAddress = "127.0.0.1";

//check to make sure an ip address was provided
if (!string.IsNullOrEmpty(ipAddress))
{
 // Create an instance of IPAddress for the specified address string (in
 // dotted-quad, or colon-hexadecimal notation).
 if (IPAddress.TryParse(ipAddress, out var address))
 {
  // Display the address in standard notation.
  return address.ToString();
 }
 else
 {
  //ipAddress is not of type IPAddress
 }
}

1. Almost any characters can be used in Distinguished Names. However, some must be escaped with the backslash \ escape character.
2. The space character must be escaped only if it is the leading or trailing character in a component name, such as a Common Name. Embedded spaces should not be escaped.

2. Broken Authentication

1. Use ASP.net Core Identity. ASP.net Core Identity framework is well configured by default, where it uses secure password hashes and an individual salt. Identity uses the PBKDF2 hashing function for passwords, and they generate a random salt per user.
   
2. Set a secure password policy

//startup.cs
services.Configure<IdentityOptions>(options =>
{
 // Password settings
 options.Password.RequireDigit = true;
 options.Password.RequiredLength = 8;
 options.Password.RequireNonAlphanumeric = true;
 options.Password.RequireUppercase = true;
 options.Password.RequireLowercase = true;
 options.Password.RequiredUniqueChars = 6;


 options.Lockout.DefaultLockoutTimeSpan = TimeSpan.FromMinutes(30);
 options.Lockout.MaxFailedAccessAttempts = 3;

 options.SignIn.RequireConfirmedEmail = true;

 options.User.RequireUniqueEmail = true;
});

3. Set a cookie policy

//startup.cs
services.ConfigureApplicationCookie(options =>
{
 options.Cookie.HttpOnly = true;
 options.Cookie.Expiration = TimeSpan.FromHours(1)
 options.SlidingExpiration = true;
});

3. Sensitive Data Exposure

1. Use a strong hash to store password credentials.
2. Enforce passwords with a minimum complexity that will survive a dictionary attack i.e. longer passwords that use the full character set (numbers, symbols and letters) to increase the entropy.
3. Use a strong encryption routine such as AES-512 where personally identifiable data needs to be restored to it's original format.
4. Use TLS 1.2 for your entire site.
5. Have a strong TLS policy, use TLS 1.2 wherever possible.
6. Ensure headers are not disclosing information about your application.

app.UseHsts(hsts => hsts.MaxAge(365).IncludeSubdomains());
app.UseXContentTypeOptions();
app.UseReferrerPolicy(opts => opts.NoReferrer());
app.UseXXssProtection(options => options.FilterDisabled());
app.UseXfo(options => options.Deny());

app.UseCsp(opts => opts
 .BlockAllMixedContent()
 .StyleSources(s => s.Self())
 .StyleSources(s => s.UnsafeInline())
 .FontSources(s => s.Self())
 .FormActions(s => s.Self())
 .FrameAncestors(s => s.Self())
 .ImageSources(s => s.Self())
 .ScriptSources(s => s.Self())
 );

4. XML External Entities (XXE)

1. Use LINQ to XML. Both the XElement and XDocument objects in the System.Xml.Linq library are safe from XXE injection by default.
2. Use XmlDictionaryReader. System.Xml.XmlDictionaryReader is safe by default, as when it attempts to parse the DTD, the compiler throws an exception saying that "CData elements not valid at top level of an XML document".
3. Using XmlDocument set XmlResolver to null.

 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();
 }

4. Use XmlNodeReader. System.Xml.XmlNodeReader objects are safe by default and will ignore DTDs even when constructed with an unsafe parser or wrapped in another unsafe parser.
5. Use XmlReader. System.Xml.XmlReader objects are safe by default.
6. Use XmlTextReader. In .NET Framework versions 4.5.2 and up, XmlTextReader's internal XmlResolver is set to null by default, making the XmlTextReader ignore DTDs by default.
7. Using XPathNavigator give it a  safe parser like XmlReader (which is safe by default) in the XPathDocument's constructor.

XmlReader reader = XmlReader.Create("example.xml");
XPathDocument doc = new XPathDocument(reader);
XPathNavigator nav = doc.CreateNavigator();
string xml = nav.InnerXml.ToString();

8. Use XslCompiledTransform. System.Xml.Xsl.XslCompiledTransform (an XML transformer) is safe by default as long as the parser it's given is safe.

5. Broken Access Control

1. Ensure cookies are sent via httpOnly:

CookieHttpOnly = true,

2. Reduce the time period a session can be stolen in by reducing session timeout and removing sliding expiration:

ExpireTimeSpan = TimeSpan.FromMinutes(60),
SlidingExpiration = false

3. Protect LogOn, Registration and password reset methods against brute force attacks by throttling requests (see code below), consider also using ReCaptcha.

[HttpPost]
[AllowAnonymous]
[ValidateAntiForgeryToken]
[AllowXRequestsEveryXSecondsAttribute(Name = "LogOn",
Message = "You have performed this action more than {x} times in the last {n} seconds.",
Requests = 3, Seconds = 60)]
public async Task<ActionResult> LogOn(LogOnViewModel model, string returnUrl)

4. Authorize users on all externally facing endpoints. The .NET framework has many ways to authorize a user, use them at method level:

[Authorize(Roles = "Admin")]
[HttpGet]
public ActionResult Index(int page = 1)


// or better yet, at controller level:
[Authorize]
public class UserController

5. When you have a resource (object) which can be accessed by a reference (in the sample below this is the id) then you need to ensure that the user is intended to be there

// Insecure
public ActionResult Edit(int id)
{
  var user = _context.Users.FirstOrDefault(e => e.Id == id);
  return View("Details", new UserViewModel(user);
}

// Secure
public ActionResult Edit(int id)
{
  var user = _context.Users.FirstOrDefault(e => e.Id == id);
  // Establish user has right to edit the details
  if (user.Id != _userIdentity.GetUserId())
  {
        HandleErrorInfo error = new HandleErrorInfo(
            new Exception("INFO: You do not have permission to edit these details"));
        return View("Error", error);
  }
  return View("Edit", new UserViewModel(user);
}