OWASP Top 10 #4 – Insecure Direct Object References

Direct Object Reference

A direct object reference is a key or id which is used identify a piece or set of data. The data would typically be a record within a a table of a database.

They can be:

  • Patterns such as incrementing integers/ids
  • Natural keys such as a username or email address.
  • Discoverable data such as a national insurance number or a social security number.

What Is it?

An insecure direct object reference is when a user accesses a direct object reference when they are not permitted to.

This can be from an unsecured page and an unauthenticated user. It can also come from a secured page and authenticated user, where there is simply insufficient checks that they can view the data they have requested.

Quite often the user guesses or enumerates a direct object reference either manually or by an automated script.

If the data should only be visible to a subset of people then any requests to that data should be validated that they are permitted to see that data. Simply relying on not displaying links to the user is not secure enough.

How Can I Protect Against It?

Validate Access

The first point to address is to validate that the user has permission to access the data.

For example; is the person permitted to see the financial records of the bank account in question?

Quite often the validation checks are placed around populating the UI to navigate to the data but no checks are placed around querying the data.

Indirect Reference Maps

A normal approach to view database records on a web page is to expose the table fields and their direct object references on the URL

https://www.MySite/Foo.aspx?id=1

Here an attacker can simply increment the id to try and view other peoples Foo which are simple incrementing ids or identity indexes generated automatically by the database.

An indirect reference provides an abstraction between what is publicly observable in the URL and what is used to uniquely identify a record within a database.

A dictionary is used to store a map between the id and a randomly generated cryptically secure identifier. We then pass these secure identifier to the user as part of URLs which they can request.

https://www.MySite/Foo.aspx?id=ASWERDFMJSDGJSERSASKAWEOSDSDF

These keys should also be mapped to the user’s session to protect against the ids being leaked:

The following code shows how to generate a cryptically safe unique id using the RNGCryptoServiceProvider class.

using System.Security.Cryptography;

using( var rng = new RNGCryptoServiceProvider() )
{
    map[id] = HttpServerUtility.UrlTokenEncode(rng.GetBytes(new byte[32]));
}

// Store the map within the users session
HttpContext.Current.Session["Map"] = map;

In the example above it would be best to only create one instance of the RNGCryptoServiceProvider class regardless of how many direct object references are being made. Consider creating a class which implements IDisposable which calls Dispose upon the instance of RNGCryptoServiceProvider within it’s Dispose method.

Obfuscation via non-discoverable surrogate keys

A surrogate key is a set of fields which when used together can uniquely identify a record. These are harder to guess than a single column which has a distinctive pattern such as an identify index which increments by one each time.

Surrogate keys would require more space to index and are also slower, it is a trade off between performance and security.

A random unique identifier such as a GUID is harder to guess but does not perform as well as an integer when used to search upon. Also for tables such as logs and audits which are used to generate a high number records, the indexes will become fragmented and as such will have more overhead for de-fragmenting them regularly.

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OWASP Top 10 #5 – Cross Site Request Forgery (CSRF)

What Is It?

Before we talk about CSRF it is important to understand that all cookies created by a domain are sent back to that domain during page requests regardless which domain the page originated from. This includes cookies which contain authentication session cookies.

Hackers can simply change the action attribute of a form to be that of the domain/URL they are trying to breach. An unsuspecting site would only ensure that the authentication session token is valid. It would therefore be possible to use the users open session to perform malicious actions such as saving or reading data from a database.

How Can I Protect My Self From It?

Protection from CSRF attacks has a simple solution. Send another cryptically secure token to the user along with the authentication session token. This token is sent both on the page being generated as well as a cookie.

Upon receiving a post back from a page, the server simply reads the CSRF token from the page and the cookie and ensures that they are identical.

As only a page generated from the site would contain the token, any fake page posting back to the server will be missing the correct CSRF token.

MVC

The MVC helper method AntiForgeyToken generates a CSRF token and persists it in a cookie and a hidden input element on the page.

using (Html.BeginForm())
{
  @Html.AntiForgeyToken()
}

Validation against a CSRF attack is made by decorating the controller’s action method with the ValidateAntiForgeryToken attribute.

[HttpPost]
[Authorise]
[ValidateAntiForgeryToken]
public ActionResult Index(McModel bigMac)
{
}

If a CSRF attack is caught a System.Mvc.HttpAntiForgeryException is thrown and the user is presented with a 500 status code.

ASP.NET Web Forms

A page written in ASP.NET web forms would traditionally need to manually perform the CSRF token, its persistence and checks to ensure they are identical when the page is posted back.

However we can now make use of the AntiForgery class.

https://msdn.microsoft.com/en-us/library/system.web.helpers.antiforgery%28v=vs.111%29.aspx

Within the aspx page a call to the GrtHtml method would generate the CSRF token which is then persisted within a cookie and an input element upon the page.

<%=System.Web.Helpers.AntiForgery.GetHtml() %>

Validation against a CSRF attack would then be performed during a page post back with the Validation method.

protected void Page_Load(object sender, EventArgs e)
{
    if (IsPostBack) {
        AntiForgery.Validate();
    }
}

Securing Cookies

Http Only Cookies

Using AntiForgery.GetHtml or @Html.AntiForgeyToken should create cookies which are secured for access by the server only and not via a client side script.

However where a more manual approach has been made to CSRF protection it is important to ensure that the CSRF token cannot be read from by a client side script. If this is not the case then a malicious page could read the token and simply add it to its form data before posting back to the server; circumventing our protection.

Cookies can be set as HttpOnly via a property during their creation:

var cookie = new HttpCookie("MyCookies", DateTime.Now.ToString());
cookie.HttpOnly = true;

Cookies can be set as HttpOnly globally for a website via the web.config:

httpCookies domain="String" httpOnlyCookies="true"

Note is it probably wise to set httpOnlyCookies to true regardless of your CSRF approach. Any information stored within a cookie which could be readable on the client side is probably a bad idea.

SSL Only Cookies

Where cookies contain sensitive information such as CSRF or authentication session token, it would be wise to enforce their transportation via SSL or HTTPS, as such preventing people from spying on the network traffic and therefore stealing them.

This can be done globally for a site within the web.config:

httpCookies domain="String" httpOnlyCookies="true" requireSSL="true"

However this approach might not always be possible. We can conditionally determine when to enforce SSL communication for cookies by checking the forms authentication settings and the connection for their status of secure connections. The individual cookie can then then be secured for SSL communication only via the Secure property.

var cookie = new HttpCookie("MyCookies", DateTime.Now.ToString());
cookie.HttpOnly = true;

if( FormsAuthentication.RequireSSL && Request.IsSecureConnection) {
    cookies.Secure = true;
}

OWASP Top 10 #3 – Broken Authentication & Session Management

How to secure .NET websites from broken authentication and session management security attacks.

Sessions In A Stateless Protocol

To understand broken authentication attacks such as session hijacking it is important to understand how a user is logged in and kept logged in within the stateless protocol HTTP.

HTTP is a stateless protocol, each request is independent of every other request. Any knowledge of previous requests and their contents is maintained outside of the bounds of the protocol.

ASP.NET maintains session data either on the client side in browser cookies or on the server side in memory or within a database and ties the data to the user via session which is identified by unique identifier.

Web sites which require a user to log in make use of a unique identifier or session token which is created during then authentication process, associated or linked to the users account and then passed between the users web browser and the web server via a cookie.

What Is It?

If the authentication session token is discovered by a potential attacker, it can be used to steal the users session. This can be by either a session fixation or a session hijack.

Session hijacking is when an attacker gets hold of a session identifier and uses this to gain access to a users session and hence impersonates them.

Session fixation is when a user who is logged in to a site overrides their session identifier set in a cookie by passing in another session identifier within the query string; thus pretending to be someone else with potentially elevated user access.

Session ids can be passed between client and server by a query string parameter within a URL as well as cookies and form data.

How Can I Protect Myself From It?

Do Not Persist Session Tokens In The URL

Seeing as URLs are logged, shared and retrievable from a browser’s history; the first line of defence is not to persist session tokens in URLs, you should favour cookies.

Note that a default form post action is GET; I have fixed many security issues which were caused simply by forgetting to set the action to POST.

Persist Session Tokens In Cookies

To get .NET to persist the session token within cookies you can set the cookieless attribute of the sessionState node to UseCookies within the web.config.

&lt;system.web&gt;
  &lt;sessionState cookieless=&quot;UseCookies&quot; /&gt;

Other options of the variable are:

  • UseUri
  • Always use the URL regardless of device support
  • UreCookies (default)
  • Always use cookies regardless of device support
  • UseDeviceProfile
  • ASP.NET determines if cookies are supported in the browser and falls back to URL if not.
  • AutoDetect
  • ASP.NET determines if cookies are enabled in the browser and falls back to the URL if not.

Note there is a difference between enabled and supported.

Note you should perhaps question whether users who are using browsers with no cookie support or with cookies disabled should be allowed to use your site.

Secure Cookies

All cookies should be set to HttpOnly; this prevents the browser from reading the contents and as such reduces the chance of their contents being read by malicious code.

Cookies which contain session data should be configured to be served over SSL connections only.

Use ASP.NET Membership Provider For Authentication

Whenever security is concerned, it is important to use tried and tested frameworks in favour of writing your own.

The .NET membership provider is built into ASP and should be favoured.

It contains the following functionality:

  • Automatic schema creation upon first usage
  • Default website projects which are set up to allow
  • Creating user accounts
  • User login/log out
  • Session persistence between page requests
  • Automatic page/URL protection from non logged in users or users not within a user group
  • User groups for a more granular permissions

Automatically Logout Expired Sessions

A session token should only be considered valid for the minimum viable amount of time possible. If a session token is leaked the smaller the time window it is exploitable reduces the chance of the session being hijacked.

You can set the forms login session length in minutes within the web.config:

&amp;lt forms loginUrl=&quot;~/Account/Login&quot; timeout=&quot;30&quot; /&amp;gt

By default the timeout is sliding, it expires after the configured amount of time has passed after the last request has been made.

It is possible to change the timeout to be fixed, expiring after the configured amount of time has passed after the user has logged in.

&amp;lt forms slidingExpiration=&quot;false&quot; /&amp;gt

Other Checks