MVC ValidateAntiForgeryToken multi-tabs problem

Question

Hello,

We'd been getting "A required anti-forgery token was not supplied or was invalid." errors, and on some further investigation, I've managed to recreate the problem in its simplest form - i'm either doing something completely wrong, or this is a limitation of the anti-forgery token system.

Either way, I'd appreciate some advice!

Empty MVC 2 project: one view page, one controller

view:

<%--Sign in form:--%>
<% using(Html.BeginForm("SignIn", "Home", FormMethod.Post)) {%>
    <%= Html.AntiForgeryToken()%>
    <input type="submit" value="Sign in" />
<%}%>

Controller:

public ActionResult Index()
{
    ViewData["status"] = "Index";
    return View();
}

[ValidateAntiForgeryToken]
public ActionResult SignIn()
{
    ViewData["status"] = "Signed In!";
    FormsAuthentication.SetAuthCookie("username", false);
    return View("Index");
}

[EDIT: simplified code example]

In order to recreate the exception, open two non-signed-in tabs - sign-in on the first tab, and then sign-in on the second tab.

The second tab will always throw an anti-forgery exception, when I guess correct behaviour would be to redirect to the signed-in page (sharing the session/authentication of the original signed-in tab)

Any advice would be appreciated!

Cheers, Dave

Answer 1

You could use the same salt:

<% using(Html.BeginForm("SignIn", "Home", FormMethod.Post)) {%>
    <%= Html.AntiForgeryToken("123")%>
    <input type="submit" value="Sign in" />
<%}%>

<% using(Html.BeginForm("Protected", "Home", FormMethod.Post)) {%>
    <%= Html.AntiForgeryToken("456")%>
    <input type="submit" value="Do secret stuff" />
<%}%>

And in your controller:

[ValidateAntiForgeryToken(Salt = "123")]
public ActionResult SignIn()
{
    ViewData["status"] = "Signed In!";
    FormsAuthentication.SetAuthCookie("username", false);
    return View("Index");
}

[Authorize]
[ValidateAntiForgeryToken(Salt = "456")]
public ActionResult Protected()
{
    ViewData["status"] = "Authed";
    return View("Index");
}

Do the same with the other token but make sure to pick a different salt.

Answer 2

Looking at the MVC 2 source code it looks like the AntiForgeryToken hidden field includes the User.Identity.Name serialized, if your signed in. In line 69 of the ValidateAntiForgeryTokenAttribute it seems to then check your token with the current User.Identity.Name.

    string currentUsername = AntiForgeryData.GetUsername(filterContext.HttpContext.User);
    if (!String.Equals(formToken.Username, currentUsername, StringComparison.OrdinalIgnoreCase)) {
        // error: form token is not valid for this user
        // (don't care about cookie token)
        throw CreateValidationException();
    }

Because in your other tab you are now signed in the code above invalidates the existing token which doesn't contain the User.Identity.Name.

This could be fixed by adding a !string.IsNullOrEmpty(formToken.Username) around that check but I don't know if that will open up security issues plus it means having a custom MVC 2 Build.