While the Facebook Developer Toolkit handles most of the needs of your average Facebook application, there is not support for the Post-Remove stuff that happens when users decide to remove your application. When your application is removed by a user, Facebook submits a “Post” in the the general direction of a url you’ve specified. The post contains a “fb_sig_user” with the relevant userid and the naive developer would remove this user from their database. That — however — would make the application vulnerable to an attack where a bad guy would hammer the post-remove form with request containing random userid, causing you to falsely believe these users have removed your application.

Below is some code that computes the proper signature that you can use to compare against the one Facebook sends along with the request. The crypto-parts were lifted from the guts of the Facebook Developer Toolkit. The process of computing the signature is described here.

public static string GenerateSignature(NameValueCollection formParams, string secret)
{
	string[] keys = formParams.AllKeys;
	Array.Sort(keys);
	string prefix = "fb_sig_";

	Dictionary dict = new Dictionary();
	var dictInit = keys.Where(k => k.StartsWith(prefix))
		.Select(k =>
			new { key = k.Substring(prefix.Length), val = formParams.Get(k) });
	foreach (var p in dictInit) { dict.Add(p.key, p.val); }

	var signatureBuilder = new StringBuilder();

	// Sort the keys of the method call in alphabetical order
	List keyList = ParameterDictionaryToList(dict);
	keyList.Sort();

	// Append all the parameters to the signature input paramaters
	foreach (string key in keyList)
		signatureBuilder.Append(String.Format(CultureInfo.InvariantCulture,
			"{0}={1}", key, dict[key]));

	// Append the secret to the signature builder
	signatureBuilder.Append(secret);

	MD5 md5 = MD5.Create();
	// Compute the MD5 hash of the signature builder
	byte[] hash = md5.ComputeHash(
		Encoding.UTF8.GetBytes(signatureBuilder.ToString().Trim()));

	// Reinitialize the signature builder to store the actual signature
	signatureBuilder = new StringBuilder();

	// Append the hash to the signature
	foreach (byte hashByte in hash)
		signatureBuilder.Append(hashByte.ToString("x2", CultureInfo.InvariantCulture));

	return signatureBuilder.ToString();
}

Here’s some code that uses the method:

string sig = GenerateSignature(Request.Form,
	ConfigurationManager.AppSettings["FACEBOOK_SECRET"]);
if (sig == Request.Form["fb_sig"])
{
	string fbuid = Request["fb_sig_user"];
	if (fbuid != null && fbuid.Length > 0)
	{
		// Remove user
	}
}