Difference between revisions of "Codereview-Authentication"

Revision as of 11:55, 5 May 2009

Introduction

“Who are you?” Authentication is the process where an entity proves the identity of another entity, typically through credentials, such as a username and password.

Depending on your requirements, there are several available authentication mechanisms to choose from. If they are not correctly chosen and implemented, the authentication mechanism can expose vulnerabilities that attackers can exploit to gain access to your system.

The storage of passwords and user credentials is an issue from a defense in depth approach, but also from a compliance standpoint. The following section also discusses password storage and what to review for.

The following discusses aspects of source code relating to weak authentication functionality. This could be due to flawed implementation or broken business logic: Authentication is a key line of defence in protecting non-public data, sensitive functionality.

Weak Passwords and Password Functionality

Password strength should be enforced upon a user setting/selecting a password. Passwords should be complex in composition. Such checks should be done on the backend/server side of the application upon an attempt to submit a new password.

Bad Example

Simply checking that a password is not NULL is not sufficient:

String password = request.getParameter("Password");
if (password == Null) 
   {throw InvalidPasswordException()
   }

Good Example

Passwords should be checked for the following composition or a variance of such

• at least: 1 uppercase character (A-Z)
• at least: 1 lowercase character (a-z)
• at least: 1 digit (0-9)
• at least one special character (!"£$%&...)
• a defined minimum length (8 chars)
• a defined maximum length (as with all external input)
• no contiguous characters (123abcd)
• not more than 2 identical characters in a row (1111)

Such rules should be looked for in code and used as soon as the http request is received. The rules can be complex RegEx expressions or logical code statements:

if password.RegEx([a-z])
   and password.RegEx([A-Z])
   and password.RegEx([0-9])
   and password.RegEx({8-30})
   and password.RexEX([!"£$%^&*()])
   return true;
else
return false;

A regular expression statement for code above:

(?=^.{8,30}$)(?=.*\d)(?=.*[a-z])(?=.*[A-Z])(?=.*[!@#$%^&*()_+}{"":;'?/>.<,]).*$

.NET Authentication controls

In the .NET, there is Authentication tags in the configuration file.

The <authentication> element configures the authentication mode that your applications use.

<authentication>

The appropriate authentication mode depends on how your application or Web service has been designed. The default Machine.config setting applies a secure Windows authentication default as shown below.

authentication Attributes:mode="[Windows|Forms|Passport|None]"

<authentication mode="Windows" />

Forms Authentication Guidelines To use Forms authentication, set mode=“Forms” on the <authentication> element. Next, configure Forms authentication using the child <forms> element. The following fragment shows a secure <forms> authentication element configuration:

<authentication mode="Forms">
<forms loginUrl="Restricted\login.aspx"     Login page in an SSL protected folder
      protection="All"                      Privacy and integrity
      requireSSL="true"                     Prevents cookie being sent over http
      timeout="10"                          Limited session lifetime
      name="AppNameCookie"                  Unique per-application name
      path="/FormsAuth"                     and path
      slidingExpiration="true" >            Sliding session lifetime
</forms>
</authentication>

Use the following recommendations to improve Forms authentication security:

• Partition your Web site.
• Set protection=“All”.
• Use small cookie time-out values.
• Consider using a fixed expiration period.
• Use SSL with Forms authentication.
• If you do not use SSL, set slidingExpiration = “false”.
• Do not use the <credentials> element on production servers.
• Configure the <machineKey> element.
• Use unique cookie names and paths.

For classic ASP pages, authentication is usually performed manually by including the user information in session variables after validation against a DB, so you can look for something like:

Session ("UserId") = UserName
Session ("Roles") = UserRoles

Cookieless Forms authentication

Authentication tickets in forms are by default stored in cookies (Authentication tickets are used to remember if the user has authenticated to the system), such as a unique ID in the cookie of the HTTP header. Other methods to preserve authentication in the stateless HTTP protocol. The directive cookieless can define thet type of authentication ticket to be used.

Types of cookieless values on the <forms> element:

• UseCookies – specifies that cookie tickets will always be used.
• UseUri – indicates that cookie tickets will never be used.
• AutoDetect – cookie tickets are not used if device does not support such; if the device profile supports cookies, a probing function is used to determine if cookies are enabled.
• UseDeviceProfile – the default setting if not defined; uses cookie-based authentication tickets only if the device profile supports cookies. A probing function is not used.

cookieless="UseUri" : What may be found in the <forms> element above

When we talk about probing we are refering to the user agent directive in the HTTP header. This can inform ASP.NET is cookies are supported.

Password Storage Strategy

The storage of passwords is also of concern, as unauthorized access to an application may give rise to an attacker to access the area where passwords are stored.

Passwords should be stored using a one-way hash algorithm. One way functions (SHA-256 SHA-1 MD5, ..;) are also known as Hashing functions. Once passwords are persisted, there is no reason why they should be human-readable. The functionality for authentication performs a hash of the password passed by the user and compares it to the stored hash. If the passwords are identical, the hashes are equal.

Storing a hash of a password, which cannot be reversed, makes it more difficult to recover the plain text passwords. It also ensures that administration staff for an application does not have access to other users’ passwords, and hence helps mitigate the internal threat vector.

Example code in Java implementing SHA-1 hashing:

import java.security.MessageDigest;
public byte[] getHash(String password) throws NoSuchAlgorithmException {
      MessageDigest digest = MessageDigest.getInstance("SHA-1");
      digest.reset();
      byte[] input = digest.digest(password.getBytes("UTF-8"));

Salting: Storing simply hashed passwords has its issues, such as the possibility to identify two identical passwords (identical hashes) and also the birthday attack. A countermeasure for such issues is to introduce a salt. A salt is a random number of a fixed length. This salt must be different for each stored entry. It must be stored as clear text next to the hashed password:

import java.security.MessageDigest;
public byte[] getHash(String password, byte[] salt) throws NoSuchAlgorithmException {
      MessageDigest digest = MessageDigest.getInstance("SHA-256");
      digest.reset();
      digest.update(salt);
      return digest.digest(password.getBytes("UTF-8"));
}

Vulnerabilities related to authentication

There are many issues relating to authentication which utilise form fields. Inadequate field validation can give rise to the following issues:

• Reviewing Code for SQL Injection

SQL injection can be used to bypass authentication functionality and even add a malicious user to a system for future use.

• Reviewing Code for Data Validation

Data validation of all external input must be performed. This also goes for authentication fields.

• Reviewing Code for Cross-site scripting

Cross site scripting can be used on the authentication page to perform identity theft, Phishing, and session hijacking attacks.

• Reviewing Code for Error Handling

Bad/weak error handling can be used to establish the internal workings of the authentication functionality such as giving insight into the database structure, insight into valid and invalid user IDs, etc.

• Hashing with Java