Access Control Cheat Sheet

Introduction

This article is focused on providing clear, simple, actionable guidance for providing Access Control security in your applications.

What is Access Control / Authorization?

Authorization is the process where requests to access a particular resource should be granted or denied. It should be noted that authorization is not equivalent to authentication - as these terms and their defininitions are frequently confused.

Access Control is the method or mechanism of authorization to enfore that requests to a system resource or functionality should be granted.

Role Based Access Control (RBAC) is commonly used to manage permissions within an application. Permissions are assigned to users in a many to many relationship.

Discretioinary Access Control (DAC) is commonly used to manage permissions within an application.

Mandatory Access Control (MAC) is a classification based system of objects and subjects. To "write up", a subject's clearance level must be

Attacks on Access Control

Vertical Access Control Attacks - A standard user accessing administration functionality

Horizontal Access Control attacks - Same role, but accessing another user's private data

Business Logic Access Control Attacks - Abuse of one or more linked activities that collectively realize a business objective

Access Control Issues

• Many applications used the "All or Nothing" approach - Once authenticated, all users have equal privileges

• Authorization Logic often relies on Security by Obscurity (STO) by assuming:
  • Users will not find unlinked or hidden paths or functionality
  • Users will not find and tamper with "obscured" client side parameters (i.e. "hidden" form fields, cookies, etc.)

• Applications with multiple permission levels/roles often increases the possibility of conflicting permission sets resulting in unanticipated privileges

• Many administrative interfaces require only a password for authentication
• Shared accounts combined with a lack of auditing and logging make it extremely difficult to differentiate between malicious and honest administrators
• Administrative interfaces are often not designed as “secure” as user-level interfaces given the assumption that administrators are trusted users
• Authorization/Access Control relies on client-side information (e.g., hidden fields)

    <input type="text" name="fname" value="Derek">
    <input type="text" name="lname" value="Jeter">
    <input type="hidden" name="usertype" value="admin">

Access Control Anti-Patterns

• Hard-coded role checks in application code
• Lack of centralized access control logic
• Untrusted data driving access control decisions
• Access control that is "open by default"
• Lack of addressing horizontal access control in a standardized way (if at all)
• Access control logic that needs to be manually added to every endpoint in code

Hard Coded Roles

 if (user.isManager() ||
     user.isAdministrator() ||
     user.isEditor() ||
     user.isUser()) {
     //execute action
 }

Hard Codes Roles can create several issues including:

• Making the policy of an application difficult to "prove" for audit or Q/A purposes
• Causing new code to be pushed each time an access control policy needs to be changed.
• They are fragile and easy to make mistakes Order Specific Operations

Order Specific Operations

Imagine the following parameters

 http://example.com/buy?action=chooseDataPackage
 http://example.com/buy?action=customizePackage
 http://example.com/buy?action=makePayment
 http://example.com/buy?action=downloadData

Can an attacker control the sequence?

Can an attacker abuse this with concurency?

Never Depend on Untrusted Data

• Never trust user data for access control decisions
• Never make access control decisions in JavaScript
• Never depend on the order of values sent from the client
• Never make authorization decisions based solely on
  • hidden fields
  • cookie values
  • form parameters
  • URL parameters
  • anything else from the request

Attacking Access Controls

• Elevation of privileges
• Disclosure of confidential data - Compromising admin-level accounts often result in access to a user's confidential data
• Data tampering - Privilege levels do not distinguish users who can only view data and users permitted to modify data

Testing for Broken Access Control

• Attempt to access administrative components or functions as an anonymous or regular user
  • Scour HTML source for “interesting” hidden form fields
  • Test web accessible directory structure for names like admin, administrator, manager, etc (i.e. attempt to directly browse to “restricted” areas)
• Determine how administrators are authenticated. Ensure that adequate authentication is used and enforced
• For each user role, ensure that only the appropriate pages or components are accessible for that role.
• Login as a low-level user, browse history for a higher level user’s cache, load the page to see if the original authorization is passed to a previous session.
• If able to compromise administrator-level account, test for all other common web application vulnerabilities (poor input validation, privileged database access, etc)

Defenses Against Access Control Attacks

• Implement role based access control to assign permissions to application users for vertical access control requirements
• Implement data-contextual access control to assign permissions to application users in the context of specific data items for horizontal access control requirements
• Avoid assigning permissions on a per-user basis
• Perform consistent authorization checking routines on all application pages
• Where applicable, apply DENY privileges last, issue ALLOW privileges on a case-by-case basis
• Where possible restrict administrator access to machines located on the local area network (i.e. it’s best to avoid remote administrator access from public facing access points)
• Log all failed access authorization requests to a secure location for review by administrators
• Perform reviews of failed login attempts on a periodic basis
• Utilize the strengths and functionality provided by the SSO solution you chose

Best Practices

Best Practice: Code to the Activity

  if (AC.hasAccess(ARTICLE_EDIT)) {
      //execute activity
  }

• Code it once, never needs to change again
• Implies policy is persisted/centralized in some way
• Avoid assigning permissions on a per-user basis
• Requires more design/work up front to get right

Best Practice: Centralized ACL Controller

• Define a centralized access controller

ACLService.isAuthorized(ACTION_CONSTANT) ACLService.assertAuthorized(ACTION_CONSTANT)

• Access control decisions go through these simple API’s
• Centralized logic to drive policy behavior and persistence
• May contain data-driven access control policy information
• Policy language needs to support ability to express both access rights and prohibitions

Best Practice: Using a Centralized Access Controller

• In Presentation Layer

      if (isAuthorized(VIEW_LOG_PANEL))
      {
         Here are the logs
         <%=getLogs();%/>
      }

• In Controller

      try (assertAuthorized(DELETE_USER))
      {
         deleteUser();
      }

Best Practice: Verifying policy server-side

• Keep user identity verification in session
• Load entitlements server side from trusted sources
• Force authorization checks on ALL requests
  • JS file, image, AJAX and FLASH requests as well!
  • Force this check using a filter if possible

SQL Integrated Access Control

Example Feature

   http://mail.example.com/viewMessage?msgid=2356342

This SQL would be vulnerable to tampering

 select * from messages where messageid = 2356342

Ensure the owner is referenced in the query!

 select * from messages where messageid = 2356342 AND messages.message_owner = 

Access Control Positive Patterns

• Code to the activity, not the role
• Centralize access control logic
• Design access control as a filter
• Deny by default, fail securely
• Build centralized access control mechanism
• Apply same core logic to presentation and server-side access control decisions
• Determine access control through Server-side trusted data

Data Contextual Access Control

Data Contextual / Horizontal Access Control API examples

   ACLService.isAuthorized(EDIT_ORG, 142)
   ACLService.assertAuthorized(VIEW_ORG, 900)

Long Form

   isAuthorized(user, EDIT_ORG, Organization.class, 14)

• Essentially checking if the user has the right role in the context of a specific object
• Centralize access control logic
• Protecting data a the lowest level!

Authors and Primary Editors

Jim Manico = jim [manico] dot net

Fred Donovan - fred.donovan [at] owasp dot org