Introductoin:

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) In Role-Based Access Control (RBAC), access decisions are based on an individual's roles and responsibilities within the organization or user base. The process of defining roles is usually based on analyzing the fundamental goals and structure of an organization and is usually linked to the security policy. For instance, in a medical organization, the different roles of users may include those such as doctor, nurse, attendant, nurse, patients, etc. Obviously, these members require different levels of access in order to perform their functions, but also the types of web transactions and their allowed context vary greatly depending on the security policy and any relevant regulations (HIPAA, Gramm-Leach-Bliley, etc.).

An RBAC access control framework should provide web application security administrators with the ability to determine who can perform what actions, when, from where, in what order, and in some cases under what relational circumstances. http://csrc.nist.gov/rbac/ provides some great resources for RBAC implementation. The following aspects exhibit RBAC attributes to an access control model.

Roles are assigned based on organizational structure with emphasis on the organizational security policy
Roles are assigned by the administrator based on relative relationships within the organization or user base. For instance, a manager would have certain authorized transactions over his employees. An administrator would have certain authorized transactions over his specific realm of duties (backup, account creation, etc.)
Each role is designated a profile that includes all authorized commands, transactions, and allowable information access.
Roles are granted permissions based on the principle of least privilege.
Roles are determined with a separation of duties in mind so that a developer Role should not overlap a QA tester Role.
Roles are activated statically and dynamically as appropriate to certain relational triggers (help desk queue, security alert, initiation of a new project, etc.)
Roles can be only be transferred or delegated using strict sign-offs and procedures.
Roles are managed centrally by a security administrator or project leader

Discretioinary Access Control (DAC) Discretionary Access Control (DAC) is a means of restricting access to information based on the identity of users and/or membership in certain groups. Access decisions are typically based on the authorizations granted to a user based on the credentials he presented at the time of authentication (user name, password, hardware/software token, etc.). In most typical DAC models, the owner of information or any resource is able to change its permissions at his discretion (thus the name). DAC has the drawback of the administrators not being able to centrally manage these permissions on files/information stored on the web server. A DAC access control model often exhibits one or more of the following attributes.

Data Owners can transfer ownership of information to other users
Data Owners can determine the type of access given to other users (read, write, copy, etc.)
Repetitive authorization failures to access the same resource or object generates an alarm and/or restricts the user's access
Special add-on or plug-in software required to apply to an HTTP client to prevent indiscriminant copying by users ("cutting and pasting" of information)
Users who do not have access to information should not be able to determine its characteristics (file size, file name, directory path, etc.)
Access to information is determined based on authorizations to access control lists based on user identifier and group membership.

Mandatory Access Control (MAC) Mandatory Access Control (MAC) ensures that the enforcement of organizational security policy does not rely on voluntary web application user compliance. MAC secures information by assigning sensitivity labels on information and comparing this to the level of sensitivity a user is operating at. In general, MAC access control mechanisms are more secure than DAC yet have trade offs in performance and convenience to users. MAC mechanisms assign a security level to all information, assign a security clearance to each user, and ensure that all users only have access to that data for which they have a clearance. MAC is usually appropriate for extremely secure systems including multilevel secure military applications or mission critical data applications. A MAC access control model often exhibits one or more of the following attributes.

Only administrators, not data owners, make changes to a resource's security label.
All data is assigned security level that reflects its relative sensitivity, confidentiality, and protection value.
All users can read from a lower classification than the one they are granted (A "secret" user can read an unclassified document).
All users can write to a higher classification (A "secret" user can post information to a Top Secret resource).
All users are given read/write access to objects only of the same classification (a "secret" user can only read/write to a secret document).
Access is authorized or restricted to objects based on the time of day depending on the labeling on the resource and the user's credentials (driven by policy).
Access is authorized or restricted to objects based on the security characteristics of the HTTP client (e.g. SSL bit length, version information, originating IP address or domain, etc.)

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)
Web and application server processes run as root, Administrator, LOCALSYSTEM or other privileged accounts
Some web applications access the database via sa or other administrative account (or more privileges than required)
Some applications implement authorization controls by including a file or web control or code snippet on every page in the application

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
non-anonymous entry point DO NOT have an access control check
No authorization check at or near the beginning of code implementing sensitive activities

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

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

Java

if ( authenticated ) {

request.getSession(true).setValue(“AUTHLEVEL”) = X_USER;

}

.NET (C#)

if ( authenticated ) {

Session[“AUTHLEVEL”] = X_USER;

}

PHP

if ( authenticated ) {

$_SESSION[‘authlevel’] = X_USER; // X_USER is defined elsewhere as meaning, the user is authorized

}

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 at the lowest level!