Accessing Functionality Not Properly Constrained by ACLs
Attack Pattern ID: 1 (Standard Attack Pattern Completeness: Complete)Typical Severity: HighStatus: Draft
+ Description


In applications, particularly web applications, access to functionality is mitigated by the authorization framework, whose job it is to map ACLs to elements of the application's functionality; particularly URL's for web apps. In the case that the application deployer failed to specify an ACL for a particular element, an attacker may be able to access it with impunity. An attacker with the ability to access functionality not properly constrained by ACLs can obtain sensitive information and possibly compromise the entire application. Such an attacker can access resources that must be available only to users at a higher privilege level, can access management sections of the application or can run queries for data that he is otherwise not supposed to.

Attack Execution Flow

  1. Survey:

    The attacker surveys the target application, possibly as a valid and authenticated user

    Attack Step Techniques

    IDAttack Step Technique DescriptionEnvironments

    Spidering web sites for all available links


    Brute force guessing of resource names


    Brute force guessing of user names / credentials


    Brute force guessing of function names / actions



    IDtypeIndicator DescriptionEnvironments

    ACLs or other access control mechanisms are present in the software

    env-Web env-ClientServer

    User IDs or other credentials are present in the software

    env-Web env-ClientServer

    Operating modes with different privileges are present in the software

    env-ClientServer env-Local env-Embedded
  2. Identify Functionality:

    At each step, the attacker notes the resource or functionality access mechanism invoked upon performing specific actions

    Attack Step Techniques

    IDAttack Step Technique DescriptionEnvironments

    Use the web inventory of all forms and inputs and apply attack data to those inputs.


    Use a packet sniffer to capture and record network traffic


    Execute the software in a debugger and record API calls into the operating system or important libraries. This might occur in an environment other than a production environment, in order to find weaknesses that can be exploited in a production environment.

    env-Local env-Embedded


    IDtypeOutcome Description
    The attacker produces a list of functionality or data that can be accessed through the system.
  1. Iterate over access capabilities:

    Possibly as a valid user, the attacker then tries to access each of the noted access mechanisms directly in order to perform functions not constrained by the ACLs.

    Attack Step Techniques

    IDAttack Step Technique DescriptionEnvironments

    Fuzzing of API parameters (URL parameters, OS API parameters, protocol parameters)

    env-Web env-Local env-Embedded env-ClientServer


    IDtypeIndicator DescriptionEnvironments

    Attempts to create a catalog of access mechanisms and data have failed.



    IDtypeOutcome Description
    Functionality is accessible to unauthorized users.
+ Attack Prerequisites

The application must be navigable in a manner that associates elements (subsections) of the application with ACLs.

The various resources, or individual URLs, must be somehow discoverable by the attacker

The deployer must have forgotten to associate an ACL or has associated an inappropriately permissive ACL with a particular navigable resource.

+ Typical Likelihood of Exploit

Likelihood: Very High

+ Methods of Attack
  • Analysis
  • Brute Force
+ Examples-Instances


Implementing the Model-View-Controller (MVC) within Java EE's Servlet paradigm using a "Single front controller" pattern that demands that brokered HTTP requests be authenticated before hand-offs to other Action Servlets.

If no security-constraint is placed on those Action Servlets, such that positively no one can access them, the front controller can be subverted.

+ Attacker Skills or Knowledge Required

Skill or Knowledge Level: Low

In order to discover unrestricted resources, the attacker does not need special tools or skills. He only has to observe the resources or access mechanisms invoked as each action is performed and then try and access those access mechanisms directly.

+ Resources Required

No special resources are required for the exploit of this pattern.

+ Probing Techniques

In the case of web applications, use of a spider or other crawling software can allow an attacker to search for accessible pages not beholden to a security constraint.

More generally, noting the target resource accessed upon performing specific actions drives an understanding of the resources accessible from the current context.

+ Solutions and Mitigations

In a J2EE setting, deployers can associate a role that is impossible for the authenticator to grant users, such as "NoAccess", with all Servlets to which access is guarded by a limited number of servlets visible to, and accessible by, the user.

Having done so, any direct access to those protected Servlets will be prohibited by the web container.

In a more general setting, the deployer must mark every resource besides the ones supposed to be exposed to the user as accessible by a role impossible for the user to assume. The default security setting must be to deny access and then grant access only to those resources intended by business logic.

+ Attack Motivation-Consequences
  • Privilege Escalation
+ Related Weaknesses
CWE-IDWeakness NameWeakness Relationship Type
285Improper Access Control (Authorization)Targeted
732Incorrect Permission Assignment for Critical ResourceTargeted
276Incorrect Default PermissionsTargeted
693Protection Mechanism FailureTargeted
721OWASP Top Ten 2007 Category A10 - Failure to Restrict URL AccessTargeted
434Unrestricted Upload of File with Dangerous TypeTargeted
+ Related Attack Patterns
NatureTypeIDNameDescriptionView(s) this relationship pertains toView\(s\)
ChildOfAttack PatternAttack Pattern122Exploitation of Authorization 
Mechanism of Attack (primary)1000
ParentOfAttack PatternAttack Pattern17Accessing, Modifying or Executing Executable Files 
Mechanism of Attack1000
ParentOfAttack PatternAttack Pattern58Restful Privilege Elevation 
Mechanism of Attack1000
+ Relevant Security Requirements

All resources must be constrained to be inaccessible by default followed by selectively allowing access to resources as dictated by application and business logic

In addition to a central controller, every resource must also restrict, wherever possible, incoming accesses as dictated by the relevant ACL.

+ Related Security Principles
  • Failing Securely

  • Least Privilege

  • Reluctance To Trust

  • Complete Mediation

+ Related Guidelines
  • Use Authorization Mechanisms Correctly

  • Design Configuration Subsystems Correctly and Distribute Safe Default Configurations

+ Purposes
  • Penetration
+ CIA Impact
Confidentiality Impact: HighIntegrity Impact: MediumAvailability Impact: Low
+ Technical Context
Architectural Paradigms
+ Content History
John StevenCigital, Inc2007-02-10Initial core pattern content
Chiradeep B. ChhayaCigital, Inc2007-02-23Fleshed out pattern with extra content
Richard StruseVOXEM, Inc2007-03-26Review and feedback leading to changes in Attack Execution Flow, Attack Prerequisites, Examples and Solutions
Sean BarnumCigital, Inc2007-04-13Modified pattern content according to review and feedback
Paco HopeCigital, Inc.2007-10-20Added extended Attack Execution Flow