A01:2021 – Broken Access Control 
Factors
| CWEs Mapped | Max Incidence Rate | Avg Incidence Rate | Avg Weighted Exploit | Avg Weighted Impact | Max Coverage | Avg Coverage | Total Occurrences | Total CVEs |
|---|---|---|---|---|---|---|---|---|
| 34 | 55.97% | 3.81% | 6.92 | 5.93 | 94.55% | 47.72% | 318,487 | 19,013 |
Overview
Moving up from the fifth position, 94% of applications were tested for some form of broken access control with the average incidence rate of 3.81%, and has the most occurrences in the contributed dataset with over 318k. Notable Common Weakness Enumerations (CWEs) included are CWE-200: Exposure of Sensitive Information to an Unauthorized Actor, CWE-201: Insertion of Sensitive Information Into Sent Data, and CWE-352: Cross-Site Request Forgery.
Description
Access control enforces policy such that users cannot act outside of their intended permissions. Failures typically lead to unauthorized information disclosure, modification, or destruction of all data or performing a business function outside the user's limits. Common access control vulnerabilities include:
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Violation of the principle of least privilege or deny by default, where access should only be granted for particular capabilities, roles, or users, but is available to anyone.
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Bypassing access control checks by modifying the URL (parameter tampering or force browsing), internal application state, or the HTML page, or by using an attack tool modifying API requests.
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Permitting viewing or editing someone else's account, by providing its unique identifier (insecure direct object references)
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Accessing API with missing access controls for POST, PUT and DELETE.
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Elevation of privilege. Acting as a user without being logged in or acting as an admin when logged in as a user.
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Metadata manipulation, such as replaying or tampering with a JSON Web Token (JWT) access control token, or a cookie or hidden field manipulated to elevate privileges or abusing JWT invalidation.
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CORS misconfiguration allows API access from unauthorized/untrusted origins.
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Force browsing to authenticated pages as an unauthenticated user or to privileged pages as a standard user.
How to Prevent
Access control is only effective in trusted server-side code or server-less API, where the attacker cannot modify the access control check or metadata.
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Except for public resources, deny by default.
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Implement access control mechanisms once and re-use them throughout the application, including minimizing Cross-Origin Resource Sharing (CORS) usage.
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Model access controls should enforce record ownership rather than accepting that the user can create, read, update, or delete any record.
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Unique application business limit requirements should be enforced by domain models.
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Disable web server directory listing and ensure file metadata (e.g., .git) and backup files are not present within web roots.
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Log access control failures, alert admins when appropriate (e.g., repeated failures).
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Rate limit API and controller access to minimize the harm from automated attack tooling.
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Stateful session identifiers should be invalidated on the server after logout. Stateless JWT tokens should rather be short-lived so that the window of opportunity for an attacker is minimized. For longer lived JWTs it's highly recommended to follow the OAuth standards to revoke access.
Developers and QA staff should include functional access control unit and integration tests.
Example Attack Scenarios
Scenario #1: The application uses unverified data in a SQL call that is accessing account information:
pstmt.setString(1, request.getParameter("acct"));
ResultSet results = pstmt.executeQuery( );
An attacker simply modifies the browser's 'acct' parameter to send whatever account number they want. If not correctly verified, the attacker can access any user's account.
https://example.com/app/accountInfo?acct=notmyacct
Scenario #2: An attacker simply forces browses to target URLs. Admin rights are required for access to the admin page.
https://example.com/app/getappInfo
https://example.com/app/admin_getappInfo
If an unauthenticated user can access either page, it's a flaw. If a non-admin can access the admin page, this is a flaw.
References
List of Mapped CWEs
CWE-22 Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal')
CWE-23 Relative Path Traversal
CWE-35 Path Traversal: '.../...//'
CWE-59 Improper Link Resolution Before File Access ('Link Following')
CWE-200 Exposure of Sensitive Information to an Unauthorized Actor
CWE-201 Exposure of Sensitive Information Through Sent Data
CWE-219 Storage of File with Sensitive Data Under Web Root
CWE-264 Permissions, Privileges, and Access Controls (should no longer be used)
CWE-276 Incorrect Default Permissions
CWE-284 Improper Access Control
CWE-285 Improper Authorization
CWE-352 Cross-Site Request Forgery (CSRF)
CWE-359 Exposure of Private Personal Information to an Unauthorized Actor
CWE-377 Insecure Temporary File
CWE-402 Transmission of Private Resources into a New Sphere ('Resource Leak')
CWE-425 Direct Request ('Forced Browsing')
CWE-441 Unintended Proxy or Intermediary ('Confused Deputy')
CWE-497 Exposure of Sensitive System Information to an Unauthorized Control Sphere
CWE-538 Insertion of Sensitive Information into Externally-Accessible File or Directory
CWE-540 Inclusion of Sensitive Information in Source Code
CWE-548 Exposure of Information Through Directory Listing
CWE-552 Files or Directories Accessible to External Parties
CWE-566 Authorization Bypass Through User-Controlled SQL Primary Key
CWE-601 URL Redirection to Untrusted Site ('Open Redirect')
CWE-639 Authorization Bypass Through User-Controlled Key
CWE-651 Exposure of WSDL File Containing Sensitive Information
CWE-668 Exposure of Resource to Wrong Sphere
CWE-706 Use of Incorrectly-Resolved Name or Reference
CWE-863 Incorrect Authorization
CWE-913 Improper Control of Dynamically-Managed Code Resources