Broken Access Control: Understanding and Preventing OWASP’s Top Web Risk

In today’s digital landscape, web application security is paramount. Among the numerous threats, broken access control stands out as a critical vulnerability that can compromise sensitive data and functions across systems. Recognized as the top risk in the OWASP Top 10 for 2021, broken access control vulnerabilities affect a vast majority of web applications worldwide.

What Is Broken Access Control?

Broken access control, commonly referred to as authorization failure, occurs when an application does not properly enforce restrictions on user actions and resource access. While authentication verifies who a user is, access control determines what actions and data the authenticated user is permitted to access.

Despite the straightforward concept, implementing robust access control mechanisms is complex and prone to errors. According to the OWASP Top 10 2021 report, a staggering 94% of tested applications exhibit some form of broken access control, ranging from missing role checks to improperly protected endpoints.

Common issues include violations of the principle of least privilege—granting users more permissions than necessary—and neglecting deny-by-default policies that block unauthorized access unless explicitly allowed.

Common Types of Broken Access Control Vulnerabilities

Security teams often face a diverse range of broken access control issues arising from how and where permission validations are applied. Below are key categories:

1. Exploiting Endpoints

Attackers directly access backend APIs or functions, bypassing the intended user interface or role restrictions. Many applications expose hidden or undocumented endpoints assumed to be protected but lacking server-side validation.

Example: An attacker logged in as a standard user invokes an /admin/deleteUser API call because the server fails to verify their administrative privileges.

2. URL Manipulation

Malicious actors alter predictable URL parameters to access or modify unauthorized resources. This method relies on weak or absent server-side checks on query strings or path segments.

Example: Changing a URL from /profile?user_id=101 to /profile?user_id=102 to view another user’s profile.

3. Insecure Direct Object References (IDOR)

IDOR vulnerabilities arise when applications expose internal object identifiers via user-controlled input without verifying authorization. Unlike URL manipulation, these flaws can occur in request bodies, headers, or cookies.

Example: A mobile app sends JSON with {"invoice_id": "30925"}; an attacker modifies this to access invoices belonging to another user because server ownership checks are missing.

4. Privilege Escalation

Users gain unauthorized higher-level access due to insufficient enforcement of roles or permissions. This is typical when session tokens or role information are manipulated client-side without proper revalidation on the server.

Example: Modifying a session token’s role=user to role=admin to gain administrative control.

Real-World Breaches Due to Broken Access Control

Broken access control is not just theoretical; it has been the root cause behind significant security incidents with severe consequences.

Snowflake Customer Data Breach (June 2024)

Attackers exploited stolen credentials to access sensitive customer data in Snowflake environments. Though Snowflake’s platform remained uncompromised, weak access controls and the lack of enforced multi-factor authentication (MFA) on customer accounts facilitated unauthorized access.

This breach highlights the critical importance of enforcing MFA and strict role restrictions across all cloud environments. (Source: TechCrunch)

Kia and Hyundai Connected Vehicle API Flaw (June 2024)

Researchers discovered a severe access control weakness in Kia and Hyundai’s vehicle APIs. Attackers required only a license plate number or VIN to access and potentially remotely control vehicle functions. The root cause was inadequate authentication and permission verification.

Such vulnerabilities in IoT and automotive systems demonstrate how broken access control can create tangible safety hazards. (Source: Wired)

MOVEit Transfer Zero-Day Exploit (2023)

The Cl0p ransomware group exploited a zero-day SQL injection flaw (CVE-2023-34362) in MOVEit Transfer. However, it was the absence of authentication on a critical endpoint that allowed attackers to trigger the injection and compromise data from over 2,000 organizations, exposing information of more than 62 million individuals.

This combined vulnerability and broken access control incident underscores the dangers of endpoint mismanagement. (Source: TechCrunch)

Best Practices to Prevent Broken Access Control

Effective prevention requires consistent application of fundamental security principles and ongoing review. Key strategies include:

1. Enforce the Principle of Least Privilege

• Grant users and services only the permissions necessary for their roles.
• Limit access scope and duration to minimize potential damage from compromised accounts.
• Employ a strict deny-by-default policy, allowing access only when explicitly permitted.

2. Server-Side Access Validation

• Perform authorization checks exclusively on the server or trusted back-end APIs.
• Avoid relying on client-side enforcement, which attackers can easily bypass.
• Verify user roles, ownership of objects, and permissions on every access attempt.

3. Secure Session Management and Authentication Controls

• Implement stateful session identifiers with proper expiration and logout handling.
• Use short-lived stateless tokens like JWTs and provide revocation mechanisms following OAuth standards.
• Never embed user roles or permissions in client code.
• Prefer attribute-based access control (ABAC) models for fine-grained authorization.

4. Conduct Regular Audits and Penetration Testing

• Integrate access control testing as part of the development lifecycle.
• Perform code reviews and dynamic scanning to detect logic flaws.
• Disable unnecessary directory listings and secure sensitive files.
• Set up alerting on repeated access control failures or suspicious patterns.

5. Implement Proper Logging and Error Handling

• Avoid revealing access control logic in error messages presented to users.
• Log all failed and successful access attempts to detect anomalies.
• Utilize rate limiting and monitoring to prevent brute force and probing attacks targeting access control weaknesses.

Conclusion: Why Broken Access Control Demands Continuous Attention

Broken access control remains one of the most dangerous and widespread vulnerabilities in the cybersecurity realm. Its effects—ranging from data breaches to operational loss and reputational damage—illustrate why organizations must treat access control as a foundational security component.

As shown by real-world breaches, mistakes like missing role validations and exposed admin endpoints provide attackers direct pathways into critical systems.

To safeguard applications effectively, organizations should adopt comprehensive security testing methodologies, including dynamic application security testing (DAST). DAST tools simulate attacker behavior by scanning live environments to uncover exposed endpoints, missing authorization controls, and flawed business logic in real time, enabling teams to prioritize critical fixes efficiently.

Ultimately, robust access control is a non-negotiable security measure that requires continuous validation, thoughtful design, and adherence to evolving best practices to reduce risks in a complex threat landscape.

Frequently Asked Questions (FAQ)

What is broken access control?

Broken access control is the failure to enforce restrictions that limit user actions and data access. This weakness allows attackers to exceed their authorized permissions, potentially compromising sensitive resources.

What are the main types of access control?

The primary access control models include:

• Discretionary Access Control (DAC): Permissions set by resource owners.
• Mandatory Access Control (MAC): Uses security classifications to enforce policies.
• Role-Based Access Control (RBAC): Assigns permissions based on user roles.
• Attribute-Based Access Control (ABAC): Uses user attributes and context for flexible authorization.

What are common causes of broken access control?

Typical causes include:

• Overly broad permissions and lack of deny-by-default policies.
• Failure to verify ownership of requested objects.
• Scattered or inconsistent access logic across the codebase.
• Relying on client-side enforcement instead of server-side validation.
• Inadequate testing and review of authorization mechanisms.