Understanding A01:2021 – Broken Access Control in OWASP Top 10

Broken Access Control (BAC) has consistently held a prominent position in the OWASP Top 10, and for good reason. It's a vulnerability that, when exploited, can lead to severe consequences, including data breaches, financial loss, and reputational damage.  

This edition of All Things AppSec delves into the intricacies of BAC, its common manifestations, and strategies to mitigate its risks. 

Understanding Broken Access Control 

Broken Access Control occurs when an application doesn't effectively implement access controls, allowing unauthorized users to access restricted functionalities or data. This can happen due to various reasons, including: 

• Improper authorization: When an application fails to accurately verify and validate user permissions before granting access to resources. 

• Insecure Direct Object References (IDORs): When application functions or data can be accessed through predictable URLs or parameters, allowing unauthorized users to manipulate them. 

• Vertical privilege escalation: When a user with a lower privilege level can elevate their access to perform actions reserved for users with higher privileges. 

• Horizontal privilege escalation: When a user can access data or functionalities intended for other users at the same privilege level. 

The impact of Broken Access Control 

The consequences of BAC extend far beyond mere inconvenience. Organizations that fall victim to this vulnerability face a multitude of risks: 

• Data breaches: Unauthorized access to sensitive information can lead to significant data loss and exposure of personal or financial data. 

• Financial loss: Data breaches can result in substantial financial losses due to legal penalties, customer churn, and remediation costs. 

• Reputational damage: A data breach caused by BAC can severely damage an organization's reputation, leading to loss of customer trust and business opportunities. 

• Business disruption: Exploits targeting BAC can disrupt normal operations, leading to downtime and financial losses. 

• Compliance violations: Failure to protect sensitive data can result in violations of industry regulations and hefty fines. 

Common manifestations of Broken Access Control 

• Missing or bypassed authentication: Lack of proper authentication mechanisms or bypassing existing ones. 

• Insufficient authorization checks: Weak or absent authorization controls allowing access to unauthorized resources. 

• Predictable resource identifiers: Using easily guessable or predictable identifiers for resources. 

• Session management flaws: Improper handling of user sessions, leading to session hijacking. 

• Elevation of privileges: Exploiting vulnerabilities to gain higher-level access. 

A striking example of the severe consequences of Broken Access Control is evident in the Snapchat username exploit of 2014.  

The platform's feature allowing users to suggest usernames to friends inadvertently created a vulnerability. Attackers capitalized on this by rapidly submitting numerous username suggestions to a target, overwhelming the system and gaining access to a list of previously rejected usernames.  

This breach resulted in the exposure of millions of user accounts, including usernames, phone numbers, and locations, causing significant damage to Snapchat's reputation and trust among its user base.  

Mitigating Broken Access Control risks 

To effectively address BAC, organizations must implement a comprehensive security strategy: 

Robust access control mechanisms: 

• Implement strong authentication and authorization processes. 

• Use role-based access control (RBAC) to define user permissions accurately. 

• Enforce the principle of least privilege, granting users only the necessary permissions. 

Secure input validation: 

• Validate and sanitize all user input to prevent injection attacks and other vulnerabilities. 

• Avoid using sensitive data directly in URLs or query parameters. 

Secure session management: 

• Implement secure session management practices, including using strong session cookies, timeouts, and regular invalidation. 

Regular security assessments: 

• Conduct thorough security assessments and penetration testing to identify vulnerabilities. 

• Stay updated on the latest threats and attack vectors. 

Employee training: 

• Educate employees about the risks of BAC and how to recognize and report suspicious activities. 

Secure coding practices: 

• Adhere to secure coding standards and guidelines to prevent vulnerabilities from being introduced into the application code. 

Wrapping up 

Broken Access Control remains a persistent threat to web applications. A combination of technical controls, security awareness, and ongoing monitoring is essential to safeguard against BAC and protect sensitive information. 

The evolving threat landscape introduces new challenges to BAC mitigation. The increasing complexity of applications, the rise of cloud computing, and the proliferation of APIs create new attack surfaces. Additionally, the growing reliance on third-party components introduces additional risks. Organizations must stay ahead of these challenges by adopting emerging security technologies and best practices.