Web Application Security Interview Questions: OWASP Top 10 & API Security: The Interview Survival Guide

Imagine this scenario: A large, modern organization suffers a catastrophic data breach. The cause? A simple flaw in their web application allowed a hacker to access customer records by just changing a number in a URL parameter. It wasn’t a complex, high-tech hack; it was a failure of basic access control.

This isn’t a fictional tale. It’s a recurring nightmare for countless organizations, and it perfectly illustrates why Web Application Security is a non-negotiable skill. If you’re preparing for a security interview, you must be ready to discuss not just what the vulnerabilities are, but how you build systems that make these failures impossible.

This guide will equip you with the essential knowledge on the OWASP Top 10 and API Security, framed perfectly for your next technical interview. We’ll move beyond textbook definitions and focus on practical defense strategies, using concepts like zero trust security and the least privilege principle to demonstrate your expertise.

The Foundation of Defense: Deciphering the OWASP Top 10

The Open Web Application Security Project (OWASP) Top 10 list is the universally recognized standard for the most critical security risks facing web applications. Think of it as the ‘Most Wanted’ list for application flaws. When an interviewer brings this up, they’re assessing your situational awareness of the current threat landscape. Knowing this list is the entry ticket; understanding the remedy for each item is what earns you the job.

The list is continuously refined, reflecting the evolution of attack techniques and application architectures. A sophisticated security candidate doesn’t just recite the categories; they explain the root cause and advocate for secure-by-design mitigation.

Injection: The Code Whisperer

Injection remains one of the most potent threats. It occurs when untrusted data is sent to a code interpreter—like an SQL database, operating system shell, or LDAP server—without proper validation. The classic example is SQL Injection, where a malicious string is inserted into a data input field, tricking the database into running the attacker’s commands instead of just retrieving data.

The Mitigation Strategy: The silver bullet against Injection is parameterized queries (prepared statements). These techniques fundamentally separate the code (the SQL command) from the data (the user input). The input is always treated as a data variable, never as an executable command, neutralizing the threat. Always validate, sanitize, and ideally, completely avoid dynamic queries where possible.

Broken Access Control: The Gatekeeper Failure

This is the vulnerability from our opening story. Broken Access Control is a pervasive flaw where an authenticated user is able to gain access to resources or perform actions outside of their intended permissions. This can include accessing other users’ data (known as Insecure Direct Object Reference, or IDOR), viewing administrative pages, or changing settings they shouldn’t be able to touch.

This risk is a direct violation of the Principle of Least Privilege (PoLP).

The Mitigation Strategy: Access control must be enforced on the server-side for every single request. Never trust client-side controls (like disabled buttons or hidden URLs). The server must check the user’s session token or identity against a comprehensive policy to verify that they are authorized to perform the specific action on the specific resource being requested. This is the cornerstone of robust permission management.

Bridging Vulnerability and Architecture: The Zero Trust Mandate

A common mistake in interviews is to focus only on fixing vulnerabilities in isolation. Top candidates connect the tactical fix (e.g., preventing an Injection) to a strategic, architectural philosophy. This is where concepts like Zero Trust Security and identity security come into play.

In a modern, cloud-native application, the traditional “hard shell, soft interior” network defense model is obsolete. Applications are distributed, users are accessing services from anywhere, and API calls traverse complex network paths. This necessitates a fundamental shift: you must operate under the assumption that a breach is inevitable and that no user, device, or system can be trusted implicitly. Every access must be verified.

This mindset is the essence of Zero Trust Security. It’s not a single product; it’s a security framework built on three core tenets: Never Trust, Always Verify; Assume Breach; and Least Privilege Access.

The Crucial Role of Identity Security

The very first step in Zero Trust is establishing a strong identity. This means validating who is making the request, where they are, and what device they are using. This comprehensive approach is known as identity security.

Enforcing Strong Identity and Access

Multi-Factor Authentication (MFA): This is non-negotiable. Requiring two or more verification factors (e.g., password and a biometric scan, or a time-based code) is the simplest and most effective way to prevent account takeover and enforce strong identity.
Principle of Least Privilege (PoLP): This is the ultimate control. A user or microservice should only be granted the minimum access control rights absolutely necessary to complete its required function. If an account is compromised, the attacker’s lateral movement is severely restricted.
Contextual Authorization: In a Zero Trust model, authorization is continuous and dynamic. Access might be revoked or downgraded if the user’s context changes—for example, if they switch from a managed corporate laptop to an unmanaged personal device.

Navigating the API Security Landscape

The majority of modern web applications function by exposing data and services through APIs (Application Programming Interfaces). These APIs are the primary targets for attackers because they often handle large volumes of sensitive data and perform critical business functions. The OWASP API Security Top 10 addresses the unique risks introduced by API-centric architectures. Being fluent in this list shows you understand the cutting edge of application defense.

Broken Object Level Authorization (BOLA/IDOR)

Often rated as the number one API risk, BOLA is essentially a type of Broken Access Control specific to API resources. It occurs when an API accepts an object identifier (like /api/v1/users/456) and processes the request without adequately checking if the authenticated user has permission to access the data belonging to object 456.

Interview Answer Key: This requires a strict, function-level access control check against the user’s credentials for every request that references an object identifier. The API endpoint must confirm that the current user ID matches the owner ID of the requested object. This is a critical example of enforcing least privilege at the data layer.

Excessive Data Exposure

This risk arises from APIs that fetch data from the backend and return too many fields to the client, relying on the client application to filter and display only what is necessary. An attacker can often intercept the full, unfiltered response and harvest sensitive information.

Mitigation: Never trust the client to filter data. The server must apply a strict data schema, ensuring that the API only retrieves and returns the specific, necessary fields required for the client-side use case. This requires meticulous design review.

Lack of Resources & Rate Limiting

APIs are vulnerable to abuse if limits aren’t placed on how often a client can make requests or how much resource (CPU, memory, bandwidth) a single request can consume. Without limits, an attacker can launch a Distributed Denial of Service (DDoS) or brute-force attack, making the service unavailable.

Mitigation: Implement rate limiting to cap the number of requests per time window (e.g., 100 requests per minute per IP or per API key). Apply proper payload size limits and timeouts to prevent single, massive requests from overwhelming the server.

Mass Assignment

Mass Assignment allows an attacker to automatically write data to properties that they shouldn’t be allowed to change. This often happens when a developer automatically binds user-supplied input (e.g., a JSON payload) to an internal object’s data model without explicitly filtering which properties can be updated. For instance, a user might try to sneak in an is_admin: true field.

Mitigation: Always whitelist the properties that a user is allowed to modify. Use specific Data Transfer Objects (DTOs) for incoming requests that only include the fields that should be user-editable. Never automatically map all input fields to the internal data model.

Conclusion:

The world of Web Application Security is a dynamic battlefield, but success in interviews and in your career hinges on mastering foundational principles. By internalizing the critical risks of the OWASP Top 10 and the architectural shift required by Zero Trust Security, you move beyond basic vulnerability remediation. Demonstrate your commitment to strong identity security, rigorous access control, and the non-negotiable standard of least privilege for every user and service. Your ability to connect tactical defenses (like preventing Injection) to strategic frameworks (like MFA implementation) proves you’re ready to build and defend the secure systems of tomorrow. Master these concepts, and you will secure your place as a leading security professional.

PoLP ensures that a user or process is only granted the minimum permissions required for their task. By strictly limiting what roles and scopes an authenticated user’s token possesses, any attempt to access unauthorized resources (the core of Broken Access Control) will fail the server-side authorization check, as they lack the necessary privilege.

In a Zero Trust Security model, we assume the API is accessed from an untrusted environment. Every request must be strictly authenticated and authorized, even if it has a valid token. We enforce strong identity security via MFA for token generation, use contextual data (like geo-location or device health) for continuous authorization, and ensure the API only grants the least privilege required for that specific call.

The most effective mitigation is the use of prepared statements or parameterized queries. This technique guarantees the separation of the executable code from the user-supplied data, ensuring that the input is always treated as a data value, never as a command to be executed by the interpreter.

You must ensure that the API backend meticulously filters the response data. Never rely on the client to filter out sensitive fields. Use specific Data Transfer Objects (DTOs) for responses that are rigorously defined to only include the data the requesting client absolutely needs.

Authentication verifies who the user is (usually by validating a login and potentially MFA). Authorization determines what that authenticated user is allowed to do or access, which is the mechanism of access control. Both are mandatory for API Security, but BOLA is a failure of Authorization, not Authentication.

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