OWASP API Security Risks Every Mobile Developer Must Know

Apps with security flaws are rejected by Apple and Google at rates that have tripled in recent years, and the main culprit behind most of these rejections is poor API security. I've been building mobile apps for a decade now, and I can tell you that API security has gone from being an afterthought to being one of the first things we discuss with every client. The truth is that most data breaches in mobile apps happen not because of sophisticated hacking techniques, but because developers overlook basic security principles that OWASP has been documenting for years. You might build the most beautiful app with perfect functionality, but if your API security has holes in it then you're essentially leaving the back door wide open for anyone who knows where to look.

Every mobile app is only as secure as its weakest API endpoint, and attackers know this better than most developers

Mobile apps today handle sensitive information that ranges from banking details to health records, and the APIs that power these apps are constantly being probed by automated tools looking for vulnerabilities. The cost of getting this wrong isn't just technical debt or a few hours of fixes, we're talking about user data being exposed, regulatory fines that can reach hundreds of thousands of pounds, and the kind of reputation damage that can sink an app before it ever gains traction. I've worked on fintech apps where a single security oversight during development could have exposed transaction data for thousands of users, and the scary part is that these vulnerabilities often hide in plain sight within code that developers write every day without thinking twice about the implications.

What is OWASP and Why Mobile Developers Should Care

OWASP stands for the Open Web Application Security Project, and it's a non-profit organisation that's been documenting security vulnerabilities and best practices since the late 1990s. They maintain several lists of top security risks, including the OWASP API Security Top 10 which specifically addresses the most common and dangerous API security flaws. The project is run by security professionals who volunteer their time to help the development community understand and prevent security issues, and their documentation has become the industry standard that security audits are based on. When Apple or Google review your app for security compliance, they're looking at many of the same issues that OWASP documents.

For mobile developers, OWASP matters because mobile apps are fundamentally different from traditional web applications in how they handle security. Your app sits on a device you don't control, communicating with APIs over networks you can't trust, and storing data in places where users might have root access or jailbroken permissions. I've probably reviewed about 80 different apps over the years for security issues, and the patterns are remarkably consistent... most vulnerabilities fall into the categories that OWASP has already documented and provided guidance on preventing.

Authentication Failures That Leave Apps Vulnerable

Authentication is the process of verifying that a user is who they claim to be, and it's the first line of defence for any mobile app that handles user accounts. The problem is that proper authentication is more complex than just checking a username and password, it involves token management, session handling, refresh token rotation, and secure storage of credentials. I've seen apps where developers stored authentication tokens in plain text in shared preferences on Android or UserDefaults on iOS, which is roughly the equivalent of leaving your house keys under the doormat and hoping nobody looks there.

One of the most common authentication failures happens when developers don't properly validate tokens on the server side for every request. They might check the token exists, but they don't verify it's still valid, hasn't been tampered with, or actually belongs to the user making the request. In one e-commerce project I worked on, the previous development team had implemented JWT tokens but never checked the expiration time server-side, which meant users could technically stay logged in forever if they just kept their token. This might seem convenient for users, but it means a stolen token remains valid indefinitely, giving attackers unlimited access to user accounts. Similar issues can arise when working with serverless architectures where authentication needs to be handled differently from traditional server setups.

Always implement token refresh mechanisms with short-lived access tokens that expire within 15-30 minutes, paired with longer-lived refresh tokens that can only be used once and are rotated with each refresh request

• Never store authentication credentials in plain text anywhere on the device
• Implement proper token expiration and refresh mechanisms on both client and server
• Use biometric authentication where available as an additional layer for sensitive operations
• Check token validity server-side for every request, not just on initial login
• Implement rate limiting on authentication endpoints to prevent brute force attacks

Data Exposure Through Broken Object Level Authorisation

Broken Object Level Authorisation happens when an API doesn't properly check whether a user should have access to a specific piece of data before returning it. The API might verify that you're logged in, but it doesn't check whether you should be able to see that particular user's profile, that specific order, or that medical record. This is probably the most common API security issue I encounter in mobile apps, and it's particularly dangerous because it's often not obvious during normal testing since your test users are accessing their own data.

The typical scenario looks like this... your app makes a request to an endpoint like /api/orders/12345 to fetch order details, and the API checks that you're authenticated but doesn't verify that order 12345 actually belongs to you. An attacker can simply change that number to 12346, 12347, and so on, pulling down every order in your system. I worked with a healthcare app where this exact issue existed in their appointment booking system, and during security testing we discovered that any logged-in user could view any other user's appointment details including medical notes just by changing the appointment ID in the request. The fix took two weeks to implement properly across all endpoints.

Mass Assignment and Excessive Data Exposure Problems

Mass assignment vulnerabilities occur when an API accepts more data than it should from the client and directly assigns it to database objects without filtering. Let's say your app has a user profile endpoint where users can update their name and email address, but the API accepts any field that exists in the user database model. An attacker could send additional fields like "isAdmin" or "accountBalance" in the request, and if your API isn't filtering these fields properly then it might just accept and save them. I've seen this happen more times than I should admit, particularly with junior developers who are using object-relational mapping frameworks that make it easy to bind request data directly to database models.

The principle of least privilege applies to API data just as much as it applies to user permissions, return only what the client needs and accept only what should be changeable

Excessive data exposure is the flip side of this problem, where your API returns more information than the mobile app actually needs. You might have an endpoint that returns complete user objects including password hashes, internal IDs, or other users' email addresses when all the mobile app needed was a display name and profile picture. This happens frequently when developers reuse the same API endpoints that were built for a web dashboard or internal tools, where returning full objects might have made sense, but it creates security risks when those same endpoints are exposed to mobile clients. The temptation is to build flexible APIs that return everything and let the client decide what to use, but this approach exposes data that attackers can intercept and exploit. These issues become even more complex when you're dealing with IoT integration where multiple data sources might be exposed through a single API.

Preventing Mass Assignment Issues

The solution requires building specific data transfer objects for each endpoint that explicitly define which fields can be read and which can be written. On a recent fintech project, we created separate request and response models for every API endpoint, which added about 20 percent more code but eliminated an entire category of security vulnerabilities. You define exactly which fields the mobile app can send when updating a profile, and exactly which fields get returned when fetching data, never allowing direct binding between API requests and database models.

Security Misconfiguration in Mobile APIs

Security misconfiguration covers a broad range of issues where APIs are deployed with settings that expose them to attack. This includes things like leaving debug endpoints enabled in production, using default passwords for admin interfaces, not implementing rate limiting, having overly permissive CORS policies, or exposing detailed error messages that reveal information about your system architecture. These issues are particularly common because they often happen during deployment rather than during development, and many developers don't have full visibility into how their APIs are configured once they reach production environments.

I've worked on projects where the staging API was accidentally exposed to the public internet with authentication disabled because a developer needed to test something quickly and forgot to revert the changes. In another case, a production API was returning full stack traces in error messages, which told attackers exactly which framework version was running and which database system was being used, making it much easier to find known vulnerabilities to exploit. The tricky thing about security misconfiguration is that these issues might not cause obvious problems during normal operation, you only discover them when someone with malicious intent starts probing your endpoints. When choosing between edge computing and cloud infrastructure, proper configuration becomes even more critical as you're managing security across multiple deployment points.

• Disable all debug endpoints and verbose logging before deploying to production
• Implement rate limiting on all API endpoints to prevent abuse and denial of service attacks
• Use environment-specific configuration files that are never committed to version control
• Return generic error messages to clients while logging detailed errors server-side
• Regularly scan your APIs with automated security tools to catch configuration drift
• Implement proper CORS policies that only allow requests from your mobile app domains

Injection Attacks and Input Validation Issues

Injection attacks happen when user input is passed directly into commands or queries without being properly validated or sanitised. SQL injection is the most well-known type, where an attacker includes database commands in input fields that get executed by your backend, but there are many other types including NoSQL injection, command injection, and LDAP injection. Mobile developers sometimes assume that because the input is coming from their own app rather than a web form that it's somehow safer, but this is a dangerous misconception. Attackers can intercept API requests, modify the data being sent, and inject malicious payloads just as easily as they can with web applications.

The fundamental problem is trusting client input. Your mobile app might have validation that prevents users from entering certain characters or formats, but those client-side checks can be bypassed completely by anyone intercepting and modifying the network requests. I worked on an educational app where the search functionality was vulnerable to SQL injection because the developers assumed the mobile app's search interface would prevent malicious input. It took about 30 minutes with a proxy tool to demonstrate that we could extract the entire user database through the search endpoint. The fix required implementing parameterised queries and input validation on the server side, which should have been there from the start. These security considerations become even more important with modern technologies like augmented reality apps where user input might come from camera data, gestures, or voice commands.

Use parameterised queries or prepared statements for all database interactions, and implement server-side input validation that checks both the format and the content of every field your API accepts

1. Never trust any data coming from the client, even if your mobile app has validation in place
2. Use parameterised queries for database access rather than concatenating strings
3. Implement allowlist validation where you define what is permitted rather than trying to block what is dangerous
4. Sanitise all user input before using it in any system commands or database queries
5. Use object-relational mapping tools correctly without raw query strings
6. Validate data types, lengths, formats, and ranges on the server side for every input field

Conclusion

The OWASP API Security risks represent years of accumulated knowledge from security professionals who have seen what actually breaks in production systems. These aren't theoretical vulnerabilities that require sophisticated attacks to exploit, they're the basic security flaws that automated scanning tools can find in minutes and that lead to real data breaches affecting real users. Building mobile apps means taking responsibility for user data, and that responsibility extends beyond just making your app work properly to making sure it can't be exploited by someone with basic security knowledge and freely available tools.

Security needs to be part of your development process from the beginning rather than something you bolt on before launch or address after an incident. The time to implement proper authentication, input validation, and authorisation checks is during initial development when your API design is still flexible, not after you've built an entire app around insecure endpoints. I've seen the difference between projects where security was considered from day one versus projects where it was treated as a final checklist item, and the cost difference in both time and money is substantial. Your users trust you with their data when they download your app, and meeting OWASP security standards is the baseline for honouring that trust.

If you're working on a mobile app and want to make sure your API security is properly handled from the start, get in touch with us and we can discuss how to build security into your development process.

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