Mobile applications don’t operate in isolation. Behind every login screen, payment flow, and push notification sits a network of APIs quietly moving data between the app, the backend, and third-party services. Yet when organizations plan mobile application security testing, API security is often treated as an afterthought, something to “get to later” once the app’s UI, permissions, and local storage have been reviewed.

This is a costly mistake. APIs are the connective tissue of modern mobile apps, and attackers know it. A mobile app may look secure on the surface, but attackers can still exploit untested APIs to steal sensitive data, take over accounts, or gain access to backend systems.

In this blog, we break down why API security deserves equal footing with mobile-specific checks, the risks organizations expose themselves to when it’s ignored, and what a genuinely thorough testing approach looks like.

How Have APIs Become the Primary Attack Surface for Mobile Apps?

A mobile app is essentially a client; most of the meaningful logic, data processing, and access control decisions happen server-side, communicated through APIs. This shifts a large part of the actual attack surface away from the device and onto the API layer.

Traditional mobile testing tends to focus heavily on:

• Insecure data storage on the device
• Weak authentication screens
• Reverse engineering and code obfuscation
• Platform-specific permissions (Android/iOS)

These checks matter, but they only cover half the picture. The other half, how the app requests data, how it authenticates each call, how it handles tokens, and how the backend validates every request, lives in the API layer. Skipping this means testing the front door while leaving the back door wide open.

The Hidden Risks of Ignoring API Security

Overlooking API security can expose mobile applications to critical vulnerabilities that attackers can exploit to gain unauthorized access, manipulate data, or disrupt business operations. Here are some of the hidden risks organizations face when APIs are not thoroughly tested:

1) Broken Object Level Authorization (BOLA)

One of the most common and risky API flaws in mobile backends is BOLA, which occurs when an API fails to verify whether the requesting user actually owns the resource they are trying to access. In a mobile context, this often means simply changing a user ID or account number in an API request to view someone else’s data. This vulnerability consistently ranks at the top of the [OWASP API Security Top 10] and is frequently missed when testing focuses only on the app’s front end.

2) Excessive Data Exposure

Mobile APIs often return more data than the app displays and rely on the client to filter the information users can see. Attackers who intercept API traffic can see this raw response, including fields like internal user IDs, email addresses, or even security-related metadata, even if the app’s UI never renders them.

3) Weak or Reused Authentication Tokens

Mobile apps frequently use token-based authentication (OAuth, JWT, API keys) to maintain sessions. Poorly implemented token handling can allow attackers to hijack sessions long after a legitimate user logs out, especially when applications use non-expiring tokens, store tokens insecurely, or reuse them across devices.

4) Lack of Rate Limiting and Abuse Controls

Without proper rate limiting on APIs, attackers can automate requests to brute-force OTPs, enumerate user accounts, or scrape data at scale. This is a backend-level issue that mobile-only testing simply won’t catch, since it requires observing how the API behaves under repeated or abnormal traffic patterns.

5) Insecure Third-Party API Integrations

Most mobile apps integrate with third-party services, payment gateways, analytics SDKs, maps, chat, or notification providers. Each integration is another API call, another authentication mechanism, and another potential point of failure. If organizations fail to properly scope and test third-party connections, attackers can exploit them as backdoors into otherwise well-secured systems.

Why Does This Gap Persist?

Despite how well-documented these risks are, API security often gets deprioritized during mobile testing for a few recurring reasons:

• Scope limitations: Engagements are sometimes scoped narrowly around the app binary, excluding backend API testing entirely.
• Tooling gaps: Static and dynamic mobile analysis tools are good at catching client-side issues but don’t inherently test server-side API logic.
• Time and cost pressure:  Comprehensive API testing, including business logic reviews, requires more manual effort than automated mobile scans, so security teams often cut it first when deadlines tighten.
• Ownership confusion: In many organizations, the mobile team and the backend/API team are separate, and security testing responsibility falls through the cracks between them.

With Kratikal’s Mobile Application Security Testing, organizations gain end-to-end visibility into application and API vulnerabilities, enabling faster remediation and a stronger security posture.

Major Security Incidents Linked to Weak API Security

API security failures aren’t theoretical; attackers have exploited poorly tested and secured APIs in several major breaches over the past few years, including incidents involving mobile-connected services.

• T-Mobile (2023): 

Attackers exploited a single exposed API to continuously pull customer data for weeks before detection, ultimately affecting tens of millions of records. The API lacked adequate rate limiting and monitoring, allowing sustained automated abuse to go unnoticed.

• Optus (2022): 

An internet-facing API endpoint that should have required authentication was left accessible, allowing large-scale extraction of customer personal data, a textbook Broken Object Level Authorization and misconfiguration failure.

• Peloton (2021): 

Researchers found that Peloton’s API allowed any authenticated user to query other users’ private account data, including age, location, and profile details, regardless of their own privacy settings, because the API didn’t verify object-level permissions.

• USPS Informed Visibility API (2018): 

A flaw in an authentication mechanism on a USPS API let any logged-in user query data belonging to other accounts, exposing tens of millions of user records.

• Facebook “View As” Breach (2018): 

A vulnerability in Facebook’s API-driven access token system allowed attackers to steal access tokens and take over user accounts, impacting tens of millions of users.

These incidents highlight a common issue: the real vulnerability existed in the API layer, which was often overlooked or insufficiently tested compared to the mobile application’s front end.

Conclusion

Mobile applications are only as secure as the APIs that power them. While securing the app’s interface, code, and local storage remains essential, ignoring API security leaves a critical gap that attackers are quick to exploit. From broken authorization and excessive data exposure to weak authentication and insecure third-party integrations, API vulnerabilities can undermine even the most well-designed mobile applications.

To build resilient mobile applications, organizations must treat API security as a core component of Mobile Application Security Testing, not a separate or optional exercise. A comprehensive security assessment should test both the mobile client and backend APIs to ensure they protect every request, response, and business workflow against modern attack techniques.

As mobile ecosystems continue to grow in complexity, integrating API security testing into every security assessment is no longer a best practice; it’s a necessity for safeguarding sensitive data, maintaining user trust, and reducing organizational risk.

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