Modern mobile apps are mostly UI.
The real logic lives in:
1. APIs
2. Backend services
3. Third party integrations
Attackers rarely attack the app itself.
They attack the API directly.
If the API is weak, the app is irrelevant.
Why Mobile APIs Are Targeted So Often
Mobile APIs are attractive because:
1. Requests are predictable
2. Authentication tokens are reusable
3. Client-side checks are easy to bypass
4. Apps expose endpoints unintentionally
Attackers don’t need the app installed.
They only need the API.
Most Common API Vulnerabilities in Mobile Apps
1. Broken Authentication
The app checks access.
The API trusts it blindly.
Real issues:
1. Tokens never expire
2. Tokens reused across devices
3. No session invalidation
Real Example
A fintech mobile app:
1. Used JWTs with no expiration
2. Stored tokens locally
Result:
1. Token reused months later
2. Account accessed without login
3. Incident discovered during fraud review
2. Broken Object Level Authorization (BOLA)
The API accepts object IDs from the client.
Example:
/api/user/12345/profile
Change the ID:
/api/user/12346/profile
If data loads, authorization is broken.
This is the #1 API vulnerability today.
Real Example
A delivery app:
1. Trusted user ID from request
2. No server-side ownership check
Result:
1. Users accessed other customers’ addresses
2. Privacy incident reported
3. Excessive Data Exposure
The API returns more data than needed.
Example response:
{
"email": "[email protected]",
"role": "user",
"is_admin": false,
"internal_notes": "VIP customer",
"password_hash": "..."
}
The app hides fields.
Attackers don’t care.
4. Client-Side Validation Only
Mobile apps often enforce:
1. Limits
2. Roles
3. Feature access
But APIs must enforce everything.
If the application does not have authorization from the API:
1. Attackers could circumvent logic.
2. Unlock features accidentally.
Practical Testing Examples
Inspect Mobile API Traffic
Button using a proxy client, assess mobile API Traffic
1. Observe requests made
2. Locate endpoint addresses
3. Analyze request headers
4. Identify request parameters
Although these tests are simple, they expose the majority of issues found with applications.
Example of: Testing for ID manipulation
1. Capture request.
GET /api/orders/88321
Authorization: Bearer <token>
2. Change ID field.
GET /api/orders/88322
If the data retrieved for 88322 is not identical to 88321, the authorization was not provided.
Example: Token Lifetime Check
Decode JWT (no secrets needed):
jwt-decode <token>
Keen attention should be paid to anything that specifies an expiration date and in addition when something has a long expiration date.
Common Tools Used For Defense & Testing
The typical tools for both defensive and testing purposes are:
1. To Test and Intercept API calls - Burp Suite, Postman and Insomnia
2. Capturing Mobile Data - Burp Mobile Assistant, mitmproxy and Charles Proxy
3. Scanning APIs, Automating Scanning - OWASP ZAP, Nuclei and OWASP Amass
4. Static (Before Running) & Runtime analysis - MobSF, APKTool and Frida (To Inspect Running Apps)
Note: All the above mentioned tools are only for TESTING (approved) purposes to NOT be used for vulnerabilities in APIs.
Practical Code Examples
Secure Authorization Check (Backend)
Bad: order = Order.get(id=request.args["id"])
Better: order = Order.get(id=request.args["id"], user_id=current_user.id)
You cannot just trust an ID.
Restrict API Response Field Fields
Bad - return user.__dict__
Better - return { "name": user.name, "email": user.email }
Send Only What You Need
Enforce Expiration on Your Tokens
Example JWT Payload - {"sub": "user123", "exp": 1710000000}
Shorter Token Lifetime = Less Chance of Replay Attack
Why Are These Issues Persisting?
The Top Four Reasons Are:
1. Rapid API Development Of Mobile Launches
2. App Development Mistakes Being Mistaken for a Security Measure
3. No Dedicated Threat Modeling for API
4. No Consideration for Risks Associated with Back-End Services from the Mobile Team
Time over Speed; however, Speed does help in getting things done quickly.
How Teams Reduce API Risk in Practice
Effective teams:
1. Test APIs independently of apps
2. Enforce authorization on every request
3. Log abnormal access patterns
4. Rotate tokens aggressively
5. Treat mobile clients as untrusted
Mobile apps are just browsers with branding.
Key Takeaways
1. Mobile apps are thin clients
2. APIs are the real target
3. Client-side checks mean nothing
4. Authorization failures cause real breaches
5. Testing APIs directly is essential
If your API is secure,
your app inherits that security.
