OWASP API Security Top 10: What External Testing Actually Catches

The OWASP API Security Top 10 lists the API security risks attackers exploit most, led by Broken Object Level Authorisation (BOLA) and Broken Authentication. External, validated testing goes further than a scanner. It discovers exposed APIs you did not know were public, then proves which flaws an attacker could actually use against you.

Application Programming Interfaces (APIs) have become the backbone of modern applications. They connect mobile apps, web applications, cloud platforms, partner integrations, and internal services. They also represent one of the fastest-growing parts of an organisation’s external attack surface.

Attackers know this. They rarely care whether an exposed service belongs to a website, a mobile application, or an API. If it is reachable from the internet and provides a path to sensitive information or functionality, it becomes a target.

That is why API security deserves dedicated attention. The OWASP API Security Top 10 provides an excellent framework for understanding where APIs commonly fail, but understanding the list is only the beginning. Security teams also need to validate which weaknesses are actually exploitable in their own environment.

What the OWASP API Security Top 10 is

The OWASP API Security Top 10 is a community-maintained list of the most significant security risks affecting APIs. Produced by the Open Worldwide Application Security Project (OWASP), it reflects common weaknesses observed during security assessments and real-world attacks rather than theoretical concerns.

You can review the latest guidance on the official OWASP API Security Top 10 project page.

Rather than memorising ten categories, security leaders should understand what attackers are trying to achieve.

Some of the highest-impact risks include:

• Broken Object Level Authorisation (BOLA)
• Broken Authentication
• Broken Object Property Level Authorisation
• Unrestricted Resource Consumption
• Broken Function Level Authorisation
• Unrestricted Access to Sensitive Business Flows
• Server Side Request Forgery (SSRF)
• Security Misconfiguration
• Improper Inventory Management
• Unsafe Consumption of APIs

Each category represents a different way attackers may abuse an exposed API. Some focus on authentication failures. Others target authorisation, business logic, or poor operational visibility.

The common thread is simple. APIs expose functionality directly. When security controls fail, attackers often gain access to sensitive data rather than simply viewing a web page.

Why external APIs are the exposure most teams underestimate

Most organisations have a reasonable understanding of their websites and internet-facing infrastructure.

APIs are different.

Development teams publish new endpoints.

Mobile applications introduce dedicated APIs.

Partners require integrations.

Cloud services expose management interfaces.

Projects end, but APIs remain online.

Over time, organisations accumulate APIs that are undocumented, forgotten, or no longer actively maintained.

These shadow endpoints frequently escape traditional asset inventories because internal documentation only reflects assets people know about.

Attackers do not rely on internal documentation.

They enumerate internet-facing infrastructure directly.

An undocumented API is still visible if it responds to requests.

This is why external API discovery forms such an important part of attack surface management. Organisations frequently discover APIs that were never included in formal inventories, particularly after acquisitions, cloud migrations, or rapid application development.

Continuous, agentless discovery can also provide significantly broader visibility than traditional inventories. Attack Insights customers commonly achieve 65% greater external footprint coverage by discovering internet-facing assets beyond manually maintained inventories.

The gap between a scanner flag and a validated exploit

Security scanners remain valuable.

They quickly assess known configurations, software versions, and common weaknesses.

However, scanners generally answer one question.

Does something appear unusual?

Attackers ask a different question.

Can I actually exploit this?

That difference matters.

A scanner might report:

• Weak authentication configuration
• Excessive API responses
• Missing security headers
• Unusual endpoint behaviour

Those findings deserve investigation, but they are not automatically exploitable.

Validation goes further.

A validated security assessment attempts to prove whether an attacker can actually abuse the weakness.

Can authentication be bypassed?

Can another user’s records be accessed?

Can sensitive fields be extracted?

Can business logic be manipulated?

These answers allow security teams to prioritise remediation based on evidence rather than assumptions.

Broken Object Level Authorisation: The API risk attackers target first

Broken Object Level Authorisation (BOLA) consistently ranks among the most serious API security issues.

Object Level Authorisation determines whether a user is permitted to access a specific record or resource.

Imagine an API request for:

/api/orders/12345

The application checks whether the user is logged in.

What it fails to check is whether that user should actually access order 12345.

An attacker changes the request to:

/api/orders/12346

If the API returns another customer’s information, the vulnerability has been proven.

From the attacker’s perspective, this is one of the simplest ways to access confidential data because authentication may work exactly as designed.

The failure lies in authorisation.

External testing validates whether these scenarios are genuinely exploitable rather than assuming access controls behave correctly.

Broken Authentication: Valid credentials should not mean unlimited access

Authentication confirms identity.

Weak authentication allows attackers to impersonate legitimate users.

Examples include:

• Predictable tokens
• Poor session handling
• Weak password policies
• Insecure authentication workflows
• Improper token validation

Attackers rarely attempt to break encryption.

They look for weaknesses in how authentication has been implemented.

External testing validates whether authentication controls withstand realistic attack techniques rather than simply confirming that authentication exists.

Excessive Data Exposure: More information than intended

Many APIs return far more information than users actually need.

For example, a mobile application may display only a customer’s name and account balance.

Behind the scenes, however, the API response might also contain:

• Internal account identifiers
• Email addresses
• Administrative fields
• Historical records
• Permission details

The application hides this information.

The API still returns it.

Attackers inspect raw API responses rather than relying on what appears in the interface.

External testing validates exactly what information leaves the server and whether unnecessary data exposure creates opportunities for further compromise.

Discover, then validate: How continuous API discovery and validation work together

API security begins with visibility.

You cannot assess APIs you do not know exist.

Modern environments change constantly.

Development teams deploy new services.

Applications evolve.

Cloud infrastructure expands.

Third-party integrations introduce additional endpoints.

Continuous API discovery helps organisations identify internet-facing APIs from the attacker’s perspective, including undocumented or shadow endpoints that internal inventories often miss.

Discovery alone is not enough.

Each newly discovered API should then be validated.

Can it be accessed anonymously?

Can authentication be bypassed?

Does authorisation function correctly?

Does it expose unnecessary data?

This combination of continuous discovery and validation provides a much clearer understanding of actual exposure than scheduled assessments alone.

Learn more about API security and how it supports continuous visibility across internet-facing applications.

If you are beginning an external visibility programme, our guide explaining what attack surface management is provides additional background on how discovery supports security operations.

Prioritising by exploitability, not raw finding count

Security teams already manage thousands of alerts.

Adding more findings rarely improves security.

Instead, focus on exploitability.

Validated testing helps separate informational observations from findings that require immediate remediation.

Across customer environments, approximately 20% of findings require urgent action because they represent genuine, exploitable conditions rather than informational issues.

That distinction allows security teams to spend less time reviewing theoretical issues and more time reducing real-world exposure.

The objective is not producing the longest report.

It is proving where attackers can actually succeed.

Build confidence through validated API testing

APIs continue to expand faster than most organisations can document them.

Every new application, integration, and cloud deployment increases the external attack surface.

Understanding the OWASP API Security Top 10 provides a useful framework, but frameworks alone do not tell you whether your APIs can actually be exploited.

That requires external testing that discovers exposed APIs, validates realistic attack paths, and confirms which findings deserve immediate attention.

Attack Insights combines continuous discovery with evidence-based validation to help organisations understand not just what APIs are exposed, but which weaknesses attackers could genuinely use.

If your organisation is deploying customer-facing APIs, partner integrations, or cloud-native applications, now is the right time to validate them from the same perspective attackers use.

Request a Demo

Unknown APIs and theoretical findings create unnecessary uncertainty.

Request a demo of our API security platform to see how Attack Insights continuously discovers internet-facing APIs, validates exploitable weaknesses, and helps your security team prioritise remediation based on proven exposure rather than assumptions.