If your business uses any app or third-party integration, then chances are it’s held together by APIs. APIs, in a nutshell, are the invisible connections that allow your systems to talk to one another by transferring data. They have become a staple in IT software and AI development, making them prime targets for cybercriminals.
The rapid expansion of AI to automate workflows and enhance productivity, which often relies on APIs, has created an unmanageable attack surface for many companies worldwide. Research from Akamai Security 2026 shows that AI-related APIs, including those tied to agents, apps, and LLMs, were involved in 42% of reported security incidents over the past year. The rate of innovation is outpacing API security maturity, and an ever-evolving digital landscape means that companies need to be hyper-aware of their API usage and, more importantly, be more proactive with their cyber security.
This blog post delves into the basics of API cyber security and penetration testing, outlining the most commonly exploited API flaws in accordance with leading guidance, whether you actually need a pen test and what kind of test to look for.
What is an API?
APIs (Application Programming Interfaces) are used widely across the digital landscape. You probably used one this morning without knowing. Imagine you’re booking a hotel room on a hotel room comparison app. How would it know if there are any rooms left? The app will use an API to check for any available rooms against the hotel’s booking system in real time. A business might use private APIs to connect their HR systems with payroll software. A retailer’s logistics partner might have their own API access to order and shipment data. This isn’t limited to systems talking to systems either. AI agents are now initiating API calls autonomously. For example, booking tables and taking payments with no person in the loop at all. The uses of APIs are endless. The most concerning part is that only about 23% of enterprises that use APIs know which ones return sensitive data (Akamai Security, 2026), which illustrates why they need to be secured. That is where pen testing comes in.
What is API Penetration Testing?
An API pen test is a simulated real-world attack on an API. In other words, a security assessment that uses the same tools and tactics as a cybercriminal. Penetration tests assess how well a business’s defences can hold up against cyber attacks. For API’s, that means probing for weaknesses in how the data and business logic are protected. Unsecured APIs can be accessed by anyone who can reach them.
There are typically three different types of penetration testing. White Box, Grey Box and Black Box.
White-Box – Testers have full knowledge of the internal workings, including source code, configuration files, and credentials.
Black-Box – Testers receive zero prior knowledge. A highly realistic representation of an outside threat, which reduces internal bias.
Grey-Box – A mix of white and black, as the colour would suggest. Partial or limited information, such as low-level architecture or standard user credentials.
Choosing which type of test is best for you depends on your security goals, time frame and budget. Black-box testing typically takes the most time, but it’s the best example of a real-world scenario. Grey-box is the best balance in most cases, while white-box is ideal for a deep, thorough look at potential bugs and configuration errors but can be expensive depending on the size of the attack surface.
For clients who are unsure, Sencode encourage grey-box testing for many API cases, as it offers the best balance between efficiency and thorough coverage.
Are APIs different from Website & Mobile Pen Testing?
APIs are commonly linked to websites and mobile applications. It’s tempting to assume that once you have pen-tested your web or mobile app, that it covers the API too. This isn’t the case. At Sencode, clients with web or mobile applications more often than not need an API test too. Modern web and mobile apps are just API-driven apps with a front-end display, and typical tests will focus on what happens in the browser or the app itself. API tests focus on the server-side logic. These differences are part of why API testing has dedicated, separate guidance from OWASP. Different from the Top 10 for web applications.
The OWASP Top 10 for API’s – The Short Version
You can read the OWASP Top 10 for API in full technical detail here. Here’s our 1-minute version.
- Broken Object Level Authorisation (BOLA) – probably the most common API flaw there is. This is where the API checks you’re logged in but doesn’t check if you’re the owner.
- Broken Authentication – login/token handling is badly implemented, meaning attackers can steal or fake someone’s identity.
- Broken Object Property Level Authorisation – quite similar to number 1 but at the level of the individual field. An API might allow you to edit specific properties you shouldn’t be able to edit.
- Unrestricted Resource Consumption – No limits on how many requests an API can be hammered with.
- Broken Function Level Authorisation – A flaw in which regular users can reach admin-only functions.
- Unrestricted Access to Sensitive Business Flows – A legitimate feature that is abused at scale by bots when technically nothing is broken.
- Server-Side Request Forgery – an API fetches a URL without checking it. Enabling attackers to trick it into reaching internal systems.
- Security Misconfiguration – basic setup mistakes that leave the door open simply due to APIs having so many moving parts.
- Improper Inventory Management – old or undocumented API versions that get forgotten about.
- Unsafe Consumption of APIs – developers trust data coming back from third-party APIs more than they should. This gives attackers a backdoor route in via someone else’s service.
For a technical breakdown of the Top 10 with some real-world examples, including code snippets and attack scenarios. You can read our comprehensive blog post on API Security Testing here.
Do you actually need an API Pen Test?
- You expose an API to third parties. This could mean partners, customers or even an app store. Even if it’s just a read-only endpoint.
- You have an app that talks to a backend API.
- Your business is integrated with external services. Any data flowing in from a third-party API can carry risks.
- You’ve never tested it. APIs are often overlooked and the last thing added to a scope.
If any of the above sound familiar, it’s worth a conversation. Sencode offers free cyber security advice and no-obligation scoping calls for all penetration testing services. You can book a call with one of our penetration testing consultants here.