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API Business Logic: What & Why they exist & how to protect

Apurva Prakash

Marketing Manager @ AppSentinels

March 23, 2024
• ⏱︎6mins Read

APIs have taken over, and that is not an exaggeration. The proof lies in the pudding (numbers):

Thanks to their ability to interconnect various apps, devices, and platforms, APIs have transformed product features, business offerings, and strategies (both technical and business-end). It is not a stretch to say that APIs are crucial in any industry that seeks a digital presence, be it on the public internet or their own isolated networks. However, as with other technologies, APIs add vulnerabilities like all other software components in existence. Untested or inadequately tested APIs present another point of attack for hackers and malicious individuals. API security disclosures are almost guaranteed to invite infiltration attempts and can harm an organization’s products, internal tech stack, communications, or network efficiency. By connecting applications, APIs can alter how they function. This may create specific vulnerabilities that API-focused hackers know of and invite them to attack said APIs to access their support data and functions. The OWASP API Top 10 List states that insufficient authentication or missing authorization checks in API endpoints can allow malicious individuals to access sensitive data they are not authorized to. Further, by invoking APIs in a distinct order than built, they may be able to access functionality unintended for them, i.e., gain control of the application business logic – a hacker’s dream.

What are the Application’s business logic attacks?

Since every proprietary API is unique, so are its vulnerabilities. Protecting its business logic adds a layer of difficulty for security mechanisms when defending against them. Let’s understand this with a simple example. Below is an example of a BOLA attack scenario against a famous social network in 2019:

As you see from this example, a group of two APIs and two objects (highlighted) were involved in the breach. The two APIs were chained/connected. Individually each of the API calls is benign. However, the change of object value while calling the second API in the malicious scenario results in the attack. If the second API implementation doesn’t do sufficient security checks, it will result in unauthorized access, breaching the logic built across the two APIs. Current generation products can’t protect against such an attack due to a variety of reasons. First, their architecture limits them to look at every single network session in silos. They don’t understand application behavior or context. Second, they are mostly based on signature while one cannot write a signature to protect the above attempt. Third and a very important point, these attacks are extremely easy to carry out and don’t require deep knowledge of computer systems or network stacks, thereby attracting newbies into attacking APIs. Finding and blocking such API attacks require building a deep understanding of the application behavior and user context that current generation security products lack.

Frequently Asked Questions

What is API business logic, and why is it described as the “most valuable and most vulnerable” component of modern applications?+–

API business logic is the decision-making engine encoded in APIs like the rules governing who can do what, in what sequence, under what conditions. It represents the competitive differentiator of the application: unique workflow implementations, pricing models, and feature behaviors that took significant time and investment to develop. It’s simultaneously the most vulnerable component because it’s unique to each organization (no generic patch can fix it), requires deep application knowledge to protect (which security tools lack), and is increasingly the target of sophisticated attackers who’ve moved beyond simple technical exploits to exploit the business rules themselves.

Why do untested or inadequately tested APIs present a particularly high business logic risk compared to well-tested ones?+–

Untested APIs have never been systematically probed for unintended workflow combinations or edge cases, meaning their business logic vulnerabilities remain entirely unknown to the development team. Developers build APIs to handle expected inputs and workflow sequences; without adversarial testing that specifically tries to abuse the business rules, these edge cases remain undiscovered until attackers find them. Adequately tested APIs, especially those subjected to negative testing and abuse scenario simulation, have surfaced their business logic weaknesses before deployment and enabling remediation before production exposure. The gap between tested and untested API security posture grows with application complexity, as more complex rules create more exploitable interactions.

How do APIs alter application functionality when they connect different systems, and what new business logic risks does this create?+–

APIs enable applications to chain functionality across multiple systems like a single user action might trigger API calls across authentication services, inventory systems, pricing engines, payment processors, and fulfillment systems. Each integration point creates a new business logic surface where cross-system rule interactions may produce unintended outcomes. A pricing rule that functions correctly in isolation may interact with a discount rule in an adjacent system in ways neither team anticipated. These cross-system logic interactions are particularly hard to test because they require understanding the combined business rules of multiple systems simultaneously and complexity that grows exponentially with integration depth.

What is the appropriate security mindset shift for protecting API business logic versus traditional vulnerability management?+–

Traditional vulnerability management asks: “What known vulnerability patterns exist in this code?” Business logic security requires a fundamentally different question: “How could an adversary use legitimate functionality in unintended ways to gain advantage?” This shift requires thinking adversarially about application workflows rather than defensively cataloguing implementation flaws. Security teams need business context – understanding what the application is supposed to do and not just technical knowledge of what it actually does. This often requires collaboration between security teams and product owners who understand the business rules, creating a cross-functional security capability that most organizations haven’t historically needed to develop.

How should organizations build API security disclosures into their incident response plan given that “API security disclosures almost guarantee infiltration attempts”?+–

API security disclosures create a public signal that attracts opportunistic attackers who probe disclosed vulnerability patterns against other organizations potentially affected by the same issue. Incident response plans should anticipate this by: monitoring for new API security disclosures affecting frameworks and libraries used in the API portfolio, immediately assessing exposure when relevant disclosures occur, deploying temporary compensating controls (enhanced monitoring, IP restrictions, rate limiting) while permanent fixes are implemented, and maintaining a rapid-response API patching capability that can deploy emergency fixes within hours rather than standard change management windows. The window between disclosure and attacker exploitation is shrinking and is measured in hours, not days.