APIs are the backbone of modern applications, powering core “behind the scenes” interactions in mobile apps, SaaS platforms, and various microservices. But as API rollouts to production environments have exploded, so has the attack surface, which makes API security mistakes a consequential challenge.
Tight deadlines and the need to keep integrations simple are leading developers to take shortcuts that quietly introduce serious risk.
In the first half of 2025 alone, researchers observed more than 40,000 API-related security incidents globally. In 2026, API attacks will continue not because of novel zero-day attacks, but because the same API security mistakes persist across organizations of all sizes. Here are some of the most common ones.
1. Skipping authentication, or treating it as optional
Skipping authentication is the most common security issue with APIs, with industry research showing that nearly half (47%) of all APIs are exposed without any form of authentication. This means the API receives and processes requests without verifying who or what is making them.
By bypassing the most basic security control there is, attackers can interact with endpoints directly and at scale. They can try to enumerate resources, extract data, or abuse other flaws in application logic to escalate access and expand the impact of an attack.
Authentication should be enforced by default as an API security measure in all situations. The most widely accepted and battle-tested approach is OAuth 2.0 with Bearer access tokens, issued by a centralized identity provider (IdP). Any exception must be explicitly justified and tightly monitored.
2. Broken authorization
Broken authorization flaws dominate the OWASP API Top 10. They occur when authentication is present, but the API fails to properly enforce what an authenticated user is allowed to access or do.
The most common form of this issue is object-level authorization failures, also known as BOLA (Broken Object Level Authorization) or IDOR (Insecure Direct Object References). These issues exist when the API allows users to access or modify resources simply by changing an identifier in the request, such as an order ID or user ID, without verifying ownership.
Function-level flaws are also common. This type of broken authorization is when the API doesn’t enforce proper role and permission checks for administrative or privileged actions. Any authenticated user may be able to invoke actions intended only for admins or other privileged accounts.
The solution is to enforce authorization checks server-side, on every request, using the authenticated identity as the source of truth. Authorization logic should be centralized, consistent across services, and treated as a mandatory step before any business logic executes.
3. Ignoring credential abuse and token weakness
Enforcing authentication is not a magic solution to all credential-related security issues. It is just the start. Weak credentials and long-lived or reused access tokens also introduce vulnerabilities that a dedicated attacker can break.
Excessively long time-to-live (TTL) values can lead to situations where, if a token is stolen, someone could use it for hours or even days. Poor token design can also be an issue. APIs frequently rely on JSON Web Tokens (JWTs) that contain far more data than necessary, sometimes including user identifiers or sensitive attributes. If these tokens are exposed, the attacker gains reusable credentials rather than just a single compromised request.
Strong authentication, therefore, also means strong token hygiene. As a best practice, access tokens should have short lifetimes, with validation of expiration (exp) and not-before (nbf) claims server-side. Additionally, proper scoping, rotation, and revocation mechanisms must be in place.
4. Excessive data exposure
Too often, APIs return more information than they should to fulfill a request. It is a widespread issue in modern APIs, especially those built quickly or generated directly from backend models, where entire database objects are serialized and returned by default.
Developers often assume the frontend will control what users can see, hiding fields in the UI or ignoring unused response attributes. However, attackers do not interact with APIs through the frontend. They call endpoints directly and receive the full response payload, which may include sensitive fields such as internal identifiers, account metadata, permission flags, or other data never intended to be exposed externally.
To prevent excessive data exposure, developers should design APIs around the principle of least privilege. Each endpoint should return only the minimum data required for its function, with explicit response schemas and field-level access controls. Any additional data exposure must have explicit justification.
5. No rate limiting or other abuse controls
Many APIs allow unlimited requests, which significantly increases the risk for credential stuffing and brute forcing against login pages and password reset endpoints, as well as scraping, enumeration, and denial-of-service-style abuse against search and lookup APIs, or any resource-intensive interface.
Even if rate limiting is in place, it may not be as effective as developers think. WAF limits may be set too high in the fear of blocking legitimate traffic, or applied only at the IP level, which attackers can easily rotate.
There are several proven algorithms and mechanisms for implementing rate limiting effectively:
- Fixed Window Counter: Limits the number of requests allowed within a fixed time interval. For example, 100 requests per minute.
- Sliding Window: Looks at recent requests instead of fixed time blocks, making it harder for attackers to flood an API when limits reset.
- Token Bucket: Allows requests as long as tokens are available in a bucket that refills at a defined rate. It enables short bursts of traffic while still enforcing an overall request limit.
- Leaky Bucket: Allows requests at a constant pace while slowing down bursts automatically.
6. Weak validation controls
Weak validation occurs when APIs trust client input too much, without enforcing what data is allowed and where it can appear. Attackers regularly send manipulated requests to find these gaps. Common targets include SQL, NoSQL, LDAP, and command injection, as well as XML parsing, many of which are explicitly highlighted in the OWASP API Top 10 as high-impact risks.
Several validation failures are fairly common. The API may not enforce the expected Content-Type header or payload format, allowing the processing of incompatible or malicious data. Resource exhaustion attacks are also possible if there is no limit to payload size or parser behavior.
Even error codes can contribute to the problem when APIs return generic messages or incorrect responses. For example, returning a 500 Internal Server Error instead of a 400 Bad Request hides validation failures and makes it harder to detect malicious input.
APIs must strictly validate all input server-side and reject anything that doesn’t belong. This includes enforcing content types and data formats, setting length and size limits, rejecting unexpected or duplicate fields, blocking unsupported HTTP methods, and returning precise 400 Bad Request responses with field-level error details.
7. Under-logging and poor monitoring
Poor logging and monitoring for APIs is very common, unfortunately. Even if logging exists, it is usually just there to help with debugging and doesn’t include a key security context that can help with detecting malicious activity.
Security logging and active monitoring can make up for a lot of bad practices in API design. All of the mistakes we covered so far produce meaningful logging events and should feed detection and alerting workflows.
Some of the main indicators to track include who is making the request (user, token, or API key), how often requests are made, which endpoints are being accessed, repeated authentication or authorization failures, validation errors, and abnormal request patterns over time.
Conclusion
Most API security mistakes in 2026 will not result from sophisticated exploits. Roughly 90% of the work is already done for attackers by common API security mistakes and weak design choices. It is crucial for developers to start closing these gaps and make security a top priority by default.
Photo by Markus Spiske; Unsplash
Rashan is a seasoned technology journalist and visionary leader serving as the Editor-in-Chief of DevX.com, a leading online publication focused on software development, programming languages, and emerging technologies. With his deep expertise in the tech industry and her passion for empowering developers, Rashan has transformed DevX.com into a vibrant hub of knowledge and innovation. Reach out to Rashan at [email protected]
