Note: This is a part of our API Security 101 series where we solve common developer queries in detail with how-to guides, common examples and code snippets. Feel free to visit the smaller guides linked later in this article on topics such as authentication methods, rate limiting, API monitoring and more.
APIs are the connective tissueof modern software - every integration, mobile app, and automated workflowdepends on them. But every API endpoint is also a potential entry point forattackers, which makes understanding API security risks essential for any teamthat builds or relies on APIs.
The most common API securityrisks and threats include unauthorized access from weak or brokenauthentication, injection attacks, excessive data exposure, lack of ratelimiting, vulnerabilities introduced by third-party dependencies, human errorsuch as leaked credentials, and shadow APIs - undocumented endpoints thatbypass security review entirely. Many of these map directly to the OWASP API Security Top 10, the industry-standard framework for API vulnerabilities, whichthis guide's companion post on API Security 101 best practices covers in full.
In this post, we break down eachof these risks - what it looks like in practice, how it's typically exploited,and how to mitigate it - so you can assess where your own APIs (and thethird-party APIs you depend on) might be exposed.
TABLE OF CONTENTS
1. Unauthorized access
2. Broken authentication tokens
3. Injection attacks
4. Data exposure
5. Rate limiting and Denial of Service (DoS) attacks
6. Third-party dependencies
7. Human error
8. Shadow and undocumented APIs
9. Consequences of API security breaches
10. FAQs
Common risks to API security developers must know of
1. Unauthorized access
Unauthorized access happens whensomeone — a malicious actor, a former employee, or even another customer'sapplication — gains access to API endpoints or data they shouldn't be able toreach. This is typically the result of weak or missing authentication, overlypermissive access controls, or endpoints that don't verify a user's identityand permissions on every request.
For example, an endpoint like/api/users/12345/invoices might only check that a request includes a validtoken — not that the token actually belongs to user 12345. An attacker whosimply changes the ID in the URL could retrieve another customer's invoices.This is a textbook case of Broken Object Level Authorization, the #1 risk inthe OWASP API Security Top 10.
Mitigation: every request needsto be authenticated and authorized — not just at login, but on every call, witha check that the authenticated user actually has permission to access thespecific object being requested. Knit's API Security 101 guide coversauthentication and authorization methods in depth, including how to enforcethese per-object access checks.
2. Broken authentication tokens
Broken authentication tokensoccur when the tokens used to verify a user's identity — session tokens, APIkeys, or JWTs — are predictable, long-lived, improperly stored, or not properlyi nvalidated. If an attacker obtains a valid token, they can impersonate thatuser for as long as the token remains valid.
A common real-world example issession hijacking: if a token doesn't expire, isn't tied to a specific deviceor IP address, or is exposed in a URL or log file, an attacker who captures itcan use it to access the account indefinitely — often without triggering anyalerts, since the requests look like normal traffic from an authenticated user.
Mitigation: issue short-livedaccess tokens with refresh tokens rather than long-lived credentials, storetokens securely, and invalidate tokens on logout or password change. Knit'sauthentication best practices section covers token lifecycle management forboth the APIs you build and the third-party APIs you integrate with.
3. Injection attacks
Injection attacks happen whenuntrusted input — from a URL parameter, form field, or API request body — is passed directly into a database query, command, or interpreter without beingvalidated or sanitized. The most common form is SQL injection, where anattacker manipulates input to run unintended database commands.
For example, if an API endpointbuilds a query like SELECT * FROM users WHERE id = '<input>' withoutsanitizing the input, an attacker could pass a value like ' OR '1'='1 toretrieve every row in the table instead of just one user's record.
Mitigation: always useparameterized queries or prepared statements instead of string concatenation,validate and sanitize every input against an expected format, and apply theprinciple of least privilege to database accounts used by APIs. Knit's sectionon input validation and parameter sanitization covers this pattern in moredepth.
4. Data Exposure
Data exposure — sometimes calledexcessive data exposure — happens when an API returns more information than theclient actually needs, relying on the front end to filter out sensitive fieldsrather than restricting them at the API level. Because API responses can beinspected directly, in browser dev tools, intercepted traffic, or by callingthe endpoint outside the intended app, any sensitive field included in aresponse is effectively exposed.
A common example: an endpointmeant to show a user's name and avatar in a directory actually returns the fulluser object, including email address, phone number, internal role, or otherfields the front end simply chooses not to display.
Mitigation: define explicitresponse schemas that include only the fields a given endpoint needs, neverrely on the client to filter sensitive data, and encrypt sensitive fields atrest and in transit. Knit's section on secure data transmission, encryption,and HTTPS covers the encryption side of this in detail.
5. Rate Limiting and Denial of Service (DoS) Attacks
Without rate limiting, an APIhas no way to distinguish between normal usage and abuse — whether that's aclient accidentally retrying a failed request in a tight loop, a scraperpulling data far faster than intended, or a deliberate Denial of Service (DoS)attack designed to overwhelm the API with traffic until it becomes slow orunavailable for everyone.
The impact isn't limited todowntime: APIs without limits are also more vulnerable to brute-force attacksagainst login or token endpoints, since an attacker can attempt unlimitedpassword or token guesses without being throttled.
Mitigation: set per-client ratelimits based on an API key, token, or IP address, return an HTTP 429 (Too ManyRequests) response with a Retry-After header when limits are exceeded, andcombine hard limits with throttling so the API degrades gracefully under loadrather than failing outright. Knit's guide to API rate limiting and throttlingcovers these patterns — including how to handle 429 responses from third-partyAPIs — in more detail.
6. Third-party dependencies
Most modern applications don'toperate in isolation — they call out to third-party APIs for payments,authentication, data enrichment, communications, and more. Each of thosedependencies extends your application's attack surface: a vulnerability, outage,or data breach at a third-party provider can compromise your application evenif your own code has no flaws at all.
This risk is easy tounderestimate because it's invisible until something goes wrong — a third-partyAPI that suddenly changes its authentication requirements, gets compromised, orquietly starts storing more of your users' data than you realized can allbecome your problem with little warning.
Mitigation: vet third-party APIsbefore integrating with them — review their security certifications, datahandling policies, and incident history — grant them the minimum access anddata they need, and prefer providers that are explicit about not retaining acopy of your data. Knit's section on third-party API security considerationscovers what to look for when evaluating an integration partner.
7. Human error
Many of the most damaging APIsecurity incidents don't involve a sophisticated exploit at all — they comedown to a misconfiguration or a mistake. Common examples include API keys orcredentials accidentally committed to a public code repository, overly permissivedatabase or IAM permissions left in place after testing, debug or stagingendpoints left accessible in production, and documentation or API collectionsshared externally that contain live credentials.
Because these mistakes are ofteninvisible until they're exploited — a leaked key can sit in a public repositoryfor months before anyone notices — they're frequently the root cause behindincidents that initially look like sophisticated attacks.
Mitigation: use secret-scanningtools on your repositories, rotate any credentials that may have been exposed,apply least-privilege access by default, and review permissions and exposedendpoints on a regular cadence rather than only at launch. The API securitychecklist in Knit's API Security 101 guide is designed to catch exactly thesekinds of configuration gaps.
8. Shadow and undocumented APIs
Shadow APIs are endpoints thatexist and are reachable in production but aren't tracked in your API inventory,documentation, or security review process. They're typically created duringrapid development — a developer spins up an endpoint for testing, an old APIversion is left running after a new one ships, or a feature is deprecated butits underlying endpoint is never decommissioned.
Because shadow APIs fall outsidenormal monitoring and review, they often lack the authentication, ratelimiting, and logging applied to documented endpoints — making them an attractive target. Attackers actively scan for these endpoints, since they're frequentlyless protected than an organization's primary, well-documented APIs.
Mitigation: maintain a complete, current inventory of every API endpointin production — including internal and deprecated ones — and decommissionunused endpoints rather than leaving them reachable. Knit's section on APIlifecycle management and decommissioning covers how to build this inventory andretirement process into your API lifecycle
9. Consequences of API security breaches
The consequences of an APIsecurity breach extend well beyond the immediate incident. Depending on whatdata and systems are exposed, organizations can face direct financial losses,regulatory fines under frameworks like GDPR, costly incident response andremediation work, and reputational damage that affects customer trust longafter the breach itself is resolved.
For B2B platforms, a breach in one customer's data can also expose every other customer connected through thesame API — which is why addressing these risks isn't optional once an API ishandling real user data. The API security checklist in Knit's API Security 101guide is a practical starting point for working through these riskssystematically.
Take your API security to the next level
If you are dealing with a large number of API integration and looking for smarter solutions, check out unified API solutions like Knit. Knit ensures that you have access to high quality data faster in the safest way possible.
There are 3 ways Knit ensures maximum security.
- Knit is the only unified API in the market that does NOT store a copy of your end user data in its severs or share it with any third party. All of our syncs are event-based and happens via webhooks to ensure that your data is not subjected to any external threats during the transfer. Learn more about Knit's secure data sync here
- Knit complies with industry best practices and security standards. We are SOC2, GDPR and ISO27001 certified and always in the process of adding more security badges to our collection.
- We monitor Knit's infrastructure continuously with the finest intrusion detection systems. Plus, our super responsive support team is available 24*7 across all time zones to make sure if at all a security issue occurs, it is resolved immediately.
If you want to learn more about Knit Security Practices, please talk to one of our experts. We would love to talk to you
FAQs
What are the common API security risks?
The most common API securityrisks include unauthorized access from weak or broken authentication, injectionattacks such as SQL injection, excessive data exposure through endpoints thatreturn more data than needed, lack of rate limiting that leaves APIs open todenial-of-service attacks, vulnerabilities introduced through third-partydependencies, human error such as leaked API keys or misconfigured permissions,and shadow APIs — undocumented endpoints that bypass security review. Thisguide covers each of these in depth, including concrete examples of how they'reexploited and how to mitigate them. For teams managing many third-partyintegrations, several of these risks — particularly third-party dependency andauthentication risk — are reduced by default through Knit's unified API, whichhandles authentication and doesn't retain a copy of integrated platforms' data.
What is the difference between an API vulnerability and an API attack?
An API vulnerability is aweakness in an API's design, code, or configuration — such as a missingauthorization check, an unvalidated input field, or an API key exposed in apublic repository — that could potentially be exploited. An API attack is theact of exploiting that vulnerability, such as an attacker manipulating arequest to access another user's data, injecting malicious input into a query,or flooding an endpoint with traffic to cause a denial of service. In practice,most vulnerabilities exist for some time before they're attacked, which is whyproactive steps like input validation, rate limiting, and regular securityaudits matter — they close the gap between a vulnerability existing and itbeing exploited, often before an attacker ever finds it.
What are shadow APIs and why are they a security risk?
Shadow APIs are API endpointsthat are live and reachable in production but aren't tracked in anorganization's API inventory, documentation, or security review process — oftenleft over from testing, old API versions, or deprecated features that were neverdecommissioned. They're a significant security risk because they typically lackthe authentication, rate limiting, and monitoring applied to documentedendpoints, making them an easier target for attackers who actively scan forexactly this kind of unmonitored access point. Shadow APIs are also harder topatch quickly, since a vulnerability can't be fixed in an endpoint the securityteam doesn't know exists. Maintaining a complete, current inventory of everyAPI endpoint — and decommissioning unused ones — is the most effective way toclose this gap, an approach covered in Knit's API lifecycle managementguidance.
How does broken authentication lead to API security breaches?
Broken authentication occurswhen the tokens, API keys, or session credentials used to verify a user'sidentity are predictable, long-lived, improperly stored, or not properlyinvalidated after logout or a password change. Knit removes a common source ofthis risk for the 100+ platforms it connects by handling OAuth flows, tokenrefresh, and credential storage automatically, so integrating teams don't haveto manage long-lived third-party credentials themselves. If an attacker obtainsa valid token through any of these weaknesses in your own API, they canimpersonate that user and access their data for as long as the token remainsvalid — often without triggering alerts, since the request looks like normaltraffic. Best practices include issuing short-lived access tokens with refreshtokens, storing tokens securely, and invalidating them on logout or passwordchange.
What is excessive data exposure in an API?
Excessive data exposure happenswhen an API response includes more information than the requesting clientactually needs — relying on the front end to filter out sensitive fields ratherthan restricting them at the API level itself. Because API responses can beinspected directly through browser developer tools, intercepted traffic, or bycalling the endpoint outside the intended application, any sensitive fieldincluded in a response — such as an email address, internal ID, or role — iseffectively exposed regardless of whether the interface displays it. Knitaddresses this for the data it processes by encrypting sensitive fields,including PII and credentials, with an additional layer of application-levelencryption beyond standard transport security. The fix at the API level is todefine explicit response schemas that return only the fields each endpointgenuinely needs.
How can third-party API dependencies introduce security risks?
Every third-party API yourapplication integrates with — for payments, authentication, data enrichment, orcommunications — extends your attack surface, because a vulnerability, outage,or data breach at that provider can compromise your application even if yourown code is secure. Knit addresses this directly: as a pass-through integrationlayer, Knit does not store a copy of your end users' data on its servers orshare it with any third party, syncing data via event-based webhooks instead ofretaining it in a database. This removes an entire category of third-party risk— a vendor's data retention practices can't expose your users' data if thatvendor never holds a copy of it. When evaluating any third-party API, reviewits security certifications, data retention policy, and incident history, andgrant it the minimum access it needs.
How does Knit reduce third-party API security risk for the integrations itconnects?
Knit reduces third-party APIsecurity risk in two ways: first, it handles authentication, token refresh, andcredential storage for every one of the 100+ HRIS, ATS, CRM, and otherplatforms it connects, removing a common source of broken-authentication riskfrom your integration layer. Second, Knit is built as a pass-through proxy — itdoes not store a copy of your end users' data on its servers or share it withany third party, and all syncs happen via event-based webhooks rather thanretained databases. Knit is also SOC2, GDPR, and ISO27001 certified, withcontinuously monitored infrastructure, intrusion detection, and 24/7 support,documented in full at getknit.dev/security. For teams managing dozens ofintegrations, this consolidates third-party API risk review into a single,audited layer instead of one per integration.
Does Knit follow the OWASP API Security Top 10 framework?
Knit's own API securitypractices align with the OWASP API Security Top 10 — the industry-standardframework covering risks like broken object-level authorization, brokenauthentication, and excessive data exposure, the same risks covered in thisguide. Knit's API Security 101 guide includes a full mapping of its securitybest practices to each of the ten OWASP categories, showing how each risk isaddressed in Knit's architecture and in the practices recommended for the APIsKnit connects to. While OWASP's list is aimed at anyone building or securingAPIs, it's a useful checklist for evaluating any third-party API — includingKnit — as part of a vendor security review.
