API Penetration Testing
API penetration testing
Your APIs carry your data and your authorisation decisions. We test them the way attackers use them: directly, at the endpoint, with the frontend out of the way. Part of Atoro’s penetration testing practice.
REST and GraphQL, validated findings, an audit-ready report, and a retest included to confirm the fixes.
Built for modern software companies · Rated 4.8/5 on G2
REST and GraphQL
OWASP API Security Top 10
Authorisation-first testing
Retest included
API penetration test
API
TEST
Authorisation-firstObject-level and function-level access control, tested role by role.
Token and auth handlingKeys, JWTs and OAuth flows verified useless once stolen or expired.
Full-surface coverageIncluding the endpoints no screen ever calls.
Validated findingsEvery issue reproduced, rated and explained by a tester.
Retest includedFixes confirmed, closure evidence provided.
What usually triggers the call
- An enterprise security review asks about API testing specifically.
- Your audit evidence needs to reach beyond the web layer.
- The API surface has grown faster than anyone has tested it.
- A public or partner API is about to ship.
- Your product’s data moves through endpoints nobody has attacked yet.
02 Why this test
The frontend is polite. Attackers are not.
Your web app enforces rules in the browser: which buttons appear, which fields validate, which records a user can see. Your API is where those rules are actually decided, and attackers skip the browser entirely. They call the endpoint directly, replay tokens, change IDs and ask for more than the interface would ever offer.
Public API, partner API or the private API behind your own frontend: if it moves customer data, it is in scope for someone. Better you than them.
Client proof
“I loved the more hands-on contact they gave via Slack direct messages.”
Lee Percox, COO, Silktide · 5.0 verified review on Clutch
The engagement: platform, API and entry-network testing for Silktide, with a detailed report of the issues found, their severity and remediation guidance. Read the Silktide case study.
03 Coverage
What the test covers
We test against the OWASP API Security Top 10 as a floor, with authorisation as the centre of gravity, because that is where APIs actually break.
Broken object-level authorisation
The number one API flaw in the wild. Can user A fetch user B’s record by changing an identifier? Can a read-only integration write? We enumerate objects and roles systematically, because one missed check on one endpoint is a data breach.
Authentication and token handling
API keys, JWTs, OAuth flows and service-to-service credentials. Expiry, revocation, audience checks, algorithm confusion and token reuse across environments. We verify that a stolen or expired credential is actually useless, rather than merely supposed to be.
Excessive data exposure
APIs that return the whole object and trust the client to display only some of it. We inspect what actually comes over the wire, including fields your frontend never shows, because attackers read responses, not screens.
Function-level authorisation and mass assignment
Beyond object-level checks sit two quieter flaw classes. Function-level authorisation asks whether a regular user can call admin functions at all. Mass assignment asks whether the API accepts fields it should ignore: a harmless-looking profile update becomes privilege escalation the moment a role or account field rides along in the request body and the server binds it.
Rate limiting and abuse
Brute-force resistance on authentication endpoints, enumeration through predictable identifiers, and cost-amplification in expensive operations. GraphQL gets particular attention here: introspection exposure, query depth and batched-query abuse.
Workflow abuse and race conditions
APIs enforce sequences the interface only implies: create, verify, pay, fulfil. We drive workflows out of order and in parallel, including concurrent requests against the same resource, because limits that are checked at read time and spent at write time are how balances go negative and single-use vouchers get used twice. These findings never appear in a scanner report, and they are among the most commercially painful.
Webhooks and outbound integrations
Webhook receivers that skip signature validation, callbacks that can be replayed, and URL-fetching features that reach internal services. Integration surface is API surface, and it is tested as such.
REST and GraphQL, documented or not
An OpenAPI spec or Postman collection speeds the work and sharpens coverage, and we ask for one in scoping. No documentation? We map the API from traffic and testing, the way an attacker would.
One missed authorisation check on one endpoint is a data breach. That is why we test them all.
04 Compare
API test or web application test?
| Web application test | API test |
|---|---|
| Attacks through the interface users see | Attacks the endpoints directly, no interface in the way |
| Covers the API only where the frontend happens to call it | Covers the full API surface, including endpoints no screen uses |
| Finds flaws in flows the UI exposes | Finds authorisation flaws the UI quietly hides |
| Right when the product is the web app | Right when the API carries the data and the decisions |
Most software products need both, and they pair naturally in one engagement with one report. Scoping tells you honestly whether one, the other or both fits your situation. See our web application penetration testing, or test the whole product as one scope with SaaS penetration testing.
05 Why Atoro
Why software companies test with Atoro
Authorisation-first.
We spend the testing budget where APIs actually fail: object-level and function-level access control, tested role by role.
Human-validated. Always.
Every finding is reproduced and severity-rated by a tester. No raw scanner output, no false-positive triage left to your team.
Fluent in modern stacks.
REST, GraphQL, microservices and third-party integrations, tested by people who build against the same patterns.
Scope before price.
Endpoints, roles and environments agreed first, then a fixed quote, usually on the first call.
06 Plan
A managed test from scoping to retest
We run the engagement with clear scoping, a defined testing window, and support before and after the test.
1
Scope
We agree the API surface, roles, environments and documentation available, then fix the price.
2
Test
Manual, authorisation-focused testing of the agreed surface, tool-assisted for coverage, human-driven for logic.
3
Validate
Every candidate finding reproduced, rated and stripped of false positives.
4
Report
Validated findings with request-level evidence, impact and remediation guidance, structured for engineers and auditors.
5
Remediate
We walk your team through the findings and support the fixes.
6
Retest
Once fixed, we confirm it, with closure evidence for your audit or customer review.
07 Deliverables
What you receive
Every engagement is scoped first and priced to what it covers. You get the number before any agreement, usually on the first call.
Included
Scoping
A clear view of what is in scope, what is out, and what the test needs to prove.
Included
Expert penetration test
Manual and tool-assisted testing by experienced security professionals.
Included
Human validation
Findings reviewed, prioritised, and explained by testers, not exported from a scanner.
Included
Audit-ready report
Structured for engineering teams, auditors, and customer security reviews.
Included
Remediation support
Help for your team to understand and close the findings.
Included
Retest included
Confirms fixes and provides closure evidence.
No raw scanner output sold as a pen test. No vulnerability dump with no priorities. No report that leaves your engineers guessing.
08 Compliance
Evidence for ISO 27001 and SOC 2
API testing extends the same evidence story as web application testing: technical vulnerability management for ISO 27001 (Annex A 8.8) and vulnerability identification for SOC 2.
For products where the API is the product, auditors and customer reviewers increasingly expect it named in scope. We write the report so the mapping is explicit.
09 FAQ
API penetration testing FAQs
How is an API penetration test different from a web application test?
A web application test attacks your product through its interface; an API test attacks the endpoints directly, with no interface in the way. That reaches the full API surface, including endpoints your frontend never calls, and focuses on authorisation, token handling and data exposure. Most products benefit from both, scoped together.
Do you test GraphQL as well as REST?
Yes, both, in the same engagement where your product uses both. GraphQL gets specific attention on introspection exposure, query depth and complexity abuse, and authorisation on resolvers, which behave differently from REST endpoint checks.
What documentation do you need (OpenAPI spec, Postman collection)?
An OpenAPI spec or Postman collection is ideal and speeds coverage, but neither is required. Without documentation we map the API from application traffic and systematic testing, exactly as an attacker would.
Can you test APIs that sit behind our mobile or web app?
Yes. Private APIs that only your own clients call are a common blind spot, because teams assume the frontend is the only consumer. Attackers make no such assumption, and these APIs frequently carry the weakest authorisation checks.
How are findings reported and retested?
Each finding comes with the exact requests to reproduce it, severity, impact in your product’s terms and remediation guidance. After your team fixes, we retest each finding and issue closure evidence suitable for auditors and customer security teams.
Does this support ISO 27001 or SOC 2?
Yes. The report maps findings to the technical vulnerability management story your ISMS or SOC 2 audit needs, and the retest provides the closure evidence auditors look for.
Our API is internal only, do we still need testing?
Internal-only is a network position, not a security control. Internal APIs are reachable through SSRF, a compromised dependency, a leaked credential or any foothold inside your environment, and they tend to have the weakest authorisation because nobody expected hostile callers. If it moves sensitive data, it deserves testing.
Can we run this alongside a web application test?
Yes, and most clients do. One scoping call covers both, testing runs as one engagement, and you get one report and one retest cycle. It is the most efficient way to cover a modern web product properly.
10 Push
Request API testing pricing
Tell us what your API does and who calls it. You get a clear scope and a fixed quote before any agreement, usually on the first call.
No raw scan sold as a pen test. No vague “starting from” proposal. No report your engineers cannot use.
We’ll review
What the test is for: product security, ISO 27001, SOC 2, customer review, or a deal
The systems, applications, APIs, and infrastructure in scope
Your timeline and any audit or customer deadline
The reporting format your reviewer or internal team needs
The remediation and retest process
The frameworks you may need next