hunt-spa-api skill

by elementalsouls·elementalsouls/Claude-BugHunter·4.7k stars·MIT

Discover a single-page-app's hidden backend API from its public JS bundle, then test that API for broken access control / missing authentication. One of the highest-yield web plays in modern recon — SPAs ship their entire backend route map to the browser, and the API behind them is frequently missing the auth middleware the login page implies. Built from an authorized engagement where this play found an unauthenticated financial API that an ASM scan reporting hundreds of "Criticals" completely missed. Use whenever a target serves a JS-heavy SPA (React/Vue/Angular/Next), an "app"/"console"/"dashboard"/"portal" subdomain, or any `*api*` host shows up in recon. Leaked build artifacts (source maps / .env / .git / asset-manifest) are owned by hunt-source-leak; API version-inventory and behavioral diffing by hunt-shadow-api; this skill owns mapping a live SPA's backend routes from its JS bundle and testing them for broken access control / missing auth.

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Install the hunt-spa-api skill

A skill is a folder. Copy it into your agent's skills folder and the agent loads it when the task matches its description.

git clone --depth 1 https://github.com/elementalsouls/Claude-BugHunter.git /tmp/Claude-BugHunter
mkdir -p ~/.claude/skills
cp -r /tmp/Claude-BugHunter/skills/hunt-spa-api ~/.claude/skills/hunt-spa-api

available in every project

In the Claude apps, zip the folder and upload it from the Skills settings. The folder on GitHub

The instructions your agent would load

SKILL.md as published, without the frontmatter. Read it on GitHub

When to use this skill

Trigger when:

A target host returns a tiny HTML shell + big /static/js/.js or /next/static/* bundles (React/Vue/Angular/Next/Svelte SPA)
You see a subdomain named console, app, dashboard, portal, admin, panel, manage, internal
Recon surfaces any api, -api, api.* host
A login page is OAuth/SSO-gated (the frontend auth tells you nothing about whether the API enforces auth)

The core insight: a SPA is a client to a backend API, and it ships the full map of that API — hosts, routes, sometimes keys — to anyone who views source. The login page being protected says nothing about whether the API behind it checks tokens. Auth is frequently enforced on the gateway/login and missing on a route group of the API.

DO NOT skip this because "the app needs login" — that's exactly when this pays off.

The play (5 steps)

1. Pull the shell + enumerate the bundles

curl -s https://console.target.com/ -o index.html
# React/CRA:
grep -oE '/static/js/[^"]+\.js' index.html
# Next.js:
grep -oE '/_next/static/[^"]+\.js' index.html
# generic:
grep -oiE 'src="[^"]+\.js[^"]*"' index.html

Download every bundle (they can be multi-MB — that's fine, it's all route data):

mkdir bundles
for j in $(grep -oE '/static/js/[^"]+\.js' index.html | sort -u); do
  curl -s "https://console.target.com$j" -o "bundles/$(echo "$j"|tr '/' '_')"
done

2. Harvest API hosts, routes, and secrets from the bundles

B=bundles/*.js
# Backend API hosts (incl. dev/beta/staging variants — often weaker auth)
grep -ohiE 'https://[a-z0-9.-]*(api|console|backend|service)[a-z0-9.-]*\.target\.com[a-z0-9/_-]*' $B | sort -u
# Versioned API base paths
grep -ohiE '/api/v[0-9]+/?' $B | sort -u
# Route literals — minified bundles store routes as STRING segments, not full URLs.
# Grep for quoted "resource/action" strings:
grep -ohiE '"[a-z0-9_-]+/[a-z0-9_/-]+"' $B | tr -d '"' \
  | grep -iE '(login|user|account|order|billing|invoice|payment|deal|report|token|otp|password|reset|admin|profile|auth|upload|export|role|permission|dashboard|wallet|finance|sales)' | sort -u
# Secrets (validate before trusting — most AIza keys are Maps/analytics, not Auth)
grep -ohiE '(AIza[0-9A-Za-z_-]{35}|AKIA[0-9A-Z]{16}|sk_live_[0-9A-Za-z]+|eyJ[A-Za-z0-9_-]{10,}\.[A-Za-z0-9_-]{10,}|apiKey["'"'"']?\s*[:=]\s*["'"'"'][^"'"'"']+)' $B | sort -u

Note: minifiers store routes as concatenated string segments (e.g. "account/payment/list"), NOT full /api/v2/... URLs — so a naive /api/v grep returns nothing. Grep for the resource-word route strings** and prepend the base yourself.

Lazy-loaded (async) chunks — don't stop at HTML-referenced bundles. CRA/webpack SPAs reference only runtime+main+vendor in index.html; numbered async route chunks are named inside main.js's chunk map and loaded at runtime, so grepping only /static/js/*.js from the shell truncates route coverage to eager chunks.

grep -oE '[0-9]+:"[a-f0-9]+"' bundles/*main*.js        # {chunkId:"hash"} pairs
# reconstruct /static/js/<id>.<hash>.chunk.js, download each, re-run route/host/secret harvest
# asset-manifest.json (if present) lists them all -> hunt-source-leak

3. Establish a CONTROL — find an endpoint that IS gated

Before declaring anything vulnerable, send an unauthenticated request to an endpoint you expect to be protected, and capture what correct rejection looks like:

curl -s -X POST https://api.target.com/api/users -H 'Content-Type: application/json' -d '{}'
# secure → {"error":"Missing or invalid authorization header"} or HTTP 401

This is your differential. A sibling API (e.g. a second API host, or a different route group on the same host) is the ideal control — same stack, so a different response = real authz gap, not a quirk.

4. Test each route family UNAUTHENTICATED, both methods

For every discovered route, send it with no Authorization header and compare to the control:

for r in <routes>; do
  curl -s -o /tmp/r -w "[%{http_code}] $r\n" -X POST -H 'Content-Type: application/json' -d '{}' "https://api.target.com/api/v2/$r"
done

Interpret:

401/"Missing authorization" → gated (correct). Move on.
200 with data → unauthenticated data exposure. Finding.
400 "field X is mandatory" → the route processed your request and reached business-logic validation without an auth check → auth bypass; supply the field minimally to confirm.
200 + verbose DB/stack error (e.g. PROCEDURE dbx.spy does not exist) → reached the data layer unauthenticated; also a SQLi-surface signal.
Mandatory fields named like isadmin / isinternal / requestedby / roleid / accounttype → authorization derived from client-supplied parameters** — set the privilege flag and you self-elevate. Critical-class.

5. Pivot & prove (minimally)

IDs returned by one endpoint (accountid, orderid, dealid) are the keys the other* endpoints consume — they prove the whole router is reachable, not just one route.
Test dev-/beta-/staging- API variants — they frequently have weaker/disabled auth.
Check the response headers: Access-Control-Allow-Origin: * compounds the issue (any web origin reads it from a victim's browser).
STOP at minimum-necessary proof. A handful of records (or a totalCount) confirms the missing check. Do NOT enumerate the table — see redteam-mindset data-minimization boundary. The finding is the absent auth, not the data volume.

What "the API behind the SSO login" really means

A common, dangerous architecture:

console.target.com (the SPA) → login is Entra/Okta/Google OAuth (looks airtight).
api.target.com (the backend) → some route groups enforce the bearer token, some route groups forgot the middleware.

The frontend login is theatre if the API doesn't independently validate the token on every route. Always test the API directly, bare, regardless of how locked-down the login UI is.

Anti-patterns

"The app requires login, so the API must be protected." No — test the API directly, unauthenticated. The whole point.
"Minified bundle, can't read it." You don't need to read it — grep it for hosts/routes/secrets. 5 minutes.
"/api/v1/foo returned 404, so no API here." Wrong base or wrong method. Try /api/, /api/v2/, POST not GET, and the exact route strings from the bundle (Express's 404 echoes the path — use it to calibrate).
"AIza key found → critical secret." Validate first — most are Maps/analytics keys (CONFIGURATIONNOTFOUND on identitytoolkit = not Auth-enabled). Don't over-claim.
Dumping the whole dataset once you get a 200. Stop at PoC. (redteam-mindset.)
Account-creation / write endpoints as "proof". Read endpoints prove the auth gap without creating state. Never POST a create/signup/upload to "demonstrate" — that's a destructive write needing explicit per-action authorization.

Related Skills & Chains

hunt-api-misconfig — once the API is mapped, run the broader misconfig matrix (method tampering, mass assignment, JWT alg confusion) per route.
hunt-idor — the accountid/orderid pivots feed straight into IDOR testing across discovered routes.
hunt-source-leak — sourcemaps (*.js.map) reconstruct original source for deeper route/secret extraction; same harvesting muscle.
hunt-nextjs — for Next.js targets, layer the middleware-bypass (x-middleware-subrequest) and /_next/data route tests on top of this.
redteam-mindset — the data-minimization boundary governs step 5: prove the missing check, don't exfiltrate the table.
recon-scope-triage — verify the API host actually belongs to the target before testing (don't pop a same-named third party's API).

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