/hunt-cache-poison

name: hunt-cache-poison
description: Hunting skill for cache poison vulnerabilities. Built from 10 public bug bounty reports including X-Forwarded-Host poisoning, X-HTTP-Method-Override / GCS cache, reflected→stored XSS via cache, classic Omer-Gil Web Cache Deception, Cloudflare Cache Deception Armor bypass, session-token cache deception, Akamai hop-by-hop smuggling → server-side edge poisoning, and Kettle's 2024 path-normalization WCD against Cloudflare/Fastly/GCP. Host/X-Forwarded-Host injection that reaches app logic (reset-link poisoning, routing SSRF, OAuth issuer) is owned by hunt-host-header; this skill owns the case where the poisoned response is CACHED and served to other users. Use when hunting cache poisoning, Web Cache Deception, CDN-fronted apps.
sources: github, hackerone_public, portswigger_research, omergil_research, youstin_research
report_count: 8

Crown Jewel Targets

Cache poisoning is high-value because a single poisoned cache entry can affect thousands or millions of victims simultaneously — one request, mass exploitation. Payout scales with blast radius.

**Highest-value targets:**

• **CDN-served assets** (cdn.shopify.com, cloudfront distributions, Fastly/Akamai edges) — poisoning these affects every visitor globally
• **E-commerce platforms** with affiliate/referral flows (Shopify, WooCommerce storefronts) — session hijack or affiliate fraud potential
• **Gaming platforms with update servers** (rockstargames updates.* domains) — DoS on update delivery = widespread client breakage
• **Authentication endpoints** served through caches — leads to account takeover (the highest severity variant)
• **Asset CDNs** (JS/CSS delivery) — XSS payload delivery at scale
• **SaaS multi-tenant platforms** — one poisoned response bleeds into all tenants sharing a cache key

**Asset types that pay most:** CDN hostnames, subdomain-per-tenant patterns, update/download servers, login/account pages cached incorrectly, affiliate link shorteners.

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Autonomous Testing Priority

**Two distinct attacks live under this skill — target the simpler one first.**

**Attack 1 — Password Reset Poisoning (Host header injection):**

The app uses the `Host` header to construct the password reset link in the email. Inject an attacker-controlled hostname; the victim's reset email contains a link to your server.

POST /forgot-password
Host: attacker.com
X-Forwarded-Host: attacker.com
X-Host: attacker.com

email=victim@target.com
Content-Type: application/x-www-form-urlencoded

Use a distinctive hostname you control or can identify in the response. **Proof:** the injected hostname appears in the response body (some apps reflect the generated reset link), or the action succeeds (2xx with a "reset email sent" message) after injection — confirming the poisoned link would be sent to the victim.

Try multiple host headers — apps vary in which one they trust (`X-Forwarded-Host` is most common, but `Host` itself also works when the proxy passes it through).

**Attack 2 — Web Cache Poisoning:**

Inject the attacker-controlled hostname into `X-Forwarded-Host` on a GET request for a cacheable page. If the hostname is reflected in the response body AND the response gets cached, subsequent visitors receive the poisoned response.

Check for cache signals in the response: `X-Cache: HIT`, `CF-Cache-Status: HIT`, `Age: <nonzero>`, or `Via: cloudfront/varnish/fastly`.

**Proof for both:** injected value reflected in response body, or action completed successfully despite the manipulated header.

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Attack Surface Signals

**URL patterns to look for:**

• `cdn.`, `assets.`, `static.`, `updates.`, `downloads.` subdomains
• URL path structures with extensions that look static: `/path/to/page.css`, `/account.php/nonexistent.jpg`
• Affiliate/link shortener endpoints: `/link/`, `/go/`, `/ref/`, `/out/`
• Paths that mix dynamic content with cacheable-looking URLs

**Response headers that signal a cache:**

X-Cache: HIT / MISS
X-Cache-Status: HIT
CF-Cache-Status: HIT / MISS (Cloudflare)
Age: <nonzero>
Via: 1.1 varnish / cloudfront / fastly
Cache-Control: public, max-age=...
Surrogate-Control: max-age=...
X-Served-By: cache-...

**JS/tech stack signals:**

• Fastly, Varnish, Cloudfront, Akamai, Nginx proxy_cache in response headers
• Shopify/Linkpop stacks with third-party integrations
• Platforms using path-based routing without normalizing trailing segments
• Servers that reflect unvalidated headers into responses (Host, X-Forwarded-Host, X-Original-URL)

**Dangerous header candidates (unkeyed inputs):**

X-Forwarded-Host
X-Host
X-Forwarded-Scheme
X-Original-URL
X-Rewrite-URL
Forwarded
X-HTTP-Method-Override

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Step-by-Step Hunting Methodology

1. **Map cache infrastructure.** Send a GET to the target and inspect response headers. Identify the caching layer (Cloudflare, Fastly, Varnish, Nginx). Note `Age`, `X-Cache`, `CF-Cache-Status` headers.

2. **Identify cache key components.** Send two identical requests — if `Age` increments, the response is cached. Vary headers one-by-one (e.g., add `X-Forwarded-Host`) to determine which headers are NOT included in the cache key (unkeyed).

3. **Test unkeyed header reflection.** Add `X-Forwarded-Host: evil.com` and check if the value appears in the response body (redirects, canonical links, CSP headers, JS src attributes, meta tags). Append a unique cache-busting query parameter (e.g. `?cb=<random>`) so the probe lands on a cache MISS under a throwaway key — this verifies reflection without prematurely storing a live poison entry under the real, victim-shared cache key. (Param Miner's "Guess headers" mode is the canonical Burp tool for discovering these unkeyed headers/parameters automatically.)

4. **Test URL path manipulation (Web Cache Deception).** Append fake static extensions to dynamic endpoints:

• `GET /account/profile.css`
• `GET /dashboard/settings.jpg`
• `GET /affiliate-link/target.js`

Check if the server returns dynam