SSRF is when an attacker makes your server send HTTP requests to unintended destinations. Since the request comes from your server (inside the firewall), it can access internal services, cloud metadata endpoints, and other resources the attacker cannot reach directly.

Why is SSRF dangerous in cloud environments?

Cloud instances have metadata endpoints (169.254.169.254) that return IAM credentials, API keys, and configuration data. SSRF can access these endpoints from inside the cloud network, potentially compromising the entire cloud account.

How do I prevent SSRF?

Use URL allowlists, block internal IP ranges (10.x, 172.16.x, 192.168.x, 169.254.x), disable HTTP redirects, and use cloud-specific protections like AWS IMDSv2. Never trust user-supplied URLs without validation.

What Is SSRF (Server-Side Request Forgery)?

Server-Side Request Forgery (SSRF) is a vulnerability where an attacker tricks your server into making HTTP requests to unintended destinations — typically internal services, cloud metadata endpoints, or other systems behind the firewall. The server acts as a proxy, giving the attacker access to resources they cannot reach directly.

Types of SSRF (Server-Side Request Forgery)?

Basic SSRF

The attacker provides a URL that the server fetches, targeting internal resources.

POST /api/fetch-url { "url": "http://169.254.169.254/latest/meta-data/iam/security-credentials/" }

Blind SSRF

The server makes the request but does not return the response. The attacker infers information from timing or out-of-band channels.

SSRF via redirects

The attacker provides a URL that redirects to an internal resource, bypassing URL validation.

How to Prevent It

# 1. Allowlist approach — only permit known-safe destinations
ALLOWED_HOSTS = ['api.example.com', 'cdn.example.com']
def fetch_url(url):
    parsed = urlparse(url)
    if parsed.hostname not in ALLOWED_HOSTS:
        raise ValueError("Host not allowed")

# 2. Block internal IP ranges
import ipaddress
def is_internal(hostname):
    ip = ipaddress.ip_address(socket.gethostbyname(hostname))
    return ip.is_private or ip.is_loopback or ip.is_link_local

# 3. Disable redirects
requests.get(url, allow_redirects=False)

# 4. Use a metadata firewall for cloud environments
# AWS: IMDSv2 requires a PUT request with hop limit
# GCP: Requires Metadata-Flavor: Google header

Real-World Impact

In the 2019 Capital One breach, SSRF was used to access AWS metadata endpoints, stealing credentials that led to 106 million customer records being exposed. SSRF can read cloud credentials, access internal databases, scan internal networks, and pivot to other services.