# Server-Side Request Forgery — OWASP Top 10

Source: https://www.skillbyai.com/en/owasp-top-10/a-ssrf

> When the server fetches a URL for you.

## Attacker-controlled destinations

**SSRF** occurs when an application fetches a URL supplied by a user (webhooks, image imports, PDF renderers, link previews) without restricting the destination. Attackers point it at internal services, admin panels or cloud **metadata endpoints** (such as 169.254.169.254) to steal credentials. Defences: allow-list destinations or schemes, resolve the hostname and block private, loopback and link-local addresses (and re-check after redirects), disable unneeded URL schemes, isolate fetchers in a restricted network segment, and require IMDSv2-style protections on cloud metadata services.

## Validating a user-supplied URL

Python sketch; also restrict network egress at the infrastructure level.

```python
import ipaddress, socket
from urllib.parse import urlparse

ALLOWED_SCHEMES = {"https"}

def is_safe_url(url: str) -> bool:
    parts = urlparse(url)
    if parts.scheme not in ALLOWED_SCHEMES or not parts.hostname:
        return False
    for info in socket.getaddrinfo(parts.hostname, parts.port or 443):
        ip = ipaddress.ip_address(info[4][0])
        if ip.is_private or ip.is_loopback or ip.is_link_local or ip.is_reserved:
            return False
    return True

# Fetch with redirects disabled (or re-validate each hop), a short timeout,
# and connect to the IP you validated to avoid DNS rebinding.
```

## Block egress by default

A fetcher service that can reach only the public internet, not internal networks, contains SSRF even if validation has a gap.

**Quiz:** Why are cloud metadata endpoints a common SSRF target?

- [ ] They are always rate limited
- [ ] They host the public website
- [ ] They store user passwords in plain text
- [x] They can return credentials for the server's cloud identity

*Answer:* They can return credentials for the server's cloud identity. Stolen instance credentials enable further access.