# Server-Side Request Forgery (SSRF) (CWE-918)

The web server receives a URL or similar request from an upstream component and retrieves the contents of this URL, but it does not sufficiently ensure that the request is being sent to the expected destination.

**Stack:** Go

- Prevalence: Medium 3 languages covered
- Impact: High 4 high-severity rules
- Prevention: Documented 4 fix examples

**OWASP:** Server-Side Request Forgery (A10:2021-Server-Side Request Forgery) - #10

## Description

By providing URLs to unexpected hosts or ports, attackers can make it appear that the server is sending the request, possibly bypassing access controls such as firewalls.

## Prevention

Prevention strategies for Server-Side Request Forgery based on 1 Shoulder detection rules.

### Go

Parse URL and validate host against domain allowlist

## Warning Signs

- [HIGH] user input flowing to HTTP client requests, enabling Server-Side Request Forgery attacks

## Consequences

- Read Application Data
- Bypass Protection Mechanism
- Execute Unauthorized Commands

## Mitigations

- Use an allowlist of allowed destinations
- Disable unnecessary URL schemes (file://, gopher://)
- Use network-level segmentation

## Detection

- Total rules: 4
- Languages: go, javascript, typescript, python

## Rules by Language

### Go (1 rules)

- **Server-Side Request Forgery (SSRF)** [HIGH]: Detects user input flowing to HTTP client requests, enabling Server-Side Request Forgery attacks.
  - Remediation: Validate URLs against an allowlist of permitted domains.

```go
allowed := map[string]bool{"api.example.com": true}
parsed, _ := url.Parse(targetURL)
if !allowed[parsed.Host] {
    return errors.New("domain not allowed")
}
```

Learn more: https://shoulder.dev/learn/go/cwe-918/ssrf