Fix SSRF (Server Side Request Forgery) in Revel
SSRF in the Revel framework typically occurs when a controller action accepts a URL from a user and fetches it using the default `http.Get` without validation. This allows attackers to scan internal networks, hit loopback services, or exfiltrate cloud metadata (IMDS). In Go, the standard library is powerful but dangerous if you don't restrict the transport layer.
The Vulnerable Pattern
func (c App) FetchContent(url string) revel.Result {
// DANGER: Directly fetching user-supplied URL
resp, err := http.Get(url)
if err != nil {
return c.RenderError(err)
}
defer resp.Body.Close()
body, _ := io.ReadAll(resp.Body)
return c.RenderText(string(body))
}The Secure Implementation
To kill SSRF, you must implement a multi-layered defense. First, parse the input using 'url.Parse' and enforce a strict 'https' scheme. Second, use an allowlist for hosts; never use a denylist as it is easily bypassed with DNS tricks or decimal IP encoding. Third, configure a custom 'http.Client' that disables redirects ('CheckRedirect') to prevent attackers from bypassing host checks via 302 jumps. Finally, use 'io.LimitReader' to prevent resource exhaustion (DoS) from oversized backend responses.
func (c App) FetchContent(target string) revel.Result { allowedHosts := map[string]bool{"api.trusted.com": true, "images.trusted.com": true} u, err := url.Parse(target) if err != nil || u.Scheme != "https" { return c.Forbidden("Invalid protocol") } if !allowedHosts[u.Host] { return c.Forbidden("Unauthorized host") }// Use a hardened client with short timeouts and no redirect following client := &http.Client{ Timeout: 5 * time.Second, CheckRedirect: func(req *http.Request, via []*http.Request) error { return http.ErrUseLastResponse // Block redirect-based SSRF }, } resp, err := client.Get(u.String()) if err != nil { return c.RenderError(err) } defer resp.Body.Close() body, _ := io.ReadAll(io.LimitReader(resp.Body, 1024*1024)) // Limit response size return c.RenderText(string(body))
}
Your Revel API
might be exposed to SSRF (Server Side Request Forgery)
74% of Revel apps fail this check. Hackers use automated scanners to find this specific flaw. Check your codebase before they do.
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