Ssrf Blind in Buffalo (Go)

Server-Side Request Forgery (SSRF) is a web security vulnerability that allows an attacker to induce the server-side application to make HTTP requests to an arbitrary domain. In the Buffalo framework for Go, SSRF often arises when user-controlled input is used to construct HTTP requests without adequate validation or network segregation. A blind SSRF occurs when the application does not return data from the induced request directly in the response, making it harder to detect but still exploitable through side channels such as DNS lookups, file retrieval, or interaction with metadata services.

Buffalo applications typically rely on the net/http client or third-party HTTP libraries to perform outgoing requests. If request URLs are derived from parameters such as url, host, or path without strict allowlisting, an attacker can supply internal endpoints (e.g., http://169.254.169.254 for AWS metadata) or internal services that are not exposed externally. The blind nature means the attacker must infer success through timing, DNS history, or external logs, which can still lead to sensitive data exposure or further compromise within a trusted network.

In a Buffalo application, routes that accept a target URL and perform a fetch without validating the host or scheme are susceptible. For example, a diagnostic endpoint that retrieves a remote resource to confirm connectivity can be abused if the target is user-supplied. Because SSRF can facilitate attacks against internal services, cloud metadata, or SMTP relays, it is important to restrict outbound destinations and sanitize inputs. middleBrick can detect such blind SSRF patterns during its unauthenticated scan by observing network interactions and identifying requests to sensitive internal IPs, helping teams identify missing network-level controls before exploitation occurs.

Remediation focuses on strict input validation, allowlisting permitted hosts and schemes, and avoiding user-controlled URLs for outbound requests. In Buffalo, you can enforce these controls at the handler level and centralize HTTP client creation to ensure consistent security.

Example: Unsafe Handler (Before)

// Unsafe: directly uses user input as the fetch URL
func FetchRemoteResource(c buffalo.Context) error {
    target := c.Param("url") // user-controlled
    resp, err := http.Get(target)
    if err != nil {
        return c.Render(500, r.String(err.Error()))
    }
    defer resp.Body.Close()
    body, _ := io.ReadAll(resp.Body)
    return c.Render(200, r.Bytes(body))
}

Example: Safe Handler with Allowlist and Custom Client (After)

var allowedHosts = map[string]bool{
    "api.example.com": true,
    "data.service.com": true,
}

var safeClient = &http.Client{
    // Configure timeouts and transport as needed
    Timeout: 10 * time.Second,
}

func isAllowedHost(u *url.URL) bool {
    return allowedHosts[u.Hostname()]
}

func FetchRemoteResource(c buffalo.Context) error {
    target := c.Param("url")
    parsed, err := url.ParseRequestURI(target)
    if err != nil {
        return c.Render(400, r.String("invalid URL"))
    }
    // Enforce HTTPS and allowed hosts
    if parsed.Scheme != "https" || !isAllowedHost(parsed) {
        return c.Render(403, r.String("forbidden destination"))
    }
    // Ensure no redirect to disallowed locations
    safeClient.CheckRedirect = func(req *http.Request, via []*http.Request) error {
        if !isAllowedHost(req.URL) {
            return http.ErrUseLastResponse
        }
        return nil
    }
    resp, err := safeClient.Get(parsed.String())
    if err != nil {
        return c.Render(502, r.String("upstream error"))
    }
    defer resp.Body.Close()
    body, _ := io.ReadAll(resp.Body)
    return c.Render(200, r.Bytes(body))
}

Additional Controls

• Use an HTTP transport with a custom dial context to restrict network destinations or apply firewall rules at the infrastructure level.
• Disable redirects by default or validate each redirect target against an allowlist.
• Prefer using service-specific clients with scoped credentials instead of generic HTTP fetching when integrating with cloud providers.
• Apply network segmentation so that backend services cannot reach sensitive metadata endpoints.

By combining input validation, allowlisting, and controlled HTTP clients, Buffalo applications can mitigate SSRF while maintaining required functionality. middleBrick supports this posture by identifying SSRF indicators during scans and mapping findings to relevant frameworks and compliance controls.