Ssrf Blind in Buffalo

Blind SSRF occurs when an attacker can cause the server to make an outbound request to an arbitrary URL, but the response is not directly returned to the attacker. This can still be exploited for internal network scanning, accessing cloud metadata services, or exfiltrating data via out-of-band techniques. In Buffalo applications, this often arises when user-supplied input is used to construct a URL for an HTTP request without proper validation.

Buffalo provides convenient methods like c.Param, c.Query, and c.Bind to access request data. A common vulnerable pattern is using these values directly in an HTTP client call. For example, an action that fetches a screenshot or proxies a request based on a user-provided URL:

func (v *ProxyHandler) Fetch(c buffalo.Context) error {
    target := c.Query("url")
    resp, err := http.Get(target)
    if err != nil {
        return c.Error(500, err)
    }
    defer resp.Body.Close()
    // Process response, maybe return status only
    return c.Render(200, r.JSON(map[string]string{"status": "ok"}))
}

Here, the url query parameter is passed directly to http.Get. An attacker can set url to http://169.254.169.254/latest/meta-data/ (AWS EC2 metadata) or internal services like http://localhost:6379/ (Redis) or http://internal-api:8080/admin. Even if the application does not return the response body (making it blind), the attacker can infer information via timing (e.g., a slow response might indicate a closed port) or trigger out-of-band interactions (e.g., DNS exfiltration if the URL contains a malicious domain).

Buffalo's framework does not enforce URL validation for outbound requests. The vulnerability is introduced by the developer when they trust user input for network operations. Additionally, if the application uses c.Bind to deserialize JSON containing a URL field, the same risk applies:

type Request struct {
    URL string `json:"url"`
}

func (v *ProxyHandler) FetchJSON(c buffalo.Context) error {
    var req Request
    if err := c.Bind(&req); err != nil {
        return c.Error(400, err)
    }
    resp, err := http.Get(req.URL) // Vulnerable
    // ...
}

In both cases, Buffalo's context methods simply retrieve the data; they do not sanitize it for SSRF.

Detecting blind SSRF in Buffalo applications requires a combination of static code review and dynamic testing. Static analysis focuses on identifying code paths where user-controlled input (from c.Param, c.Query, c.Bind, etc.) flows into network operations like http.Get, http.Post, database drivers, or third-party libraries that perform I/O.

For dynamic testing, a black-box scanner like middleBrick actively probes the running endpoint. middleBrick submits a variety of SSRF payloads, including known internal service addresses (e.g., http://169.254.169.254/latest/meta-data/, http://localhost:22) and attacker-controlled domains with unique identifiers. It then observes behavioral differences that indicate a blind SSRF vulnerability:

• Timing discrepancies: Requests to internal IPs or closed ports may cause delays or timeouts compared to normal external requests.
• Out-of-band detection: middleBrick uses a collaborator server to detect if the target URL triggers a DNS lookup or HTTP request to an attacker-controlled domain, which would indicate that the server is making the request.
• Error message analysis: Sometimes the application returns different error messages or status codes when accessing internal vs. external resources, leaking information.

To scan a Buffalo application with middleBrick, simply provide the endpoint URL that accepts user-supplied URLs:

middlebrick scan https://your-buffalo-app.com/api/fetch

The scan runs in 5–15 seconds and produces a report with a security score (A–F), per-category breakdowns, and prioritized findings. If blind SSRF is detected, it will be listed under the SSRF category with severity and remediation guidance specific to Buffalo.

Remediating SSRF in Buffalo applications involves validating and sanitizing any user-supplied URLs before they are used in network requests. Buffalo's validation system provides a clean way to enforce this.

1. Use an allowlist of permitted hosts. Parse the URL and check its host against a known list. Also restrict schemes to http and https to prevent file:// or gopher:// attacks. For example:

func (v *ProxyHandler) Fetch(c buffalo.Context) error {
    target := c.Query("url")
    u, err := url.Parse(target)
    if err != nil {
        return c.Error(400, errors.New("invalid URL"))
    }
    // Allow only specific hosts
    allowedHosts := map[string]bool{
        "api.example.com": true,
        "images.example.com": true,
    }
    if !allowedHosts[u.Host] {
        return c.Error(400, errors.New("host not allowed"))
    }
    if u.Scheme != "http" && u.Scheme != "https" {
        return c.Error(400, errors.New("invalid scheme"))
    }
    resp, err := http.Get(target)
    // ...
}

2. Leverage Buffalo's custom validators. Define a validator that can be reused across actions. In your Buffalo app setup:

// validators.go
var allowedHosts = map[string]bool{
    "api.example.com": true,
    "images.example.com": true,
}

func init() {
    app.Validator.Add("ssrf_url", func(value string, params ...string) error {
        u, err := url.Parse(value)
        if err != nil {
            return errors.New("invalid URL")
        }
        if !allowedHosts[u.Host] {
            return errors.New("URL host not allowed")
        }
        if u.Scheme != "http" && u.Scheme != "https" {
            return errors.New("invalid scheme")
        }
        return nil
    })
}

Then in your action, use c.Bind with the validator tag:

type Request struct {
    URL string `json:"url" validate:"ssrf_url"`
}

func (v *ProxyHandler) FetchJSON(c buffalo.Context) error {
    var req Request
    if err := c.Bind(&req); err != nil {
        return c.Error(400, err)
    }
    resp, err := http.Get(req.URL) // Safe: validated
    // ...
}

3. Use a safe HTTP client. Consider using a library like github.com/kennylevinsen/ssrf to create a client that restricts outbound requests to allowed hosts and networks. Wrap your HTTP calls with this client.

4. Set timeouts. Configure timeouts on your HTTP client to mitigate slow-response attacks. Buffalo allows setting timeouts on the context or using a custom client.

By implementing these measures, you can effectively prevent SSRF vulnerabilities in Buffalo applications.