Dns Cache Poisoning in Buffalo with Basic Auth

Buffalo is a popular Go web framework for building rapid web applications. When Basic Authentication is used without additional protections, certain network and application-layer conditions can enable DNS cache poisoning, particularly when the application resolves hostnames dynamically or uses HTTP redirects based on request headers.

In Buffalo, developers often rely on the framework’s built-in mechanisms for routing and request handling. If an endpoint performs a redirect (e.g., using Redirect.Status(302).To(url)) where the target URL includes a hostname derived from user input or a request header (such as Referer or a custom X-Forwarded-Host), and that hostname is resolved at runtime, an attacker on a shared network may inject a malicious DNS response. This can cause the application to resolve a legitimate-looking hostname to an IP under the attacker’s control, effectively enabling a DNS cache poisoning scenario that redirects authenticated sessions.

Basic Authentication transmits credentials with each request via the Authorization header (e.g., Authorization: Basic base64(username:password)). If the application uses these credentials to construct URLs or to select backend services (e.g., tenant-based routing via hostname), poisoned DNS entries can direct credentials to an attacker who can capture or manipulate the authenticated session. Because the authentication itself does not encrypt the hostname resolution path, the combination of Basic Auth and dynamic hostname resolution increases the attack surface.

Consider a scenario where a Buffalo app uses a subdomain derived from the user’s organization name to route requests, such as https://{{.OrgName}}.example.com. If an attacker can poison the DNS cache for a subdomain within the application’s namespace (e.g., via a compromised upstream resolver or a local resolver), they can redirect the user’s authenticated session to a malicious server that also presents a valid certificate for that subdomain. The Basic Auth credentials are then sent to the attacker, who can replay them.

To detect such risks, tools like middleBrick can scan the Buffalo application’s unauthenticated endpoints, including those using Basic Auth, and analyze OpenAPI specs and runtime behavior for host resolution and redirect patterns. It checks for insecure use of dynamic hostnames, missing certificate validation, and improper handling of the Authorization header in the context of SSRF and DNS-related attack vectors. The scanner generates a security risk score and provides prioritized findings with remediation guidance, without attempting to fix or block traffic.

Remediation focuses on eliminating dynamic hostname resolution for authenticated flows and ensuring that redirects and URL construction do not expose credentials to poisoned DNS paths. Avoid using user-controlled input to determine hostnames or ports, and prefer fixed, trusted endpoints.

1. Avoid dynamic hostnames in redirects

Do not construct redirect URLs from request headers or parameters that can be influenced by an attacker. Instead, use hardcoded or configuration-based URLs. For example, replace this unsafe pattern:

// UNSAFE: redirect target derived from user input
func (rc AppController) RedirectToExternal(c buffalo.Context) error {
    target := c.Param("url")
    return c.Redirect(http.StatusFound, target)
}

With a controlled redirect:

// SAFE: redirect to a fixed, trusted host
func (rc AppController) RedirectToDashboard(c buffalo.Context) error {
    const dashboard = "https://app.example.com/dashboard"
    return c.Redirect(http.StatusFound, dashboard)
}

2. Do not embed Basic Auth credentials in URLs or logs

Ensure that the Authorization header is not used to build URLs that may be logged or resolved dynamically. Use standard client libraries that handle headers securely and avoid string concatenation that could lead to SSRF or cache poisoning.

3. Validate and restrict host resolution

If you must work with hostnames, validate them against an allowlist and avoid runtime DNS lookups for authenticated paths. For example, if you need to call an external service, configure the base URL via environment variables and perform no runtime substitution based on request data:

// Configure a trusted client
func newExternalClient() *http.Client {
    return &http.Client{
        Transport: &http.Transport{
            TLSClientConfig: &tls.Config{InsecureSkipVerify: false},
        },
    }
}

func callExternalService(c buffalo.Context) error {
    client := newExternalClient()
    req, _ := http.NewRequest("GET", "https://api.trusted.example.com/v1/data", nil)
    req.Header.Set("Authorization", "Basic "+os.Getenv("EXTERNAL_BASIC_AUTH"))
    resp, err := client.Do(req)
    if err != nil {
        return c.Render(http.StatusInternalServerError, r.String("request failed"))
    }
    defer resp.Body.Close()
    // process response
    return nil
}

4. Use secure session management alongside Basic Auth

Basic Auth lacks built-in session mechanisms. Pair it with secure cookies (with HttpOnly and Secure flags) and CSRF protection to reduce reliance on credentials in every URL, which in turn limits exposure in DNS resolution paths.

middleBrick can support remediation validation by scanning the updated endpoints for insecure redirects, host resolution patterns, and improper use of the Authorization header, helping teams verify that fixes align with secure coding practices.