Dns Rebinding in Buffalo with Bearer Tokens

Buffalo is a convention-over-configuration web framework for Go, and Bearer Tokens are commonly used for HTTP authorization via the Authorization: Bearer <token> header. When these two are combined, a Dns Rebinding attack can bypass authorization assumptions by manipulating DNS resolution after the initial request, exposing endpoints that should remain protected.

During a scan, middleBrick tests unauthenticated attack surfaces and checks whether authentication and authorization checks are tied to the request origin or to the resolved host. With Bearer Tokens, the framework typically validates the token presence and scope but may not enforce strict host or referer checks. An attacker can register a domain, point it to a local or internal IP, then serve a page that rapidly changes DNS to pivot the victim’s browser to a sensitive Buffalo endpoint. Because the request still carries a valid Bearer Token—often stored in JavaScript or browser storage—the server sees an authenticated session and may allow access based on token validity alone, not on origin or network context.

Consider a Buffalo app that uses token-based auth but allows any host header to reach routes guarded by EnsureAuthenticated. An attacker crafts a page that first resolves to a benign IP, then rebinds to the server’s IP via a controlled DNS update. The victim’s browser, with a cached Bearer Token, sends an authenticated request to the rebinded host. Buffalo processes the request, sees a valid token, and may expose sensitive data or allow state-changing operations. This demonstrates BOLA/IDOR and Authentication weaknesses, where the framework’s trust in the token bypasses host-based controls. middleBrick’s 12 checks run in parallel and would flag this as a high-severity finding under Authentication and Property Authorization, noting that token validation must be bound to more than just the Authorization header.

In OpenAPI/Swagger spec analysis, middleBrick resolves $ref paths and cross-references runtime behavior. If the spec defines security schemes using Bearer Tokens but does not require host or referer constraints, and runtime tests show access without origin validation, a concrete finding is generated. The scanner does not assume internal architecture but reports that Bearer Token usage in Buffalo must include strict host binding and anti-rebinding controls to prevent unauthorized access through DNS manipulation.

Remediation centers on ensuring that Bearer Token validation in Buffalo includes host and origin checks, and that tokens are not treated as sufficient alone. Below are concrete code examples showing secure patterns.

1. Host header validation middleware

Add middleware to verify the request Host against an allowlist before processing authentication. This prevents DNS-rebounded requests from being accepted even with a valid Bearer Token.

// middleware/host_check.go
package middleware

import (
    "net/http"
    "strings"
)

func HostCheck(allowedHosts []string) func(http.Handler) http.Handler {
    return func(next http.Handler) http.Handler {
        return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
            host := r.Host
            allowed := false
            for _, h := range allowedHosts {
                if host == h || (strings.HasPrefix(h, "*") && strings.HasSuffix(host, h[1:])) {
                    allowed = true
                    break
                }
            }
            if !allowed {
                http.Error(w, "host not allowed", http.StatusForbidden)
                return
            }
            next.ServeHTTP(w, r)
        })
    }
}

// In main.go
// app.Use(middleware.HostCheck([]string{"api.example.com", "*.example.com"}))

2. Binding token validation to request context

Ensure that token validation checks not only the Authorization header but also that the request’s server name matches expected origins. This can be done within a custom auth handler used by Buffalo.

// middleware/auth_bearer.go
package middleware

import (
    "net/http"
    "strings"
)

func AuthenticateBearer(expectedAudience string) func(http.Handler) http.Handler {
    return func(next http.Handler) http.Handler {
        return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
            auth := r.Header.Get("Authorization")
            if auth == "" {
                http.Error(w, "authorization header required", http.StatusUnauthorized)
                return
            }
            parts := strings.Split(auth, " ")
            if len(parts) != 2 || parts[0] != "Bearer" {
                http.Error(w, "invalid authorization header format", http.StatusUnauthorized)
                return
            }
            token := parts[1]
            // validate token with your identity provider; include audience/issuer checks
            if !isValidToken(token, expectedAudience) {
                http.Error(w, "invalid token", http.StatusUnauthorized)
                return
            }
            // enforce host binding as additional context
            if !isAllowedHost(r.Host) {
                http.Error(w, "host not authorized", http.StatusForbidden)
                return
            }
            next.ServeHTTP(w, r)
        })
    }
}

func isValidToken(token, audience string) bool {
    // integrate with your OAuth2/JWT validation logic
    return true // placeholder
}

func isAllowedHost(host string) bool {
    allowed := map[string]bool{
        "api.example.com": true,
        "app.example.com": true,
    }
    return allowed[host]
}

// In routes.go
// app.Use(middleware.AuthenticateBearer("https://api.example.com"))

3. Secure cookie and token storage guidance

Advise frontend consumers to store Bearer Tokens in HttpOnly, Secure cookies or use short-lived tokens with strict CORS. Buffalo apps should set appropriate CORS headers to limit origins.

// middleware/cors.go
package middleware

import (
    "net/http"
)

func CORS(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        w.Header().Set("Access-Control-Allow-Origin", "https://app.example.com")
        w.Header().Set("Access-Control-Allow-Methods", "GET, POST, OPTIONS")
        w.Header().Set("Access-Control-Allow-Headers", "Authorization, Content-Type")
        w.Header().Set("Access-Control-Expose-Headers", "X-Rate-Limit-Remaining")
        if r.Method == "OPTIONS" {
            w.WriteHeader(http.StatusOK)
            return
        }
        next.ServeHTTP(w, r)
    })
}

// In main.go
// app.Use(middleware.CORS())

By combining these patterns, Buffalo applications can mitigate Dns Rebinding risks even when Bearer Tokens are used, ensuring that authentication is bound to trusted hosts and validated within a controlled origin context.