Dangling Dns in Buffalo with Jwt Tokens

A dangling DNS record in a Buffalo application becomes a security risk when it is referenced by logic that uses JWT tokens for access control. Buffalo uses the standard jwt-go or golang-jwt/jwt package to parse and validate tokens. If a route protected by JWT validation points internally to a hostname that no longer resolves or resolves to an unintended service, an attacker may be able to influence or bypass authorization checks depending on how the application handles token claims and hostnames.

Consider a scenario where the application uses the Host header from the request to select a tenant or API endpoint, and then validates a JWT token before proceeding. If the hostname used in the Host header resolves to a dangling DNS record, the application might route the request to an unintended location. Because JWT validation is typically performed before routing decisions, an attacker could send requests with a valid token but a manipulated hostname, potentially accessing functionality that should be restricted or causing the application to interact with an unexpected backend.

Additionally, if JWT tokens contain host-based claims (such as iss, aud, or custom claims used to restrict allowed hostnames), and the application does not strictly validate these against a known set of hostnames, a dangling DNS record can be leveraged in host header manipulation attacks. The token may appear valid, but the hostname it refers to does not point to a legitimate service, creating an inconsistent authorization boundary. This mismatch can lead to authorization issues, where a token intended for one service is accepted for another unresolved or unintended host.

The risk is amplified when the application uses subdomain-based routing without strict hostname verification. For example, if the token is accepted for any subdomain that resolves (or does not resolve), an attacker might probe for dangling entries to identify endpoints that do not properly enforce hostname-based restrictions. MiddleBrick scans detect such inconsistencies by correlating runtime behavior with OpenAPI specifications and flag cases where hostname-based access controls do not align with DNS configurations.

In summary, a dangling DNS record does not directly expose JWT validation, but it can weaken the trust boundary when hostnames are used in routing or authorization decisions. If JWT tokens are accepted without strict hostname validation, an attacker may be able to exploit DNS misconfigurations to reach unintended application logic or bypass intended access controls.

To remediate risks related to JWT tokens in a Buffalo application, enforce strict hostname validation and ensure token claims are verified against expected values. Below are concrete code examples demonstrating secure handling of JWT tokens within a Buffalo application.

1. Validate JWT issuer and audience claims

Always validate the iss (issuer) and aud (audience) claims in the token to ensure it was issued for your application and intended for the correct service. Do not rely solely on token signature validation.

package actions

import (
    "github.com/golang-jwt/jwt/v5"
    "github.com/gobuffalo/buffalo"
    "net/http"
)

func RequireValidToken(next buffalo.Handler) buffalo.Handler {
    return func(c buffalo.Context) error {
        tokenString := c.Request().Header.Get("Authorization")
        if tokenString == "" {
            return c.Error(http.StatusUnauthorized, errors.New("missing authorization token"))
        }

        token, err := jwt.Parse(tokenString, func(token *jwt.Token) (interface{}, error) {
            // Validate signing method
            if _, ok := token.Method.(*jwt.SigningMethodHMAC); !ok {
                return nil, fmt.Errorf("unexpected signing method: %v", token.Header["alg"])
            }
            return []byte("your-secret-key"), nil
        })
        if err != nil {
            return c.Error(http.StatusUnauthorized, err)
        }

        if claims, ok := token.Claims.(jwt.MapClaims); ok && token.Valid {
            // Validate issuer
            if iss, ok := claims["iss"].(string); !ok || iss != "https://expected-issuer.example.com" {
                return c.Error(http.StatusForbidden, errors.New("invalid issuer"))
            }
            // Validate audience
            if aud, ok := claims["aud"].(string); !ok || aud != "https://your-buffalo-app.example.com" {
                return c.Error(http.StatusForbidden, errors.New("invalid audience"))
            }
        } else {
            return c.Error(http.StatusUnauthorized, errors.New("invalid token claims"))
        }

        return next(c)
    }
}

2. Enforce hostname-based restrictions

If your application uses hostname-based routing or tenant isolation, explicitly validate the request hostname against a whitelist. Do not trust the Host header alone.

package actions

import (
    "github.com/gobuffalo/buffalo"
    "net/http"
)

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

func RequireValidHostname(next buffalo.Handler) buffalo.Handler {
    return func(c buffalo.Context) error {
        host := c.Request().Host
        if !allowedHosts[host] {
            return c.Error(http.StatusForbidden, errors.New("hostname not allowed"))
        }
        return next(c)
    }
}

3. Combine JWT validation with hostname checks in middleware

Apply both JWT validation and hostname validation as middleware to ensure that each request is authenticated, authorized, and directed to an expected endpoint.

package actions

import (
    "github.com/gobuffalo/buffalo"
)

func SecureAPI() buffalo.Handler {
    return buffalo.Chain(
        RequireValidHostname,
        RequireValidToken,
    )
}

// Usage in routes:
// app.GET("/api/resource", SecureAPI(), ResourceHandler)

4. Avoid using subdomains derived from user input without validation

If your application dynamically constructs hostnames based on user input, ensure that the resolved hostname matches an expected pattern and is not pointing to a dangling or unintended DNS record.

func ValidateSubdomain(c buffalo.Context) error {
    host := c.Request().Host
    // Example: ensure subdomain is from a known set
    allowedSubdomains := []string{"api", "app", "admin"}
    var subdomain string
    // extract subdomain logic here...
    if !contains(allowedSubdomains, subdomain) {
        return c.Error(http.StatusBadRequest, errors.New("invalid subdomain"))
    }
    return nil
}

By strictly validating JWT claims and hostnames, and by combining these checks in middleware, you reduce the risk associated with DNS misconfigurations and ensure that tokens are only accepted in expected contexts.