Dns Cache Poisoning in Buffalo with Jwt Tokens

Buffalo is a popular web framework for Go, and JWT tokens are commonly used for stateless authentication. When DNS cache poisoning occurs in the environment where Buffalo serves authenticated requests, it can undermine the integrity of JWT validation by redirecting resolution of internal service names (such as identity providers or authorization endpoints) to malicious hosts.

In a Buffalo application, JWTs are typically parsed and verified using a public key or a shared secret. If an attacker can poison the DNS cache of the host or an upstream resolver used by the application, they may redirect, for example, auth.example.com to a malicious server. Should the Buffalo app rely on DNS-based discovery for JWKS (JSON Web Key Set) or for routing tokens to a specific verification endpoint, the poisoned DNS entry can cause the application to fetch keys or validation metadata from an attacker-controlled location.

This becomes critical when the application uses dynamic discovery patterns, such as fetching a JWKS URI from an OIDC configuration endpoint whose hostname is resolved via DNS. A poisoned cache can cause the hostname in the jwks_uri field to point to an attacker, enabling token verification with malicious keys. Even if the JWT library performs hostname validation, the initial resolution step may have already been compromised, allowing an attacker to serve a valid-looking key pair and produce tokens that appear legitimate.

Moreover, Buffalo applications that make outbound HTTP calls to internal microservices (for example, to validate tokens or fetch user data) are susceptible if those calls rely on service names resolved through DNS. An attacker who poisons the cache can intercept or manipulate these calls, potentially substituting a malicious service that accepts or mishandles JWTs. Because Buffalo does not inherently prevent DNS-level redirection, developers must ensure that critical hostnames used in JWT workflows are either hardcoded, pinned, or validated via mechanisms outside the DNS layer.

The risk is compounded when the application runs in environments with shared or untrusted DNS resolvers, such as containers sharing a host’s DNS configuration. Without additional controls, JWT validation logic in Buffalo can be bypassed indirectly, not by breaking cryptographic checks, but by steering name resolution to an attacker. Detecting such issues requires correlating runtime behavior with configuration, which is where scanning tools can highlight unexpected external dependencies and untrusted resolution paths.

To mitigate DNS cache poisoning risks affecting JWT workflows, Buffalo applications should minimize reliance on mutable DNS resolution for security-critical operations. The following practices and code examples focus on hardcoding, pinning, and validating the elements involved in JWT handling.

Use a static JWKS endpoint URL. Instead of dynamically resolving a hostname at runtime, embed the full HTTPS URL of the JWKS endpoint in configuration or environment variables, avoiding runtime DNS lookups for the key location.

// config.go
package config

const JWKSURI = "https://auth.example.com/.well-known/jwks.json"

Pin the public key or use a local key set. Rather than fetching keys over DNS-dependent channels at runtime, load a known public key or certificate directly into the application.

// jwt.go
package main

import (
    "github.com/golang-jwt/go-jwt/v5"
    "io/ioutil"
)

var publicKeyBytes = []byte(`-----BEGIN PUBLIC KEY-----
MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAwIDAQAB
...
-----END PUBLIC KEY-----`)

func validateToken(tokenString string) (*jwt.Token, error) {
    keyFunc := func(token *jwt.Token) (interface{}, error) {
        return publicKeyBytes, nil
    }
    return jwt.Parse(tokenString, keyFunc)
}

Validate hostnames explicitly. If dynamic resolution is unavoidable, ensure strict hostname verification against a whitelist and avoid trusting DNS CNAME chains for security-sensitive domains.

// verify.go
package main

import (
    "crypto/tls"
    "net"
    "net/http"
)

func customDialContext(network, address string) (net.Conn, error) {
    // Enforce that only allowed hostnames are dialed
    // Example: allow only auth.example.com on port 443
    // Raise an error for any other hostname
    ...
}

func newSecureClient() *http.Client {
    tlsConfig := &tls.Config{
        ServerName: "auth.example.com",
    }
    transport := &http.Transport{
        DialContext: customDialContext,
        TLSClientConfig: tlsConfig,
    }
    return &http.Client{Transport: transport}
}

Isolate JWT verification services. Run critical identity services on IP addresses rather than hostnames, or use service meshes with mTLS to reduce DNS dependency. This approach ensures that even if DNS is poisoned, the endpoint for JWT validation remains unchanged.

By combining static configuration, key pinning, and controlled resolution, Buffalo applications can reduce the attack surface related to DNS cache poisoning in JWT workflows. These measures align with secure coding practices and help maintain the integrity of token validation regardless of external DNS manipulation.