Dns Cache Poisoning in Buffalo with Bearer Tokens

DNS cache poisoning (also referred to as DNS spoofing) in the Buffalo HTTP client occurs when an attacker forces Buffalo to resolve a hostname to a malicious IP address and caches that result. When Buffalo is configured to use Bearer tokens—typically via an Authorization header—the poisoned DNS entry can redirect those authenticated requests to an attacker-controlled server, leading to token theft or unauthorized access. This becomes a practical attack path when Buffalo resolves a hostname once and reuses the IP for subsequent requests, and when tokens are passed in headers that are not protected by additional transport-level mitigations.

Consider a scenario where your service calls an external API using a hostname like api.example.com with a Bearer token in the Authorization header. If an attacker poisons the local DNS cache for api.example.com to point to a rogue server, Buffalo may unknowingly send the Bearer token to that rogue server. Because the token is carried in the Authorization header, the attacker can capture or replay it to impersonate the client. This risk is elevated in Buffalo when DNS caching behavior is combined with long-lived tokens and unvalidated redirect handling, and it is not mitigated by the presence of TLS alone if the attacker can present a valid certificate for the rogue server.

The interaction with Bearer tokens does not change the fundamental DNS protocol weaknesses that enable cache poisoning, but it increases the impact: stolen tokens can be used to access protected resources until expiration or revocation. Because Buffalo does not inherently prevent reuse of a resolved IP for different hosts (especially when virtual hosting is involved), and because it relies on the system’s resolver behavior, developers must treat DNS as an untrusted network service when handling authentication. Attack patterns such as man-in-the-middle on local networks or compromised upstream resolvers can facilitate this, and findings from scans may map to OWASP API Top 10 A05:2023 (Security Misconfiguration) and A01:2023 (Broken Access Control) when token leakage occurs.

Remediation focuses on reducing the window of exposure for Bearer tokens when using Buffalo, ensuring tokens are not sent to unintended endpoints due to DNS manipulation. Below are concrete steps and code examples using the Go net/http client (commonly used with Buffalo) to demonstrate safer patterns.

1. Use explicit IP-based endpoints or pin certificates

Where possible, call services by IP or use certificate pinning to reduce reliance on DNS. This is not always practical but helps in high-risk scenarios.

// Example: Using a pinned server name and custom DialTLS to reduce DNS reliance
import (
    "crypto/tls"
    "net/http"
)

func newPinnedClient() *http.Client {
    return &http.Client{
        Transport: &http.Transport{
            TLSClientConfig: &tls.Config{
                ServerName: "api.example.com",
            },
        },
    }
}

req, _ := http.NewRequest("GET", "https://api.example.com/v1/resource", nil)
req.Header.Set("Authorization", "Bearer eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9...

2. Avoid caching Authorization headers on redirected or cross-origin responses

Ensure that requests with Bearer tokens do not automatically follow redirects to different hosts, and do not cache responses that include Authorization headers. Buffalo middleware or client wrappers can enforce this.

import (
    "net/http"
)

func noAuthRedirect(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        // Strip Authorization header on redirect to prevent leakage to third-party origins
        if isRedirect(r) && redirectHost(r) != originalHost(r) {
            r.Header.Del("Authorization")
        }
        next.ServeHTTP(w, r)
    })
}

func isRedirect(r *http.Request) bool {
    // simplified check
    return r.URL.Host != ""
}

func redirectHost(r *http.Request) string {
    // simplified: extract host from Location in a real implementation
    return "attacker.com"
}

func originalHost(r *http.Request) string {
    return "api.example.com"
}

3. Set short timeouts and enforce per-request token binding

Use short timeouts and bind tokens to specific request attributes (e.g., hostname, method) to limit misuse if a token is intercepted due to DNS poisoning.

import (
    "context"
    "net/http"
    "time"
)

func boundedRequest(token string) (*http.Response, error) {
    ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
    defer cancel()

    req, err := http.NewRequestWithContext(ctx, "GET", "https://api.example.com/v1/data", nil)
    if err != nil {
        return nil, err
    }
    req.Header.Set("Authorization", "Bearer "+token)
    req = req.WithContext(context.WithValue(ctx, "boundHost", "api.example.com"))

    client := &http.Client{
        Timeout: 10 * time.Second,
    }
    return client.Do(req)
}

4. Validate hostname on responses and reject mismatches

When receiving responses, validate that the effective request URL matches the expected host. This mitigates the impact of DNS poisoning redirecting token-bearing requests.

import (
    "net/http"
    "net/url"
)

func validateHost(resp *http.Response, expectedHost string) bool {
    if resp.Request.URL.Hostname() != expectedHost {
        return false
    }
    // additional checks on TLS state can be added here
    return true
}

// Usage in a client check
resp, err := http.DefaultClient.Do(req)
if err != nil {
    // handle error
}
if !validateHost(resp, "api.example.com") {
    // reject response to prevent token exposure to rogue hosts
}

5. Use short-lived tokens and rotate frequently

Short-lived Bearer tokens reduce the window of usefulness if intercepted via poisoned DNS. Combine with automatic rotation and revocation strategies.

// Example token usage with early expiration checks
func isTokenExpired(tokenString string) bool {
    // parse JWT or validate with auth server; simplified
    return false // placeholder
}

func safeAuthorizedRequest(token string) error {
    if isTokenExpired(token) {
        return fmt.Errorf("token expired; refresh required")
    }
    // proceed with request
    return nil
}