Dns Cache Poisoning in Buffalo (Go)

Buffalo is a popular Go web framework that encourages rapid development by providing routing, middleware, and HTML rendering helpers. When a Buffalo application performs DNS resolution—either directly or via an HTTP client that relies on the Go standard library’s net package—the framework does not enforce additional validation of DNS responses. This leaves the application subject to classic Dns Cache Poisoning when an attacker can inject crafted responses into the resolver cache on the operating system or within the application’s network stack.

In Buffalo, developers often use net/http clients to call external services, and these calls inherit the system’s DNS behavior. If an attacker can spoof DNS replies for a targeted hostname, they may redirect the Buffalo app to a malicious IP. This can facilitate SSRF against internal services, redirect sensitive API calls to a rogue endpoint, or enable request smuggling depending on how the app uses the resolved address. Because Buffalo does not provide built-in DNS hardening, the responsibility falls to the developer and the underlying Go resolver configuration.

The risk is especially relevant when Buffalo apps run in containers or shared network environments where an attacker can position themselves on the local network to observe or inject DNS traffic. The framework’s focus on productivity does not include automatic DNSSEC validation or source port randomization guarantees, so a vulnerable resolver cache can be poisoned via classic UDP response flooding or via off-path attacks if predictable transaction IDs and ports are used by the Go resolver in certain configurations.

To detect this using middleBrick, you can scan an unauthenticated Buffalo endpoint; the 12 security checks include Input Validation and SSRF, which surface DNS-related weaknesses in the API surface. The scan runs in 5–15 seconds and produces a per-category breakdown with severity-ranked findings and remediation guidance, helping you identify whether your Buffalo app’s external calls are exposed to manipulation via poisoned cache entries.

Remediation focuses on ensuring that DNS lookups performed by your Buffalo application are as robust as possible. You can configure the Go net resolver with custom net.Resolver settings and avoid relying solely on the operating system cache. Below are concrete patterns you can apply in a Buffalo app.

1. Use a custom resolver with stricter behavior

Instead of using the implicit resolver via http.Transport, create a resolver with a reasonable timeout and, where possible, prefer TCP for lookups to reduce the feasibility of UDP-based poisoning.

import (
    "context"
    "net"
    "time"
)

var resolver = &net.Resolver{
    PreferGo: true,
    Dial: func(ctx context.Context, network, address string) (net.Conn, error) {
        d := net.Dialer{
            Timeout:   5 * time.Second,
            KeepAlive: 30 * time.Second,
        }
        return d.DialContext(ctx, "tcp", "8.8.8.8:53")
    },
}

Then wire this resolver into your HTTP client used by Buffalo:

import (
    "net/http"
    "time"
)

func secureClient() *http.Client {
    transport := &http.Transport{
        DialContext: resolver.DialContext,
    }
    return &http.Client{
        Transport: transport,
        Timeout:   10 * time.Second,
    }
}

2. Validate and restrict hostnames and IPs

In Buffalo handlers, avoid passing unchecked user input directly to DNS-dependent functions. Validate hostnames against an allowlist or strict pattern, and prefer connecting to IPs only when they belong to explicitly permitted ranges.

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

func safeDialHandler(c buffalo.Context) error {
    host := c.Param("host")
    if !isValidHost(host) {
        return c.Render(400, r.Text("invalid host"))
    }
    conn, err := resolver.DialContext(c.Request().Context(), "tcp", host+":443")
    if err != nil {
        return c.Render(502, r.Text("upstream error"))
    }
    defer conn.Close()
    // proceed with secure communication
    return nil
}

func isValidHost(host string) bool {
    // Basic validation; adjust to your threat model
    if host == "" || len(host) > 253 {
        return false
    }
    if ip := net.ParseIP(host); ip != nil {
        // Optionally restrict private IPs
        return !ip.IsPrivate()
    }
    // Allow only alphanumeric and hyphen/dot patterns
    var ok bool
    _, ok = net.LookupHost(host)
    return ok
}

3. Enforce transport security and avoid insecure defaults

Ensure that downstream connections use TLS with verified certificates and prefer modern cipher suites. In Buffalo, you can set TLSClientConfig on your HTTP client to pin certificates or enforce TLS versions.

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

func tlsClient() *http.Client {
    return &http.Client{
        Transport: &http.Transport{
            TLSClientConfig: &tls.Config{
                MinVersion: tls.VersionTLS12,
            },
        },
        Timeout: 10 * time.Second,
    }
}

By combining a custom resolver, strict input validation, and enforced TLS, you reduce the impact of a compromised DNS cache on your Buffalo application. middleBrick can scan your Buffalo endpoints to verify that no unchecked external calls remain and to highlight areas where input validation and transport hardening are still needed.