Dangling Dns in Buffalo with Bearer Tokens

A dangling DNS record in Buffalo that points to an internal or stale endpoint, combined with Bearer Token usage, can expose protected resources when the token is accepted at a URL that should not be routable. Buffalo uses subdomain or wildcard DNS to route requests to applications; if a subdomain is not cleaned up or is reused, requests sent to that subdomain may be handled by an unintended service that still validates Bearer tokens from trusted sources.

In this scenario, an attacker who knows or guesses a subdomain that resolves to a Buffalo app can send requests with a captured or leaked Bearer token. Because Buffalo applications often rely on token validation middleware, the request may be authorized even though the network path is unintended (dangling DNS). This is especially risky when the token is scoped broadly or when the endpoint does additional routing based on hostname without verifying that the hostname is authoritative.

For example, a developer might decommission an environment but leave the DNS CNAME or A record in place. If the application still accepts Authorization headers with Bearer tokens, an external caller can reach the stale endpoint and perform authenticated actions. This combines DNS mismanagement (dangling records) with token-based authentication weaknesses, leading to unauthorized access to APIs or admin interfaces that should no longer be reachable.

The risk is amplified when the Buffalo app uses wildcard DNS and shared token validation across multiple services. A dangling wildcard entry can direct traffic to a service that trusts the same Bearer token audience, bypassing intended network segmentation. Because Buffalo apps often serve both web and API routes, a dangling DNS entry can expose administrative endpoints or internal microservice routes that should only be accessible within a private network.

middleBrick detects this class of risk by scanning the unauthenticated attack surface and cross-referencing DNS resolution with observed authentication behavior. When a dangling DNS record responds to requests that include Bearer tokens, the scan flags the exposed surface and highlights the need for strict hostname validation and token scope enforcement.

To mitigate dangling DNS risks when using Bearer Tokens in Buffalo, enforce strict hostname checks and validate token audiences per request. Do not rely solely on token validity; ensure the request’s host matches the intended service domain. Below are concrete code examples for Buffalo middleware and route handling.

1. Hostname validation middleware

Add a middleware that verifies the request host against an allowlist before token validation proceeds.

// In middleware/host_check.middleware.ts
import { Middleware, Inject, Provide } from '@midwayjs/core';
import { IMiddleware } from '@midwayjs/koa';
import { Context } from 'koa';

@Provide()
@Middleware()
export class HostCheckMiddleware implements IMiddleware {
  async resolve(ctx: Context, next: () => Promise) {
    const allowedHosts = new Set(['api.example.com', 'app.example.com']);
    if (!allowedHosts.has(ctx.hostname)) {
      ctx.status = 400;
      ctx.body = { error: 'invalid_host' };
      return;
    }
    await next();
  }
}

2. Token validation with audience and issuer checks

Ensure your token verification validates the audience (aud) and issuer (iss) claims, and reject tokens that do not match expected values for the current hostname.

// In middleware/auth.middleware.ts
import { Provide, Inject, Configuration } from '@midwayjs/core';
import { JwtService } from '@midwayjs/jwt';
import { Context } from 'koa';

@Provide()
export class AuthMiddleware {
  constructor(
    @Inject() private readonly jwtService: JwtService,
    @Configuration('jwt') private readonly jwtConfig
  ) {}

  async verifyToken(ctx: Context) {
    const authHeader = ctx.get('Authorization');
    if (!authHeader || !authHeader.startsWith('Bearer ')) {
      ctx.status = 401;
      ctx.body = { error: 'unauthorized' };
      return null;
    }
    const token = authHeader.split(' ')[1];
    try {
      const payload = await this.jwtService.verify(token, {
        audience: this.jwtConfig.audience,
        issuer: this.jwtConfig.issuer,
      });
      // Ensure the token is intended for the current host
      if (payload.host && payload.host !== ctx.hostname) {
        ctx.status = 403;
        ctx.body = { error: 'token_host_mismatch' };
        return null;
      }
      return payload;
    } catch (err) {
      ctx.status = 401;
      ctx.body = { error: 'invalid_token' };
      return null;
    }
  }
}

3. Route-level hostname guard

For sensitive routes, explicitly check the hostname and token binding together.

// In controller/admin.controller.ts
import { Provide, Inject, Post, Body, Configuration } from '@midwayjs/core';
import { Context } from 'koa';

@Provide()
export class AdminController {
  constructor(
    @Configuration('app') private readonly appConfig
  ) {}

  @Post('/settings')
  async updateSettings(@Body() body: any, ctx: Context) {
    if (ctx.hostname !== this.appConfig.trustedHostname) {
      ctx.status = 403;
      return { error: 'forbidden_host' };
    }
    const token = ctx.get('Authorization')?.split(' ')[1];
    if (!token || !this.isValidBearer(token, ctx.hostname)) {
      ctx.status = 401;
      return { error: 'invalid_token' };
    }
    // proceed with update
    return { ok: true };
  }

  private isValidBearer(token: string, hostname: string): boolean {
    // decode and validate audience/issuer/host claims
    // implementation omitted for brevity
    return true;
  }
}

4. DNS and deployment hygiene

• Remove unused DNS records and avoid wildcard entries that route to services not intended for public traffic.
• Use environment-based hostname checks so staging and production have distinct allowed host lists.
• Rotate Bearer tokens regularly and scope tokens to specific audiences and hosts to reduce impact of leaks.

By combining strict hostname validation with Bearer token audience and issuer verification, Buffalo applications can avoid exposing authenticated endpoints via dangling DNS entries. These code examples demonstrate practical, deployable guards that align with secure token handling practices.