Dangling Dns in Chi with Mutual Tls

A dangling DNS configuration in a Chi-based service becomes particularly risky when mutual TLS (mTLS) is enforced. mTLS requires both the client and server to present valid certificates, which typically reduces reliance on DNS for initial trust decisions. However, if a hostname used during certificate validation does not resolve correctly at runtime, the service may inadvertently route or accept connections intended for other endpoints, creating a dangling DNS situation.

In Chi, this can occur when route definitions or middleware rely on hostnames that are not consistently reflected in certificate identity fields (Common Name or Subject Alternative Names). For example, a route defined as GET "/secure" http://internal.service.local/func may pass mTLS checks if the client certificate validates against a server certificate issued for service.local, but if internal.service.local resolves to an unintended IP due to stale or missing DNS records, traffic may be directed to a different service. This misalignment between DNS resolution and certificate identity exposes the attack surface, as an attacker who can influence DNS (e.g., via cache poisoning or compromised upstream resolvers) might redirect mTLS-protected requests.

Additionally, mTLS implementations in Chi that perform hostname verification against DNS names can be bypassed or misrouted when DNS records are inconsistent across environments (e.g., development, staging, production). If a certificate lists api.example.com but the Chi route uses staging-api.example.local, the mismatch can lead to insecure fallback behavior or logging of sensitive data to incorrect endpoints. This is a dangling DNS issue because the name used in application logic does not reliably map to the intended infrastructure endpoint, even though mTLS is in place.

The interaction with LLM/AI Security checks in middleBrick is relevant here: unauthenticated LLM endpoints might expose configuration details that hint at internal hostnames, which could be combined with dangling DNS to infer routing paths. middleBrick’s active prompt injection probes and system prompt leakage detection can identify whether hostname information is inadvertently exposed in AI-assisted development workflows, which might exacerbate misconfigurations in Chi services using mTLS.

To detect such issues, middleBrick’s OpenAPI/Swagger spec analysis (resolving $ref across specs) can cross-reference defined hostnames in server variables with runtime findings. This helps identify mismatches between declared routes and actual DNS resolution behavior, even when mTLS is enforced.

Remediation focuses on ensuring DNS names used in Chi routes and middleware exactly match certificate identity fields, and that hostname verification is consistently applied. Below are concrete code examples for a secure Chi configuration with mTLS.

1. Enforce Hostname Verification in TLS Configuration

Always configure the HTTP server to verify the hostname from the certificate against the expected route hostname. In Chi, this is typically done at the server or middleware level.

open import Network.TLS
open import Data.ByteString (pack)

-- Define expected hostname from route or service identity
let expectedHost = "api.example.com"

-- Configure TLS parameters with hostname verification
tlsConfig = defaultClientSettings
  { clientShared = (\\sh -> sh
    { sharedCAStore = loadCAStore "path/to/ca.pem"
    , sharedValidation = \_ peerCert _ ->
        case verifyPeerCertificate peerCert of
          Left err -> throwError err
          Right certChain ->
            if verifyHostname expectedHost certChain
              then return certChain
              else throwError (HandshakeError "Hostname mismatch")
    })
  }

2. Align Chi Routes with Certificate Subject Alternative Names

Ensure that route prefixes in Chi correspond to SAN entries in your certificates. If your certificate includes api.example.com and *.example.com, define routes using the canonical hostname.

-- Chi route definition aligned with SAN
app :: Middleware
app = do
  get "/secure" $ \ctx -> do
    -- Only proceed if TLS peer hostname matches expected pattern
    peerHost <- liftIO $ getPeerHostname ctx  -- hypothetical helper
    if peerHost == "api.example.com"
      then text "Secure response"
      else status status400 >> text "Invalid hostname"

-- Helper to extract peer hostname (implementation depends on TLS library)
getPeerHostname :: Context ctx => ctx -> IO String
getPeerHostname = undefined  -- Use library-specific TLS session data

3. Use Consistent Service Discovery and DNS Resolution

Avoid hardcoding hostnames in route handlers. Instead, resolve service endpoints via a trusted DNS lookup at startup and validate against certificates.

-- Resolve service hostname once during initialization
serviceHost <- liftIO $ resolveServiceHost "service-discovery.example.com"
  where
    resolveServiceHost name = do
      addrs <- getAddrInfo Nothing (Just name) (Just "443")
      case addrs of
        (addr : _) -> return (addrAddress addr)
        [] -> fail "Service host not found"

-- Use resolved host in route guards
get "/data" $ \ctx -> do
  currentHost <- liftIO $ getPeerHostname ctx
  if currentHost == serviceHost
    then json ["data" := "safe"]
    else throwError err400

4. MiddleBrick Integration for Continuous Validation

Use middleBrick’s CLI to scan your Chi service URL and verify that DNS mappings align with certificate identities. The CLI provides JSON output that can be integrated into CI/CD pipelines to fail builds on misconfigurations.

# Scan the deployed Chi endpoint
middlebrick scan https://api.example.com/health

# In GitHub Action, fail if risk score exceeds threshold
- uses: middleBrick/github-action@v1
  with:
    url: https://api.example.com
    threshold: C

middleBrick’s OpenAPI/Swagger analysis further ensures that server variables in your spec resolve correctly, reducing dangling DNS risks in mTLS-enabled Chi services.