Stack Overflow in Chi with Api Keys

In Chi, a common pattern for securing HTTP endpoints is to use API keys passed via headers. When API keys are handled naively, the framework can inadvertently expose sensitive data in error responses or logs, especially during stack traces that reveal internal paths or variable values. This becomes a Stack Overflow risk when error details are surfaced to clients and later indexed or shared publicly.

Chi routes are often defined with nested or overlapping path patterns. If route matching logic does not enforce strict parameter constraints, an attacker may trigger ambiguous matches that lead to unhandled exceptions. These exceptions may include the API key in serialized form, either in console logs or in verbose error responses returned to the caller. Such exposure violates the principle of least privilege and can aid further reconnaissance.

Additionally, if middleware responsible for API key validation is applied inconsistently across routes, some endpoints may remain unauthenticated. An attacker can probe these gaps and cause the application to behave differently under load or malformed input, increasing the likelihood of information leakage through stack traces. The combination of Chi’s routing flexibility and improper error handling around API keys can unintentionally provide clues useful for exploitation.

middleBrick detects unauthenticated endpoints and input validation issues that may amplify this risk. Its LLM/AI Security checks specifically test for system prompt leakage and output exposure, which can occur if error messages inadvertently reveal sensitive context such as API key handling logic. By scanning the unauthenticated attack surface, middleBrick highlights misconfigurations before they become public.

Remediation guidance includes ensuring consistent middleware application, avoiding inclusion of API keys in error messages, and validating route parameters rigorously. The framework’s built-in mechanisms should be used to standardize error responses, preventing stack traces from reaching external consumers.

Secure handling of API keys in Chi requires explicit validation and consistent middleware usage. Below are concrete code examples demonstrating safe practices.

1. Validate API key presence and format early

Use a middleware that checks for the API key and rejects malformed requests before routing.

open import Http
open import Web.Server

isValidKey : String -> Bool
isValidKey key = String.length key == 32 && String.all isHexDigit key

apiKeyMiddleware : Middleware
apiKeyMiddleware req respond =
  case Request.header "x-api-key" req of
    Nothing -> respond (Response.badRequest "Missing x-api-key header")
    Just key -> if isValidKey key
                then proceed req respond
                else respond (Response.forbidden "Invalid API key")

app : Application
app = httpApp do
  middleware apiKeyMiddleware
  -- routes follow

2. Avoid exposing API keys in error responses

Ensure error serialization does not include sensitive headers. Use a custom error handler that strips sensitive data.

safeErrorHandler : Error -> Response
safeErrorHandler err =
  let status = Error.status err
      body   = Response.string (Error.message err)
  in Response.respond status body

apiWithErrorHandling : Application
apiWithErrorHandling = httpApp do
  middleware apiKeyMiddleware
  catchError safeErrorHandler
  -- routes

3. Apply middleware consistently across all routes

Define a router that enforces API key checks globally rather than per-route to prevent accidental gaps.

securedRouter : Router (Middleware -> Router (Response Unit))
securedRouter = do
  route "/users/*" $ \next -> next apiKeyMiddleware userRoutes
  route "/admin/*" $ \next -> next apiKeyMiddleware adminRoutes
  notFound404

mainApp : Application
mainApp = httpApp $ securedRouter id

4. Use environment variables for key configuration

Never hardcode keys. Load them at startup and validate presence.

main : IO ()
main = do
  maybeKey <- System.Env.lookup "API_KEY"
  case maybeKey of
    Nothing -> putStrLn "API_KEY environment variable missing" *> exitFailure
    Just key -> do
      let validated = isValidKey key
      if not validated then putStrLn "Invalid API key format" *> exitFailure
      else run 8080 mainApp