CWE-209 in Chi

CWE-209 (Generation of Error Message Containing Sensitive Information) in Chi manifests through several framework-specific patterns that expose internal system details to attackers. The most common scenario occurs when Chi's error handling middleware reveals stack traces, database connection strings, or internal API endpoints in HTTP responses.

Chi's context-based middleware chain creates unique exposure points. When a request handler panics or returns an error, the default recovery middleware may include the full error message in the response body. This becomes problematic when the error originates from database operations, external service calls, or authentication failures that contain sensitive configuration details.

Consider this vulnerable pattern in Chi applications:

func sensitiveHandler(w http.ResponseWriter, r *http.Request) {
    // Database connection string exposed in error
    db, err := sql.Open("postgres", os.Getenv("DATABASE_URL"))
    if err != nil {
        http.Error(w, err.Error(), http.StatusInternalServerError) // CWE-209: DATABASE_URL exposed
        return
    }
    defer db.Close()
    
    // Internal service URLs exposed
    svc := externalService{endpoint: os.Getenv("SERVICE_URL")}
    if err := svc.call(); err != nil {
        http.Error(w, fmt.Sprintf("Service call failed: %v", err), http.StatusInternalServerError) // CWE-209: SERVICE_URL exposed
        return
    }
    
    w.Write([]byte("OK"))
}

Chi's middleware composition model also creates exposure through chained error handling. When multiple middleware layers wrap a handler, an error in an inner layer can bubble up through outer layers, each potentially adding sensitive context to the error message. This is particularly dangerous in authentication middleware that might expose user enumeration or role information.

Another Chi-specific manifestation occurs with route parameter validation. When using Chi's chi.URLParam or chi.RouteContext, malformed input that causes parsing errors can leak internal routing structure or parameter validation logic in error messages.

RESTful API endpoints in Chi applications often suffer from verbose error responses that include stack traces when running in development mode. This is especially problematic when the same error handling code is deployed to production without proper sanitization.

Chi's support for route groups and sub-routers can compound the issue. When an error occurs in a nested router, the error message might include the full route path and handler chain, revealing the application's internal API structure to attackers.

Detecting CWE-209 in Chi applications requires a combination of static analysis and runtime scanning. The most effective approach is using middleBrick's API security scanner, which specifically tests for sensitive information disclosure in error responses.

middleBrick's scanner tests Chi applications by sending malformed requests to trigger error conditions, then analyzes the responses for sensitive content. The scanner looks for patterns like database connection strings, internal IP addresses, file paths, and stack traces in HTTP response bodies.

Here's how to scan a Chi API with middleBrick:

# Install middleBrick CLI
npm install -g middlebrick

# Scan your Chi API endpoint
middlebrick scan https://api.yourapp.com/v1/users

# For local development
middlebrick scan http://localhost:8080 --verbose

The scanner performs 12 parallel security checks, including authentication bypass attempts and error message analysis. For CWE-209 specifically, it looks for:

• Stack traces containing file paths and line numbers
• Database connection strings and credentials
• Internal service URLs and API keys
• Environment variable names in error messages
• Framework version information
• Code snippets or function signatures

middleBrick provides a security score (0-100) with letter grades and per-category breakdowns. For CWE-209, it shows the severity level and provides specific remediation guidance.

Additional detection methods include:

# Static analysis with golangci-lint
golangci-lint run --enable-all

# Custom middleware to detect sensitive error responses
func sensitiveErrorDetector(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        // Wrap response writer to capture output
        buf := &bytes.Buffer{}
        rw := middleware.NewResponseWriter(w, buf)
        
        next.ServeHTTP(rw, r)
        
        // Check response for sensitive patterns
        if strings.Contains(buf.String(), "password") ||
           strings.Contains(buf.String(), "DATABASE_URL") ||
           strings.Contains(buf.String(), "stack trace") {
            log.Printf("Potential CWE-209: %s", buf.String())
        }
    })
}

For comprehensive coverage, integrate middleBrick into your CI/CD pipeline:

# GitHub Action for API security scanning
- name: Scan API Security
  uses: middlebrick/middlebrick-action@v1
  with:
    api_url: https://api.staging.yourapp.com
    fail_below_score: 80
    scan_frequency: daily

Remediating CWE-209 in Chi applications requires implementing proper error handling that separates internal error details from user-facing messages. The key is using Chi's middleware system to create a centralized error handling layer.

Here's a production-ready error handling middleware for Chi:

package middleware

import (
    "net/http"
    "github.com/go-chi/chi/v5/middleware"
)

type errorHandler struct {
    logger Logger
    debug  bool
}

func NewErrorHandler(logger Logger, debug bool) func(next http.Handler) http.Handler {
    return func(next http.Handler) http.Handler {
        return &errorHandler{logger: logger, debug: debug}
    }
}

func (e *errorHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
    // Create a recovery middleware that catches panics
    defer func() {
        if err := recover(); err != nil {
            e.logger.Errorf("Panic recovered: %v", err)
            
            // Only show detailed error in debug mode
            if e.debug {
                http.Error(w, fmt.Sprintf("Internal Server Error: %v", err), http.StatusInternalServerError)
            } else {
                http.Error(w, "Internal Server Error", http.StatusInternalServerError)
                // Log the actual error for debugging
                e.logger.Errorf("Panic details: %v", err)
            }
        }
    }()
    
    // Use Chi's default recovery middleware
    recovery := middleware.Recoverer
    recovery.ServeHTTP(w, r)
}

// Custom error response writer that sanitizes output
type sanitizedResponseWriter struct {
    http.ResponseWriter
    statusCode int
}

func (s *sanitizedResponseWriter) WriteHeader(code int) {
    s.statusCode = code
    s.ResponseWriter.WriteHeader(code)
}

// Error type with safe message
type safeError struct {
    Message string `json:"message"`
    Code    int    `json:"code"`
}

// Helper function to return safe errors
func HandleSafeError(w http.ResponseWriter, err error, code int, debug bool) {
    safeMsg := "An error occurred"
    
    if debug {
        safeMsg = err.Error()
    }
    
    http.Error(w, safeMsg, code)
    
    // Log the actual error
    log.Printf("Error: %v (code: %d)", err, code)
}

// Example usage in a Chi route
func sensitiveHandler(w http.ResponseWriter, r *http.Request) {
    db, err := sql.Open("postgres", os.Getenv("DATABASE_URL"))
    if err != nil {
        HandleSafeError(w, err, http.StatusInternalServerError, isDebug())
        return
    }
    defer db.Close()
    
    // Process request...
    w.Write([]byte("OK"))
}

func isDebug() bool {
    return os.Getenv("DEBUG") == "true"
}

// Production error handler with structured logging
func ProductionErrorHandler(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        // Wrap response writer to capture status code
        srw := &sanitizedResponseWriter{ResponseWriter: w}
        
        next.ServeHTTP(srw, r)
        
        // Check if error occurred and log appropriately
        if srw.statusCode >= 400 {
            // Only log detailed error in development
            if isDebug() {
                log.Printf("Request failed: %d %s", srw.statusCode, r.URL.Path)
            } else {
                // In production, log minimal details
                log.Printf("Request failed: %d %s", srw.statusCode, r.URL.Path)
            }
        }
    })
}

// Register middleware in Chi application
func main() {
    r := chi.NewRouter()
    
    // Add error handling middleware
    r.Use(ProductionErrorHandler)
    r.Use(NewErrorHandler(log.NewLogger(), isDebug()))
    
    // Add routes
    r.Get("/api/users", sensitiveHandler)
    
    http.ListenAndServe(":8080", r)
}

For database operations, use parameterized queries and catch specific database errors:

func getUser(w http.ResponseWriter, r *http.Request) {
    id := chi.URLParam(r, "id")
    
    // Use parameterized query to prevent SQL injection
    row := db.QueryRow("SELECT name, email FROM users WHERE id = $1", id)
    
    var user User
    err := row.Scan(&user.Name, &user.Email)
    if err != nil {
        if err == sql.ErrNoRows {
            http.Error(w, "User not found", http.StatusNotFound)
        } else {
            // Generic error message for production
            http.Error(w, "Database error", http.StatusInternalServerError)
            // Log actual error
            log.Printf("Database error: %v", err)
        }
        return
    }
    
    // Return user data
    json.NewEncoder(w).Encode(user)
}

// Custom error type for API responses
type apiError struct {
    Error string `json:"error"`
    Code  int    `json:"code"`
}

func writeAPIError(w http.ResponseWriter, err error, code int, debug bool) {
    response := apiError{
        Error: "An error occurred",
        Code:  code,
    }
    
    if debug {
        response.Error = err.Error()
    }
    
    w.Header().Set("Content-Type", "application/json")
    w.WriteHeader(code)
    json.NewEncoder(w).Encode(response)
    
    // Always log the actual error
    log.Printf("API Error: %v (code: %d)", err, code)
}

// Example with proper error handling
func createOrder(w http.ResponseWriter, r *http.Request) {
    var order Order
    err := json.NewDecoder(r.Body).Decode(&order)
    if err != nil {
        writeAPIError(w, err, http.StatusBadRequest, isDebug())
        return
    }
    
    // Process order...
    if err := processOrder(order); err != nil {
        writeAPIError(w, err, http.StatusInternalServerError, isDebug())
        return
    }
    
    w.WriteHeader(http.StatusCreated)
}

// Centralized error handling middleware
func ErrorHandlerMiddleware(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        defer func() {
            if err := recover(); err != nil {
                // Log panic details
                log.Printf("Panic: %v\nStack: %s", err, string(debug.Stack()))
                
                // Return safe error response
                http.Error(w, "Internal Server Error", http.StatusInternalServerError)
            }
        }()
        
        next.ServeHTTP(w, r)
    })
}

// Register centralized error handler
func setupRouter() *chi.Mux {
    r := chi.NewRouter()
    
    // Add error handling middleware
    r.Use(ErrorHandlerMiddleware)
    r.Use(middleware.Logger)
    r.Use(middleware.Recoverer)
    
    // Add routes
    r.Post("/api/orders", createOrder)
    r.Get("/api/users/{id}", getUser)
    
    return r
}

// Production-ready main function
func main() {
    r := setupRouter()
    
    // Use middleBrick to scan before deployment
    if os.Getenv("SCAN_BEFORE_DEPLOY") == "true" {
        // This would be a separate scanning step in CI/CD
        // middleBrick scan http://localhost:8080
    }
    
    log.Fatal(http.ListenAndServe(":8080", r))
}