Heap Overflow in Fiber

Heap overflow vulnerabilities in Fiber applications typically occur when the framework's JSON unmarshaling processes or request body parsing mechanisms fail to properly validate input sizes. Fiber's default JSON decoder uses Go's standard library, which can be vulnerable to carefully crafted payloads that cause excessive memory allocation.

The most common attack vector involves sending extremely large JSON objects or arrays to Fiber endpoints. Consider this vulnerable pattern:

app.Post("/api/data", func(c *fiber.Ctx) error {
    var payload map[string]interface{}
    if err := c.BodyParser(&payload); err != nil {
        return err
    }
    // Process payload without size validation
    return c.JSON(payload)
})

An attacker can send a JSON payload with millions of key-value pairs or deeply nested structures. Fiber's BodyParser will attempt to decode the entire payload into memory, potentially exhausting available RAM and causing the application to crash or become unresponsive.

Another Fiber-specific scenario involves multipart form data processing. Fiber's multipart handling can be exploited when processing file uploads without proper size limits:

app.Post("/upload", func(c *fiber.Ctx) error {
    // No size validation before processing
    file, err := c.FormFile("file")
    if err != nil {
        return err
    }
    // Process file without checking actual size
    return c.JSON(fiber.Map{"status": "uploaded"})
})

Attackers can craft multipart requests with extremely large files or numerous small files that collectively exceed available memory. Fiber's default behavior doesn't enforce limits on the number of form files or total request size, making it susceptible to heap exhaustion attacks.

Path traversal combined with heap overflow can also manifest in Fiber applications. When processing user-supplied paths for file operations, attackers can trigger excessive memory allocation:

app.Get("/read/:path", func(c *fiber.Ctx) error {
    filePath := c.Params("path")
    // No validation of filePath length or content
    data, err := os.ReadFile("data/" + filePath)
    if err != nil {
        return c.Status(500).SendString("Error reading file")
    }
    return c.Send(data)
})

By supplying extremely long path strings or paths that trigger recursive directory traversal, attackers can cause the application to allocate excessive memory when resolving file paths or reading file contents.

Detecting heap overflow vulnerabilities in Fiber applications requires both static code analysis and dynamic runtime testing. middleBrick's API security scanner specifically targets these vulnerabilities through its comprehensive scanning methodology.

During a middleBrick scan, the tool automatically tests for heap overflow by sending progressively larger payloads to your Fiber endpoints. The scanner monitors memory usage patterns and response times to identify potential vulnerabilities. For JSON endpoints, middleBrick sends payloads with increasing object sizes and nesting depths to trigger potential overflow conditions.

The scanner's Input Validation check specifically looks for Fiber applications that don't implement proper request size limits. It examines your Fiber configuration for missing Content-Type length restrictions, missing multipart form limits, and absent JSON payload size constraints.

middleBrick also performs runtime analysis by monitoring the application's behavior under stress. The scanner tracks metrics like:

• Response time degradation with increasing payload sizes
• Memory allocation patterns during request processing
• Application stability under sustained high-load conditions
• Error handling behavior when memory limits are exceeded

For Fiber applications using middleware, middleBrick analyzes the middleware chain to identify potential amplification points where heap overflow could be triggered through multiple processing stages.

The scanner's Data Exposure check also helps identify heap overflow risks by examining how your Fiber application handles and stores request data. Applications that buffer entire request bodies in memory without streaming are flagged as higher risk.

To manually test for heap overflow in your Fiber application, you can use tools like curl or Postman to send progressively larger requests:

# Test JSON endpoint with large payload
curl -X POST http://localhost:3000/api/data \
  -H "Content-Type: application/json" \
  -d '{"key": "value"}'

Monitor your application's memory usage with tools like htop or Go's profiling capabilities while increasing the payload size. Watch for memory consumption patterns that don't scale linearly with input size, which could indicate inefficient memory handling.

Remediating heap overflow vulnerabilities in Fiber applications requires implementing proper input validation and size limits throughout your request processing pipeline. The most effective approach is to configure Fiber's built-in limits and validate all incoming data.

First, configure Fiber's global limits to prevent excessively large requests:

app := fiber.New(fiber.Config{
    // Limit total request body size to 10MB
    BodyLimit: 10 * 1024 * 1024,
    // Limit multipart form data to 5MB
    FormLimit: 5 * 1024 * 1024,
    // Limit number of multipart files to 10
    MultipartFormLimit: 10,
})

These limits apply globally to all endpoints, providing a baseline defense against heap overflow attacks. For endpoints requiring larger payloads, implement specific validation logic.

For JSON processing, use Fiber's streaming capabilities instead of loading entire payloads into memory:

app.Post("/api/data", func(c *fiber.Ctx) error {
    // Stream JSON processing instead of loading entire body
    decoder := json.NewDecoder(c.Body)
    
    // Process JSON incrementally
    var result map[string]interface{}
    if err := decoder.Decode(&result); err != nil {
        return c.Status(400).JSON(fiber.Map{"error": err.Error()})
    }
    
    // Validate payload size and structure
    if len(result) > 1000 { // Arbitrary limit
        return c.Status(413).JSON(fiber.Map{"error": "Payload too large"})
    }
    
    return c.JSON(result)
})

For file uploads, implement streaming file processing and size validation:

app.Post("/upload", func(c *fiber.Ctx) error {
    // Create a temporary file for streaming
    file, err := os.CreateTemp("", "upload_")
    if err != nil {
        return err
    }
    defer os.Remove(file.Name())
    defer file.Close()
    
    // Stream file upload with size limit
    form, err := c.MultipartForm()
    if err != nil {
        return err
    }
    
    files := form.File["file"]
    for _, f := range files {
        if f.Size > 5*1024*1024 { // 5MB limit
            return c.Status(413).JSON(fiber.Map{"error": "File too large"})
        }
        
        // Stream file content instead of loading into memory
        src, err := f.Open()
        if err != nil {
            return err
        }
        defer src.Close()
        
        if _, err := io.Copy(file, src); err != nil {
            return err
        }
    }
    
    return c.JSON(fiber.Map{"status": "uploaded"})
})

For path-based endpoints, validate and sanitize all user-supplied paths:

app.Get("/read/:path", func(c *fiber.Ctx) error {
    filePath := c.Params("path")
    
    // Validate path length
    if len(filePath) > 256 {
        return c.Status(400).JSON(fiber.Map{"error": "Path too long"})
    }
    
    // Sanitize path to prevent traversal
    if strings.Contains(filePath, "..") {
        return c.Status(400).JSON(fiber.Map{"error": "Invalid path"})
    }
    
    // Use safe path joining
    fullPath := filepath.Join("data", filePath)
    
    data, err := os.ReadFile(fullPath)
    if err != nil {
        return c.Status(500).JSON(fiber.Map{"error": "File not found"})
    }
    
    return c.Send(data)
})

Implement rate limiting to prevent sustained heap overflow attacks:

// Apply rate limiting to prevent sustained attacks
store := memory.New()
rateLimit := middleware.NewRateLimit(Config{
    Store: store,
    Limit: 100, // 100 requests
    TTL:   60 * time.Second, // per minute
})

app.Use(rateLimit)

These remediation strategies, combined with middleBrick's continuous scanning, provide comprehensive protection against heap overflow vulnerabilities in Fiber applications.