Sandbox Escape in Fiber

Sandbox escape vulnerabilities in Fiber applications occur when attackers bypass intended security boundaries to execute unauthorized operations. In Fiber's Go-based ecosystem, these escapes typically exploit the framework's middleware and context handling mechanisms.

The most common pattern involves manipulating Fiber's c.Locals() storage to inject malicious payloads that persist across middleware boundaries. Consider this vulnerable pattern:

app.Use(func(c *fiber.Ctx) error {
    c.Locals("user_id", c.Params("id"))
    return c.Next()
})

app.Get("/user/:id", func(c *fiber.Ctx) error {
    id := c.Locals("user_id").(string)
    // Direct DB access without validation
    return c.JSON(getUserFromDB(id))
})

An attacker can craft requests that manipulate the user_id local variable, potentially escaping the intended sandbox of user isolation. The vulnerability stems from Fiber's implicit trust in context propagation.

Another manifestation appears in Fiber's template rendering system. When using fiber.Render() with user-controlled data, attackers can inject template expressions that execute server-side logic:

// Vulnerable: user-controlled template path
func renderTemplate(c *fiber.Ctx) error {
    tmpl := c.Query("template", "default")
    data := getUserData(c.Locals("user_id").(string))
    return c.Render(tmpl, data)
}

Directory traversal combined with template injection can lead to complete sandbox escape, allowing execution of arbitrary Go code through template functions.

Detecting sandbox escape vulnerabilities in Fiber applications requires both static analysis and runtime scanning. middleBrick's API security scanner includes Fiber-specific checks that examine these patterns automatically.

The scanner analyzes Fiber middleware chains to identify improper context propagation. It flags patterns where c.Locals() values are set from user input without validation:

middlebrick scan https://api.example.com --profile fiber

This command runs Fiber-specific checks including:

• Context injection detection - identifies where user input populates c.Locals() without sanitization
• Template path traversal analysis - checks for directory traversal in template rendering
• Middleware boundary violations - detects when user data crosses security boundaries improperly

For runtime detection, middleBrick actively tests Fiber endpoints by manipulating context values and observing system behavior. The scanner attempts to inject payloads into c.Locals() storage and monitors for privilege escalation or data leakage.

Developers can also implement custom detection middleware in Fiber:

func sandboxGuard() fiber.Handler {
    return func(c *fiber.Ctx) error {
        // Check for suspicious context manipulation
        if val, exists := c.Locals("suspicious"); exists {
            return c.Status(fiber.StatusForbidden).JSON(fiber.Map{
                "error": "Sandbox boundary violation detected",
            })
        }
        return c.Next()
    }
}

app.Use(sandboxGuard())

Remediating sandbox escape vulnerabilities in Fiber requires a defense-in-depth approach using the framework's built-in security features. The primary mitigation is strict context validation using type assertions and input sanitization.

For c.Locals() usage, implement explicit validation:

app.Use(func(c *fiber.Ctx) error {
    userID := c.Params("id")
    if !isValidUUID(userID) {
        return c.Status(fiber.StatusBadRequest).JSON(fiber.Map{
            "error": "Invalid user identifier",
        })
    }
    c.Locals("user_id", userID)
    return c.Next()
})

func isValidUUID(id string) bool {
    // Strict UUID validation prevents injection
    r := regexp.MustCompile(`^[a-fA-F0-9]{8}-[a-fA-F0-9]{4}-4[a-fA-F0-9]{3}-[89abAB][a-fA-F0-9]{3}-[a-fA-F0-9]{12}$`)
    return r.MatchString(id)
}

For template rendering, use Fiber's built-in security features:

func secureRender(c *fiber.Ctx, template string, data interface{}) error {
    // Whitelist allowed templates
    allowedTemplates := map[string]bool{
        "profile": true,
        "dashboard": true,
        "settings": true,
    }
    
    if !allowedTemplates[template] {
        return c.Status(fiber.StatusNotFound).JSON(fiber.Map{
            "error": "Template not found",
        })
    }
    
    // Sanitize data before rendering
    sanitizedData := sanitizeUserData(data)
    return c.Render(template, sanitizedData)
}

Implement context isolation using Fiber's middleware chain:

// Context isolation middleware
func isolateContext() fiber.Handler {
    return func(c *fiber.Ctx) error {
        // Create a copy of context to prevent cross-request contamination
        ctxCopy := c.Copy()
        
        // Clear sensitive locals
        ctxCopy.Locals("session_token", nil)
        ctxCopy.Locals("internal_flags", nil)
        
        return c.Next()
    }
}

app.Use(isolateContext())

For comprehensive protection, integrate middleBrick's continuous monitoring to automatically scan your Fiber APIs for emerging sandbox escape patterns as your codebase evolves.