Format String in Fiber

Format string vulnerabilities in Fiber applications typically emerge through improper handling of user-controlled data in logging, error messages, and response generation. Unlike traditional web frameworks, Fiber's high-performance design and Go-based architecture create specific patterns where format string issues commonly occur.

The most prevalent manifestation appears in logging operations. Developers often use fmt.Sprintf or fmt.Printf with user-supplied data without proper sanitization:

func handleRequest(c *fiber.Ctx) error {
    userID := c.Query("user_id")
    log.Printf("User %s accessed resource", userID) // Vulnerable
    return c.SendString("OK")
}

When an attacker supplies a value like %x%x%x, they can extract memory contents through the logging mechanism. Fiber's default logging middleware, which uses Go's standard log package, doesn't sanitize format specifiers, making this particularly dangerous.

Another Fiber-specific pattern involves error message construction. The framework's error handling often uses fmt.Errorf with dynamic data:

func getUser(c *fiber.Ctx) error {
    id := c.Params("id")
    user, err := db.GetUser(id)
    if err != nil {
        return fmt.Errorf("Error fetching user %s: %v", id, err) // Vulnerable
    }
    return c.JSON(user)
}

Database query construction in Fiber applications also presents format string risks. While Fiber itself doesn't directly handle database operations, developers frequently combine it with raw SQL queries:

func searchUsers(c *fiber.Ctx) error {
    search := c.Query("search")
    query := fmt.Sprintf("SELECT * FROM users WHERE name LIKE '%%%s%%'", search) // Vulnerable
    rows, _ := db.Query(query)
    // ...
}

Format string vulnerabilities in Fiber can also appear in template rendering. The framework's template engine, when used with custom format specifiers, can expose sensitive data:

func renderTemplate(c *fiber.Ctx) error {
    data := map[string]string{"name": c.Query("name")}
    return c.Render("template", data) // If template uses unsafe formatting
}

Detecting format string vulnerabilities in Fiber applications requires both static analysis and dynamic testing approaches. Static analysis tools can identify risky patterns, but they need Fiber-specific context to avoid false positives.

Code review should focus on these Fiber-specific patterns:

// Dangerous patterns to flag:
log.Printf("User %s accessed %s", c.Query("user"), c.Query("resource"))
fmt.Errorf("Operation failed for %s: %v", c.Params("id"), err)
fprintf(writer, c.Query("format"), data)

Dynamic testing with middleBrick specifically targets format string vulnerabilities in Fiber applications. The scanner's black-box approach tests unauthenticated endpoints by injecting format specifiers into various parameters:

middlebrick scan https://api.example.com/user/profile

middleBrick's format string detection includes these specific tests:

• Basic format specifier injection (%s, %x, %p, %n)
• Sequential format string attacks (%08x.%08x.%08x)
• Format string with length modifiers (%10s, %-10s)
• Pointer format specifiers (%p, %x)

The scanner examines both HTTP responses and server logs (if accessible) to detect format string exploitation. For Fiber applications, middleBrick specifically tests:

// middleBrick tests these patterns:
// GET /api/user?id=%s%s%s%s
// GET /api/data?format=%n
// POST /api/login with JSON: {"user": "%x%x%x"}

middleBrick's LLM security module also detects format string patterns in AI-related endpoints, which is particularly relevant for Fiber applications using language models:

// Format string in AI prompts:
func generateText(c *fiber.Ctx) error {
    prompt := c.Query("prompt")
    result := model.Generate(fmt.Sprintf("You are %s", prompt)) // Vulnerable
    return c.JSON(result)
}

Remediating format string vulnerabilities in Fiber applications requires a multi-layered approach. The primary defense is eliminating direct format string usage with user-controlled data.

For logging operations, replace vulnerable patterns with safe alternatives:

// Vulnerable
log.Printf("User %s accessed %s", userID, resourceID)

// Secure
log.Printf("User %s accessed %s", sanitize(userID), sanitize(resourceID))

// Or use structured logging
zap.L().Info("user_access",
    zap.String("user_id", userID),
    zap.String("resource", resourceID))

For error message construction, use explicit string concatenation or structured error types:

// Vulnerable
return fmt.Errorf("Error fetching user %s: %v", id, err)

// Secure
return fmt.Errorf("Error fetching user %s: %w", id, err) // %w for errors

// Or use custom error types
return &UserError{ID: id, Err: err}

Database operations require parameterized queries instead of string formatting:

// Vulnerable
query := fmt.Sprintf("SELECT * FROM users WHERE name LIKE '%%%s%%'", search)

// Secure
query := "SELECT * FROM users WHERE name LIKE ?"
rows, err := db.Query(query, "%"+search+"%")

Template rendering should use safe escaping mechanisms:

// Use Fiber's template engine with auto-escaping
func renderSafe(c *fiber.Ctx) error {
    data := map[string]interface{}{
        "name": html.EscapeString(c.Query("name"))
    }
    return c.Render("template", data)
}

// Or use Go's html/template package
import "html/template"

t := template.Must(template.New("t").Parse(templateString))
t.Execute(writer, data)

For AI/ML endpoints, validate and sanitize all prompt inputs:

func generateText(c *fiber.Ctx) error {
    prompt := c.Query("prompt")
    // Remove format specifiers
    cleanPrompt := strings.NewReplacer("%", "").Replace(prompt)
    result := model.Generate("You are " + cleanPrompt)
    return c.JSON(result)
}

// Or use a whitelist approach
func validatePrompt(prompt string) bool {
    allowed := regexp.MustCompile(`^[a-zA-Z0-9\s.,!?]+$`)
    return allowed.MatchString(prompt)
}

middleBrick's continuous monitoring in the Pro plan can help verify these remediations by regularly scanning your APIs and alerting if format string vulnerabilities reappear after code changes.