Header Injection in Buffalo

Header injection vulnerabilities in Buffalo applications typically arise when user-controlled data flows into HTTP response headers without proper validation or encoding. In Buffalo's Go-based framework, this often occurs through middleware, custom response writers, or when dynamically constructing header values from request parameters.

A common Buffalo-specific pattern involves using context.Set to store user input that later gets written to headers. For example:

func MyHandler(c buffalo.Context) error {
    userID := c.Param("id")
    context.Set(c, "userID", userID)
    
    // Later in middleware
    c.Response().Header().Set("X-User-ID", userID)
    return c.Render(200, r.JSON(map[string]string{"status": "ok"}))
}

This creates a header injection vector if userID contains newline characters. An attacker could send id=123%0D%0AX-Injected: malicious, causing the browser to interpret this as two separate headers.

Buffalo's middleware chain can also introduce header injection points. Consider a middleware that adds CORS headers based on request data:

func CORSHandler(next buffalo.Handler) buffalo.Handler {
    return func(c buffalo.Context) error {
        origin := c.Request().Header.Get("Origin")
        c.Response().Header().Set("Access-Control-Allow-Origin", origin)
        return next(c)
    }
}

If an attacker sends a request with Origin: http://evil.com%0D%0AX-Injected: value, the server will add both headers to the response.

Another Buffalo-specific scenario involves populator methods that bind request data to models, which are then used in headers:

func CreateWidget(c buffalo.Context) error {
    widget := &Widget{}
    if err := c.Bind(widget); err != nil {
        return err
    }
    
    c.Response().Header().Set("X-Widget-ID", widget.ID)
    return c.Render(200, r.JSON(widget))
}

If the model binding doesn't properly validate header values, injection becomes possible.

Detecting header injection in Buffalo applications requires both manual code review and automated scanning. For manual detection, search your codebase for patterns where request parameters or user-controlled data flow into header-setting functions.

Key functions to examine:

• c.Response().Header().Set()
• c.Response().Header().Add()
• context.Set() followed by header usage
• Any middleware that constructs headers from request data
• Populator methods that bind request data to structs used in headers

Automated detection with middleBrick specifically identifies header injection vulnerabilities in Buffalo applications. The scanner tests for:

Testing for header injection in: https://your-buffalo-app.com/api/widgets
✓ Authentication bypass: PASS
✓ BOLA/IDOR: PASS
⚠ Header Injection: FAIL
  - Vulnerability: Response splitting via X-Injected header
  - Severity: High
  - Location: /api/widgets/{id}
  - Remediation: Validate and sanitize header values

middleBrick's header injection test specifically sends payloads containing CRLF sequences (%0D%0A) and monitors for additional headers in the response. This black-box approach works regardless of whether you're using Buffalo's default middleware or custom implementations.

For comprehensive coverage, scan your Buffalo API endpoints with middleBrick's CLI:

middlebrick scan https://your-buffalo-app.com/api --output json

The scanner also checks for related issues like improper CORS configuration and missing security headers that often accompany header injection vulnerabilities.

Remediating header injection in Buffalo applications requires input validation and proper header construction. The most effective approach is to validate header values before setting them.

For simple cases, use a validation function:

func sanitizeHeader(value string) string {
    // Remove CRLF characters and other control characters
    return strings.ReplaceAll(strings.ReplaceAll(value, "\r", ""), "\n", "")
}

// Usage in handlers
userID := sanitizeHeader(c.Param("id"))
c.Response().Header().Set("X-User-ID", userID)

For more robust validation, use regular expressions to allow only expected characters:

var headerValueRegex = regexp.MustCompile(`^[a-zA-Z0-9_.-]+$`)

func validateHeader(value string) (string, error) {
    if !headerValueRegex.MatchString(value) {
        return "", errors.New("invalid header value")
    }
    return value, nil
}

// Usage in middleware
origin := c.Request().Header.Get("Origin")
if validOrigin, err := validateHeader(origin); err == nil {
    c.Response().Header().Set("Access-Control-Allow-Origin", validOrigin)
} else {
    c.Response().Header().Set("Access-Control-Allow-Origin", "https://yourdomain.com")
}

Buffalo's middleware system provides another layer of protection. Create a header injection prevention middleware:

func HeaderInjectionPrevention(next buffalo.Handler) buffalo.Handler {
    return func(c buffalo.Context) error {
        // Wrap response writer to monitor header setting
        original := c.Response()
        wrapped := &secureResponseWriter{ResponseWriter: original}
        c.SetResponse(wrapped)
        
        return next(c)
    }
}

type secureResponseWriter struct {
    http.ResponseWriter
    headersSet map[string]bool
}

func (s *secureResponseWriter) Header() http.Header {
    return s.ResponseWriter.Header()
}

func (s *secureResponseWriter) WriteHeader(statusCode int) {
    // Validate headers before writing
    for key := range s.Header() {
        if !isValidHeaderKey(key) {
            http.Error(s.ResponseWriter, "Invalid header", http.StatusInternalServerError)
            return
        }
    }
    s.ResponseWriter.WriteHeader(statusCode)
}

func isValidHeaderKey(key string) bool {
    // Allow only expected header keys
    allowed := map[string]bool{
        "Content-Type": true,
        "X-User-ID": true,
        // Add your allowed headers
    }
    return allowed[key]
}

For data coming from models or structs, validate at the binding level:

type Widget struct {
    ID string `json:"id" db:"id" validate:"alphanum,max=32"`
}

// In your handler
if err := validate.Struct(widget); err != nil {
    return c.Error(400, err)
}
c.Response().Header().Set("X-Widget-ID", widget.ID)