Crlf Injection in Buffalo

CRLF injection vulnerabilities in Buffalo applications typically occur when user input is incorporated into HTTP headers or other protocol elements without proper sanitization. Buffalo's middleware stack and response handling create specific attack vectors that developers must understand.

The most common manifestation appears in the buffalo.Context response handling. When setting headers or cookies based on user input, developers might inadvertently allow carriage return (CR) and line feed (LF) characters to break out of the intended header structure. Consider this vulnerable pattern:

func setCustomHeader(c buffalo.Context) error {
    headerName := c.Param("name")
    headerValue := c.Param("value")
    
    // VULNERABLE: No validation of headerName or headerValue
    c.Response().Header().Set(headerName, headerValue)
    return c.Render(200, r.String("Header set"))
}

An attacker could craft requests like:

curl -X POST http://localhost:3000/set-header \
  -d 'name=Location%0D%0AX-XSS-Protection%3A%200' \
  -d 'value=http://evil.com'

This would create a response with injected headers, potentially leading to HTTP response splitting attacks where the attacker controls the response body content.

Buffalo's cookie handling presents another vector. The cookie.Set function accepts user input for cookie names and values:

func setCustomCookie(c buffalo.Context) error {
    cookieName := c.Param("name")
    cookieValue := c.Param("value")
    
    // VULNERABLE: No sanitization
    c.Response().Header().Add("Set-Cookie", fmt.Sprintf("%s=%s", cookieName, cookieValue))
    return c.Render(200, r.String("Cookie set"))
}

CRLF injection here could allow setting multiple cookies or injecting arbitrary Set-Cookie headers, potentially hijacking sessions or manipulating client behavior.

Buffalo's middleware chain can also be exploited. Custom middleware that logs or processes request headers without validation creates injection opportunities:

func vulnerableMiddleware(next buffalo.Handler) buffalo.Handler {
    return func(c buffalo.Context) error {
        // VULNERABLE: Directly using request headers
        customHeader := c.Request().Header.Get("X-Custom-Header")
        log.Printf("Custom header: %s\n", customHeader)
        
        return next(c)
    }
}

If the logging output is consumed by another system that processes HTTP headers, this could lead to second-order CRLF injection attacks.

Detecting CRLF injection in Buffalo applications requires both static analysis and dynamic testing. middleBrick's scanning approach is particularly effective for Buffalo APIs because it understands the framework's conventions and common vulnerability patterns.

middleBrick automatically tests for CRLF injection by sending payloads containing %0D%0A (URL-encoded CRLF) in various HTTP header contexts. For Buffalo applications, it specifically targets:

• Custom header endpoints that accept header names/values from user input
• Cookie-setting endpoints with dynamic cookie names
• Middleware that processes or reflects request headers
• API endpoints that construct Location headers or redirects
• Endpoints that generate CSV or text content with user-controlled line breaks

The scanner sends multiple test payloads and analyzes responses for signs of successful injection, such as:

# Test payload for Location header injection
Location%3A%20http%3A%2F%2Fevil.com%0D%0AContent-Type%3A%20text%2Fhtml%0D%0A%0D%0A%3Cscript%3Ealert%281%29%3C%2Fscript%3E

middleBrick's analysis includes checking whether the response contains unexpected headers or content that would indicate successful CRLF injection. The scanner also examines Buffalo's default middleware stack to identify potential injection points.

For local development, you can use curl to test specific endpoints:

# Test header injection
curl -v -X POST http://localhost:3000/api/set-header \
  -H "Content-Type: application/x-www-form-urlencoded" \
  -d 'name=Location%0D%0AContent-Type%3A%20text%2Fhtml%0D%0A%0D%0A%3Chtml%3E%3Cbody%3E%3Ch1%3EInjected%3C%2Fh1%3E%3C%2Fbody%3E%3C%2Fhtml%3E'

Look for injected content in the response body or unexpected headers in the response. middleBrick automates this testing across all endpoints and provides a security score with specific findings for each vulnerability category.

Remediating CRLF injection in Buffalo applications requires a combination of input validation, output encoding, and framework-specific best practices. Here are Buffalo-specific approaches to secure your code:

First, implement strict validation of header names and values. Buffalo applications should use a whitelist approach:

var allowedHeaders = map[string]bool{
    "content-type": true,
    "location": true,
    "x-custom-header": true,
    // Add your allowed headers
}

Then create a secure header-setting function:

func setSecureHeader(c buffalo.Context, name, value string) error {
    name = strings.ToLower(strings.TrimSpace(name))
    
    // Validate header name against whitelist
    if !allowedHeaders[name] {
        return c.Error(400, errors.New("invalid header name"))
    }
    
    // Validate header value - no CRLF characters
    if strings.ContainsAny(value, "\r\n") {
        return c.Error(400, errors.New("invalid header value"))
    }
    
    c.Response().Header().Set(name, value)
    return nil
}

For cookie handling, use Buffalo's built-in cookie utilities which provide some protection, but still validate inputs:

func setSecureCookie(c buffalo.Context, name, value string) error {
    // Validate cookie name and value
    if strings.ContainsAny(name, "\r\n=; ") || strings.ContainsAny(value, "\r\n") {
        return c.Error(400, errors.New("invalid cookie format"))
    }
    
    expiration := time.Now().Add(24 * time.Hour)
    cookie := http.Cookie{
        Name:     name,
        Value:    value,
        Expires:  expiration,
        Path:     "/",
        Secure:   true,
        HttpOnly: true,
    }
    
    http.SetCookie(c.Response(), &cookie)
    return nil
}

Buffalo's middleware system allows for centralized CRLF protection. Create a middleware that sanitizes all headers:

func crlfProtectionMiddleware(next buffalo.Handler) buffalo.Handler {
    return func(c buffalo.Context) error {
        // Sanitize request headers
        for name := range c.Request().Header {
            if strings.ContainsAny(name, "\r\n") {
                return c.Error(400, errors.New("invalid header format"))
            }
        }
        
        // Sanitize request body for form data
        if c.Request().Body != nil {
            // Read and validate body content
            body, err := io.ReadAll(c.Request().Body)
            if err != nil {
                return err
            }
            
            // Check for CRLF in body if it's form data
            if strings.ContainsAny(string(body), "\r\n") {
                return c.Error(400, errors.New("invalid body format"))
            }
            
            // Reset body for downstream handlers
            c.Request().Body = io.NopCloser(bytes.NewBuffer(body))
        }
        
        return next(c)
    }
}

Register this middleware globally in your app.go:

app := buffalo.New(buffalo.Options{
    Env: env,
})

// Apply CRLF protection to all routes
app.Use(crlfProtectionMiddleware)

For API responses, ensure you're not inadvertently injecting CRLF through user-controlled content. When generating CSV or text responses:

func generateCSV(c buffalo.Context) error {
    data := c.Param("data")
    
    // Sanitize data - escape or remove CRLF
    sanitized := strings.ReplaceAll(data, "\r", "")
    sanitized = strings.ReplaceAll(sanitized, "\n", " ")
    
    csvContent := fmt.Sprintf("Name,Value\n%s,%s", sanitized, sanitized)
    
    c.Response().Header().Set("Content-Type", "text/csv")
    return c.Render(200, r.String(csvContent))
}

middleBrick's continuous monitoring in the Pro plan can help verify that these remediations remain effective as your codebase evolves. The scanner will automatically retest all endpoints on your configured schedule and alert you if new CRLF vulnerabilities are introduced.