Crlf Injection in Buffalo (Go)

Crlf Injection occurs when an attacker can inject CRLF characters (\r\n) into HTTP headers or other protocol-sensitive surfaces, causing header injection, response splitting, or cache poisoning. In the Buffalo web framework for Go, this risk arises when user-controlled data is written into headers, redirects, or cookies without proper sanitization. Because Buffalo is built on Go’s net/http layer, any header value that includes raw line breaks can break header structure and allow an attacker to inject additional headers or a second HTTP response.

For example, if a handler takes a query parameter and places it into a Location header for a redirect without validation, an attacker can supply a URL containing \r\n to smuggle a new header or body. A vulnerable Buffalo handler might look like:

http.Redirect(w, r, "/?next=" + userInput, http.StatusFound)

If userInput contains \r\nSet-Cookie: auth=attacker, the resulting Location header may include injected header lines when the redirect is processed by the client or intermediary. Similarly, logging or debug endpoints that echo user input into response headers can unintentionally expose header-splitting paths. Because Buffalo encourages rapid prototyping, developers may omit canonicalization or strict allow-listing of header values, increasing the likelihood of Crlf Injection.

The scanner’s checks for Input Validation and HTTP Response Smuggling help surface these classes of issues by analyzing how user input propagates into headers and whether response parsing can be disrupted. Even though Buffalo applications run as native Go servers, the framework does not automatically neutralize line breaks in header contexts; developers must ensure that any data entering headers is sanitized or rejected.

Remediation centers on disallowing control characters in header inputs and using framework-safe patterns. In Go, you should treat header values as opaque and remove or replace any CR (\r) or LF (\n) characters. A robust approach is to validate early and use Buffalo’s built-in facilities for setting headers and redirects rather than constructing Location values manually.

Safe redirect example in Buffalo:

import (
    "github.com/gobuffalo/buffalo"
    "github.com/gobuffalo/buffalo/plugins/render"
    "strings"
)

func safeRedirect(c buffalo.Context) error {
    next := c.Param("next")
    // Reject or sanitize inputs that could reach headers
    if strings.ContainsAny(next, "\r\n") {
        c.Response().WriteHeader(400)
        return c.Render(400, r.JSON(map[string]string{"error": "invalid_request"}))
    }
    // Use Buffalo's redirect helper which ensures proper header formatting
    return c.Redirect(302, next)
}

When setting custom headers, always sanitize:

func setSafeHeader(c buffalo.Context) error {
    userValue := c.Param("value")
    // Remove line break characters to prevent header injection
    sanitized := strings.ReplaceAll(strings.ReplaceAll(userValue, "\r", ""), "\n", "")
    c.Response().Header().Set("X-Custom-Value", sanitized)
    return c.Render(200, r.JSON(map[string]string{"status": "ok"}))
}

For cookie values, prefer Buffalo’s cookie helpers and avoid embedding user input directly into header lines. If you must construct headers manually, use Go’s http.Header utilities that handle proper formatting and avoid concatenation that could introduce line breaks.

Additionally, integrate the CLI tool to verify fixes: middlebrick scan <url> to confirm that the endpoints no longer reflect user-controlled line breaks in headers. For teams using the GitHub Action, set a threshold to fail builds if findings related to injection or smuggling appear, and consider the Pro plan for continuous monitoring across changes.