Crlf Injection in Buffalo with Basic Auth

Crlf Injection occurs when user-controlled data is reflected in HTTP headers without sanitization, allowing an attacker to inject newline characters (\r\n) to split the header stream. In the Buffalo web framework for Go, this typically arises when dynamic values from request parameters, headers, or cookies are passed into header-setting logic such as response.Header().Set() or response.Header().Add(). When Basic Authentication is in use, the Authorization header value is often parsed by application code or middleware to extract credentials. If the extracted username or password is later reflected into other response headers without validation, an attacker can inject newline sequences to append or overwrite headers, potentially enabling HTTP response splitting, cache poisoning, or cross-site scripting via injected headers.

For example, a handler that builds a custom header reflecting the authenticated user’s name can be exploited if the name contains \r\n. Consider a route that reads the username from a Basic Auth token and sets it in a custom header:

// In a Buffalo controller action
username := getBasicAuthUsername(r)
response.Header().Set("X-User", username)

An attacker authenticating with a Basic Auth credential like alice\r\nContent-Security-Policy: default-src 'none' as the username can cause the response to include a second header, potentially altering browser security policies. This works because the framework does not inherently sanitize header values derived from authentication data. The presence of Basic Auth increases the attack surface: credentials are transmitted in the Authorization header, and if the server reuses any part of that credential in other headers, an injection vector is created. While Buffalo does not automatically expose CRLF injection, patterns where authentication data influences headers—especially in APIs or middleware that log or decorate responses—require careful handling to avoid splitting headers.

CRLF Injection can also interact with logging and monitoring systems. If Buffalo logs request headers or usernames directly to output that is later processed by other tools, injected newlines can break log formatting or enable log injection attacks. The framework’s use of standard Go HTTP primitives means header manipulation follows Go’s net/http semantics, where extraneous \r\n characters can terminate a header block and begin a new one. Therefore, validating and sanitizing any data derived from Basic Auth before it reaches header-setting code is essential to prevent injection.

Remediation focuses on never reflecting untrusted data into HTTP headers and strictly validating inputs derived from Basic Auth. Since Basic Auth credentials are typically base64-encoded and decoded server-side, ensure the decoded username and password are not directly reused in headers. Instead, treat them as opaque authentication tokens and avoid echoing them into responses.

Secure header handling pattern: Use static values or vetted identifiers that do not originate from user-controlled sources. For example, instead of using the username in a custom header, assign a sanitized user ID or UUID that is validated against your authentication store:

// Secure: use a verified user ID instead of raw auth data
userID := getUserIDFromSession(r) // validated server-side identifier
response.Header().Set("X-User-ID", userID)

Input validation and sanitization: If you must use data from Basic Auth, enforce strict allow-lists and reject any input containing carriage return or line feed characters. In Go, you can sanitize before setting headers:

import "strings"

func sanitizeHeaderValue(value string) string {
    // Remove any carriage return or newline characters
    value = strings.ReplaceAll(value, "\r", "")
    value = strings.ReplaceAll(value, "\n", "")
    return value
}

username := getBasicAuthUsername(r)
safeUsername := sanitizeHeaderValue(username)
response.Header().Set("X-User", safeUsername)

Example of a proper Basic Auth parsing flow: Decode credentials, validate format, and avoid header reflection. Below is a minimal, secure handler that authenticates against a user store and sets only safe headers:

func (c APIController) AuthenticatedUser(ctx buffalo.Context) error {
    auth := ctx.Request().Header.Get("Authorization")
    if auth == "" {
        return ctx.Response().WriteHeader(http.StatusUnauthorized)
    }
    const prefix = "Basic "
    if !strings.HasPrefix(auth, prefix) {
        return ctx.Response().WriteHeader(http.StatusUnauthorized)
    }
    payload, err := base64.StdEncoding.DecodeString(auth[len(prefix):])
    if err != nil {
        return ctx.Response().WriteHeader(http.StatusUnauthorized)
    }
    parts := strings.SplitN(string(payload), ":", 2)
    if len(parts) != 2 {
        return ctx.Response().WriteHeader(http.StatusUnauthorized)
    }
    username, password := parts[0], parts[1]
    // Validate credentials against secure store (pseudo-code)
    if !isValidUser(ctx, username, password) {
        return ctx.Response().WriteHeader(http.StatusUnauthorized)
    }

    // Safe: no user-controlled data placed in response headers
    ctx.Response().Header().Set("Content-Type", "application/json")
    return ctx.Render(http.StatusOK, r.JSON(map[string]string{"status": "ok"}))
}

Additionally, ensure that any logging or middleware that copies headers does not propagate injected sequences. Configure logging formats to treat \r and \n as invalid or escape them. By decoupling authentication state from response header generation and rigorously validating inputs, Crlf Injection risks in Buffalo applications using Basic Auth are effectively mitigated.