Crlf Injection in Buffalo with Jwt Tokens

Crlf Injection occurs when an attacker can inject a Carriage Return (CR, \r) and Line Feed (\n) sequence into a header or status-line context, causing the server to prematurely terminate a header block and inject additional headers or a second response. In the Buffalo web framework for Go, this typically arises when user-controlled data is placed into response headers, the status line, or header-like values (e.g., Location) without proper sanitization. JWT tokens can become a vector when their contents or metadata are reflected into headers, logged, or used to influence redirection or authentication flows. For example, if a handler decodes or inspects a JWT and then writes a claim (such as a user ID or role) into a header like X-User-ID or into a redirect Location, unsanitized newlines within the claim enable Crlf Injection. Similarly, an attacker-supplied JWT that contains crafted header or payload fields with newline characters may be echoed by middleware or logging, allowing injection of arbitrary headers or response splitting. In Buffalo, even though the framework provides secure defaults, developers must explicitly validate and sanitize any data derived from JWTs before using it in headers, status codes, or redirect targets. This includes claims used in Set-Cookie, Location during redirects, or custom headers added after authentication. Without such sanitization, an attacker can forge cache-poisoning responses, perform HTTP response splitting, or manipulate authentication flows by injecting a second request or response line. The risk is heightened when JWTs are parsed from untrusted sources and their contents are used to control HTTP semantics, because the newline characters in base64url-encoded payloads or header fields can bypass naive assumptions about token structure. MiddleBrick scans can detect such exposure by testing endpoints that accept or reflect JWTs in header contexts, identifying whether injected \r\n sequences are preserved or neutralized.

To prevent Crlf Injection when working with JWTs in Buffalo, ensure that any data derived from JWT claims is sanitized before being used in headers, status codes, or redirect locations. Always treat JWT payloads as untrusted input and validate/sanitize strings that may be reflected into HTTP-level constructs.

// Example: Safe handling of JWT claims in Buffalo handlers
package actions

import (
    "strings"

    "github.com/gobuffalo/buffalo"
    "github.com/golang-jwt/jwt/v5"
)

func ProfileHandler(c buffalo.Context) error {
    tokenString := c.Param("token")
    // Parse and validate the JWT using a trusted method and key
    token, err := jwt.Parse(tokenString, func(token *jwt.Token) (interface{}, error) {
        // TODO: provide your key or verification logic
        return myKey, nil
    })
    if err != nil || !token.Valid {
        return c.Render(401, r.JSON(map[string]string{"error": "invalid_token"}))
    }
    // Extract claims safely
    if claims, ok := token.Claims.(jwt.MapClaims); ok {
        // Sanitize any string claim before using it in a header or redirect
        userID := sanitizeHeaderValue(claims["user_id"])
        // Set a safe header; do not allow raw user input here
        c.Response().Header().Set("X-User-ID", userID)
        // If using claims for redirects, validate and sanitize the target
        target := sanitizeHeaderValue(claims["redirect_to"])
        if target != "" && isValidRedirectTarget(target) {
            return c.Redirect(302, target)
        }
        // Alternatively, use a hard-coded or enumerated set of destinations
        return c.Render(200, r.JSON(map[string]string{"user_id": userID}))
    }
    return c.Render(400, r.JSON(map[string]string{"error": "invalid_claims"}))
}

// sanitizeHeaderValue removes CR and LF characters to prevent header injection
func sanitizeHeaderValue(v interface{}) string {
    s := ""
    if v == nil {
        return s
    }
    s = strings.TrimSpace(v.(string))
    // Remove any carriage return or newline characters
    s = strings.ReplaceAll(s, "\r", "")
    s = strings.ReplaceAll(s, "\n", "")
    // Optionally enforce length or allowed characters
    return s
}

// isValidRedirectTarget ensures the target is safe for a Location header
func isValidRedirectTarget(target string) bool {
    // Allow only relative paths or approved hostnames to prevent open redirects
    return strings.HasPrefix(target, "/") || target == "https://app.example.com"
}

Additional measures include avoiding the use of raw JWT claims in Set-Cookie without further sanitization, and ensuring any logging or error messages that include JWT data are scrubbed of newline characters. The Buffalo CLI can scaffold secure handler patterns, and using the middleBrick CLI to scan your endpoints will surface any remaining injection risks in header or redirect usage.