Request Smuggling in Echo Go with Bearer Tokens

Request smuggling occurs when an HTTP proxy or server processes requests differently depending on how headers are framed and buffered, allowing an attacker to smuggle a request across boundaries intended for separate users. In Echo Go, this risk is heightened when bearer tokens are handled inconsistently between front-end and back-end parsing, particularly when header ordering, chunked transfer encoding, or content-length mismatches are present.

Bearer tokens are typically passed via the Authorization header (e.g., Authorization: Bearer eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9...). If Echo routes requests through middleware that buffers or re-parses headers differently than the upstream service, an attacker can craft a request where the Authorization header is associated with one user but processed under another. For example, a request can be split so that the first line and headers are interpreted by one handler, while the body of the request (possibly containing a second Authorization header) is interpreted by another, leading to privilege escalation or unauthorized access.

Echo Go applications that use multiple layers—such as a reverse proxy terminating TLS and forwarding to an Echo instance—may inadvertently allow smuggling when header parsing diverges. Consider a scenario where the proxy uses Transfer-Encoding: chunked while the backend expects Content-Length. An attacker can inject a second request within the body of the first request. If the Authorization header appears late in the smuggled request, the backend might associate the bearer token with a different user context, bypassing intended access controls.

A concrete attack pattern involves an authenticated user sending a request with two Authorization headers separated by carefully crafted whitespace and chunk boundaries. The first header contains a valid Bearer token for User A, while the second, smuggled header contains a token for User B. If Echo’s routing or middleware fails to enforce strict header parsing, the backend may process the second header and execute actions as User B, despite the initial token belonging to User A.

To detect this class of issue, scanning with tools that understand both protocol-level request splitting and framework-specific routing behavior is essential. middleBrick runs checks that simulate smuggling attempts across common deployment topologies, identifying whether Authorization headers are consistently interpreted across request boundaries in Echo Go services. Findings include evidence of inconsistent header parsing, improper handling of chunked versus content-length-based requests, and missing safeguards that would prevent token association across smuggled requests.

Remediation focuses on ensuring consistent header parsing, strict authorization enforcement, and eliminating ambiguity in how requests are interpreted by Echo and any intermediate proxies.

• Always extract and validate the Authorization header early in middleware, and reject requests with multiple Authorization headers.

func AuthMiddleware(next echo.HandlerFunc) echo.HandlerFunc {
    return func(c echo.Context) error {
        authHeaders := c.Request().Header["Authorization"]
        if len(authHeaders) > 1 {
            return echo.NewHTTPError(http.StatusBadRequest, "multiple authorization headers")
        }
        auth := authHeaders[0]
        if !strings.HasPrefix(auth, "Bearer ") {
            return echo.NewHTTPError(http.StatusUnauthorized, "invalid auth scheme")
        }
        token := strings.TrimPrefix(auth, "Bearer ")
        // validate token, e.g., jwt.Parse(token)
        return next(c)
    }
}

• Enforce strict host and header canonicalization in front of Echo to prevent smuggling via header reordering.

func CanonicalizeHeaders(next echo.HandlerFunc) echo.HandlerFunc {
    return func(c echo.Context) error {
        req := c.Request()
        // ensure only one Authorization header is present
        if req.Header.Get("Authorization") != "" {
            // strip any duplicate headers
            req.Header.Del("Authorization")
            req.Header.Set("Authorization", req.Header.Get("Authorization"))
        }
        return next(c)
    }
}

• Configure your reverse proxy (e.g., Nginx, HAProxy) to use the same message parsing behavior as Echo, avoiding chunked-to-content-length mismatches.

# Nginx configuration snippet to prevent smuggling
location /api/ {
    proxy_pass http://echo-backend;
    proxy_set_header Authorization $http_authorization;
    chunked_transfer_encoding off;
    proxy_buffering off;
}

• Use structured tokens (e.g., JWT) with audience and issuer validation inside Echo handlers to ensure tokens are bound to the correct service and identity.

func ValidateToken(next echo.HandlerFunc) echo.HandlerFunc {
    return func(c echo.Context) error {
        auth := c.Request().Header.Get("Authorization")
        if auth == "" {
            return echo.NewHTTPError(http.StatusUnauthorized, "missing authorization")
        }
        parts := strings.Split(auth, " ")
        if len(parts) != 2 || parts[0] != "Bearer" {
            return echo.NewHTTPError(http.StatusBadRequest, "malformed authorization header")
        }
        token, err := jwt.Parse(parts[1], func(token *jwt.Token) (interface{}, error) {
            return []byte("your-secret-key"), nil
        })
        if err != nil || !token.Valid {
            return echo.NewHTTPError(http.StatusUnauthorized, "invalid token")
        }
        return next(c)
    }
}