Crlf Injection in Echo Go with Hmac Signatures

Crlf Injection occurs when an attacker can inject CRLF sequences (\r\n) into a header or value that is later reflected into the HTTP response. In Echo Go, this typically happens when user-controlled input is placed into a custom header or cookie without proper sanitization. When Hmac Signatures are used—commonly to sign request parameters or headers for integrity verification—the interaction between the signature generation and unchecked input can amplify the impact of Crlf Injection.

Consider an endpoint that accepts a X-Request-ID header and includes it in the data that is signed using Hmac Signatures. If the application constructs the signing string by concatenating raw header values without validation, an attacker can inject newline characters to create multiple logical lines in the signed payload. Because Hmac Signatures bind integrity to the exact byte sequence, the injected CRLF may not break the signature verification but can change how servers or intermediaries interpret the signed message. This can lead to response splitting, header smuggling, or cache poisoning depending on how the signed data is later used.

In Echo Go, developers sometimes use middleware that copies headers into the context for signing. If a header like X-Data contains value\r\nSet-Cookie: injected=1, and the application signs X-Data as part of the Hmac payload, the signature may still validate while the injected line is processed by downstream handlers or logging systems. This creates a scenario where the attacker can influence protocol-level parsing without invalidating the cryptographic assurance, because the signature covers the tainted value but does not prevent its interpretation as control characters.

Another pattern involves query parameters that are included in the Hmac sign string. An input such as id=123\r\nX-Admin: true can be reflected into headers if the application reuses parameter-derived values to construct response headers. Even with Hmac Signatures confirming data integrity, the reflected CRLF can trick naive header-parsing logic into treating injected text as a new header. This is particularly relevant when the signed data is later serialized into a Set-Cookie or Location header, as the newline can terminate the original header prematurely.

Because Echo Go encourages structured handlers and middleware chains, developers may assume that Hmac Signatures provide end-to-end integrity, inadvertently neglecting output encoding and input validation. The framework does not automatically neutralize CRLF characters in values that are signed and later reflected. This gap between cryptographic integrity and safe data representation means that Crlf Injection remains viable in signed contexts, where the attacker manipulates how the signed content is interpreted rather than breaking the signature itself.

Real-world parallels include response splitting and HTTP smuggling techniques that abuse unchecked newlines in headers. While Hmac Signatures protect against tampering, they do not sanitize content for safe transmission over the wire. Therefore, when using Hmac Signatures in Echo Go, it is essential to treat signed inputs as potentially hostile and enforce strict validation and encoding before any reflected use in headers or cookies.

Remediation focuses on two areas: preventing CRLF injection in inputs that are used for signing or reflection, and ensuring that signed values are safely encoded before being placed into headers or cookies. Below are concrete code examples demonstrating secure patterns in Echo Go.

First, validate and sanitize any user input that participates in Hmac Signatures. Reject or transform inputs containing carriage return or line feed characters before they are included in the signing string or echoed into headers.

// sanitizeInput removes or rejects CRLF characters
func sanitizeInput(value string) (string, error) {
    if strings.ContainsAny(value, "\r\n") {
        return "", fmt.Errorf("invalid input: contains newline characters")
    }
    return value, nil
}

func handler(c echo.Context) error {
    raw := c.QueryParam("id")
    safe, err := sanitizeInput(raw)
    if err != nil {
        return echo.NewHTTPError(http.StatusBadRequest, "invalid parameter")
    }
    // Use safe value in signing and response
    return c.String(http.StatusOK, safe)
}

Second, when constructing data for Hmac Signatures, canonicalize the input by trimming and normalizing whitespace. This prevents attackers from smuggling newlines that do not survive trimming but still affect signature bytes.

func canonicalizeAndSign(input string, key []byte) (string, error) {
    trimmed := strings.TrimSpace(input)
    if strings.ContainsAny(trimmed, "\r\n") {
        return "", fmt.Errorf("input contains line breaks after trimming")
    }
    mac := hmac.New(sha256.New, key)
    mac.Write([]byte(trimmed))
    signature := hex.EncodeToString(mac.Sum(nil))
    return signature, nil
}

Third, when reflecting signed values into headers, explicitly set headers using Echo’s structured methods rather than raw concatenation. This avoids accidental injection through values that have passed Hmac verification.

func dataHandler(c echo.Context) error {
    userData := c.QueryParam("data")
    safe, err := sanitizeInput(userData)
    if err != nil {
        return echo.NewHTTPError(http.StatusBadRequest, "data invalid")
    }
    // Use Echo's header setter which does not interpret CRLF
    c.Response().Header().Set("X-Signed-Data", safe)

    // Example Hmac signing of the safe value
    key := []byte(os.Getenv("HMAC_KEY"))
    signature, err := canonicalizeAndSign(safe, key)
    if err != nil {
        return echo.NewHTTPError(http.StatusInternalServerError, "signing error")
    }
    c.Response().Header().Set("X-Data-Signature", signature)
    return c.JSON(http.StatusOK, map[string]string{"data": safe, "sig": signature})
}

Fourth, if you use middleware that copies headers into the context for signing, ensure that copied values are also sanitized. Do not propagate raw header values that may contain CRLF sequences into downstream handlers or logging.

func signingMiddleware(next echo.HandlerFunc) echo.HandlerFunc {
    return func(c echo.Context) error {
        raw := c.Request().Header.Get("X-Custom")
        safe, err := sanitizeInput(raw)
        if err != nil {
            return echo.NewHTTPError(http.StatusBadRequest, "header invalid")
        }
        c.Set("signedValue", safe)
        return next(c)
    }
}

Finally, apply output encoding for any data that is used to construct Set-Cookie or Location headers, even when covered by Hmac Signatures. Echo provides cookie utilities that handle safe attribute formatting and reduce the risk of CRLF injection through cookie values.

cookie := &http.Cookie{
    Name:     "session",
    Value:    safeValue,
    Path:     "/",
    HttpOnly: true,
    Secure:   true,
}
cookieSanitized := sanitizeCookieValue(cookie.Value)
c.SetCookie(cookie)