Poodle Attack in Echo Go with Hmac Signatures

The Poodle attack (Padding Oracle On Downgraded Legacy Encryption) targets block cipher modes like CBC by exploiting error messages returned during padding validation. In Echo Go, using HMAC signatures without additional safeguards can inadvertently create an environment where an attacker can leverage a padding oracle to recover plaintext. This occurs when an endpoint accepts requests with encrypted payloads and HMACs but returns distinct errors for padding failures versus MAC verification failures.

Consider an API that expects an encrypted and HMAC-protected payload. The server first verifies the HMAC and then decrypts. If the server responds with a padding error before rejecting the MAC, or if the timing or message content differs between padding failures and MAC failures, an attacker can iteratively modify ciphertext and observe responses to decrypt data. This is a classic CBC padding oracle scenario. Even with HMAC signatures intended to ensure integrity, if the application does not use constant-time comparison and does not defer all error handling to a generic failure, the HMAC does not prevent the oracle.

In Echo Go, if routes process encrypted payloads and rely on standard library crypto operations without care, the combination of CBC-mode decryption and HMAC verification can expose a padding oracle. For example, an endpoint that deciphers using cipher.NewCBCDecrypter and then checks an HMAC generated with hmac.New may leak information through error paths. An attacker can send modified ciphertexts and observe differences in HTTP status codes or response times, enabling decryption of sensitive data such as authentication tokens. This violates confidentiality even though integrity checks are present.

Real-world impact includes recovery of session cookies or authentication tokens. The attack does not require breaking the HMAC; it bypasses it by exploiting the decryption layer’s behavior. Tools can automate this by sending many crafted requests and analyzing response codes, demonstrating why unauthenticated scan coverage includes such attack patterns.

To mitigate Poodle-style padding oracles in Echo Go when using HMAC signatures, ensure that decryption and verification errors are handled uniformly and that constant-time operations are used. The following patterns reduce risk.

1. Use Constant-Time MAC Verification

Always compare MACs in constant time to prevent timing leaks. In Go, use hmac.Equal instead of simple byte comparison. This prevents attackers from learning partial information about the MAC through timing differences.

package main

import (
	"crypto/hmac"
	"crypto/sha256"
	"net/http"

	"github.com/labstack/echo/v4"
)

func verifySignature(expectedMAC, receivedMAC []byte) bool {
	return hmac.Equal(expectedMAC, receivedMAC)
}

func secureHandler(c echo.Context) error {
	payload := c.Request().Body // assume encrypted payload handled separately
	receivedMAC := c.Request().Header.Get("X-Signature")
	if receivedMAC == "" {
		return echo.NewHTTPError(http.StatusBadRequest, "missing signature")
	}

	macKey := []byte("super-secret-key") // use secure key management in practice
	mac := hmac.New(sha256.New, macKey)
	// In practice, MAC is computed over a canonical representation of the data
	mac.Write([]byte("canonical-data-here"))
	expectedMAC := mac.Sum(nil)

	if !verifySignature(expectedMAC, []byte(receivedMAC)) {
		return echo.NewHTTPError(http.StatusUnauthorized, "invalid signature")
	}

	// Proceed only after constant-time verification
	return c.String(http.StatusOK, "verified")
}

2. Defer to a Generic Error Path

Ensure that any decryption or verification failure returns the same HTTP status and response body to avoid leaking whether the failure was due to padding or MAC. This removes the oracle distinction.

func handleDecryptAndVerify(c echo.Context, ciphertext, receivedMAC string) error {
	// Decrypt using CBC or other mode; capture any error
	plaintext, err := decryptCBC(ciphertext)
	if err != nil {
		// Log internally if needed, but return generic error to client
		return echo.NewHTTPError(http.StatusBadRequest, "invalid request")
	}

	// Compute MAC over plaintext or associated data
	macKey := []byte("super-secret-key")
	mac := hmac.New(sha256.New, macKey)
	mac.Write([]byte(plaintext))
	expectedMAC := mac.Sum(nil)

	if !hmac.Equal(expectedMAC, []byte(receivedMAC)) {
		return echo.NewHTTPError(http.StatusBadRequest, "invalid request")
	}

	// Use plaintext
	return c.JSON(http.StatusOK, map[string]string{"data": plaintext})
}

// decryptCBC is a placeholder for actual decryption logic.
// Ensure it does not distinguish between padding errors and other failures
// in its error returns to the caller.
func decryptCBC(ciphertext string) (string, error) {
	// Implement decryption with proper error handling that returns
	// a generic error on failure to avoid leaking padding oracles.
	return "decrypted-data", nil
}

3. Use Authenticated Encryption

Where possible, prefer authenticated encryption modes such as GCM or use an encrypt-then-MAC pattern with strict rules. This reduces reliance on padding and inherently binds confidentiality and integrity.

import (
	"crypto/aes"
	"crypto/cipher"
	"crypto/rand"
	"io"
)

func encryptGCM(plaintext, key []byte) ([]byte, error) {
	block, err := aes.NewCipher(key)
	if err != nil {
		return nil, err
	}
	gcm, err := cipher.NewGCM(block)
	if err != nil {
		return nil, err
	}
	nonce := make([]byte, gcm.NonceSize())
	if _, err = io.ReadFull(rand.Reader, nonce); err != nil {
		return nil, err
	}
	return gcm.Seal(nonce, nonce, plaintext, nil), nil
}

By following these patterns, Echo Go services can use HMAC signatures securely while minimizing the risk of padding oracle attacks like Poodle.