Padding Oracle in Buffalo with Hmac Signatures

A Padding Oracle attack leverages the behavior of a server that distinguishes between padding errors and other failures (e.g., MAC verification). When Buffalo applications use Hmac Signatures to protect requests or webhook payloads, a misconfigured verification flow can create an oracle: the server reveals whether the padding is valid before (or instead of) confirming the HMAC integrity, allowing an attacker to iteratively recover plaintext or forge authenticated messages.

Consider a Buffalo API endpoint that receives an encrypted+sized payload and an Hmac signature header. If the server first decrypts the ciphertext (which may produce invalid padding during decryption), then computes the HMAC over the resulting plaintext, and only after that compares the signature, the server may return a 400/403 for bad padding versus a 401 for a bad signature. This timing or status-code difference becomes an oracle. An attacker who can submit chosen ciphertexts can use the oracle to decrypt data without knowing the key, or to forge valid Hmac-signed messages by iteratively adjusting bytes based on server responses.

In Buffalo, this often occurs when using securecompare incorrectly or when decryption and signature verification steps are ordered poorly. For example, if the application decrypts first and then verifies the Hmac, but leaks the decryption or padding error through HTTP status codes, response content, or timing, the Hmac Signature mechanism does not prevent the oracle because the oracle exists at the decryption/padding layer. Even when Hmac Signatures are used to bind a payload to a secret key, an attacker can bypass the intended integrity protection by exploiting the padding verification oracle to eventually produce valid Hmac-signed plaintexts.

Real-world impact: an attacker can recover sensitive tokens, session cookies, or API keys that were encrypted and Hmac-protected, or they can forge requests that appear legitimately signed. This violates confidentiality and integrity expectations and may map to OWASP API Top 10:2023 —2: Broken Authentication and Session Management and A03:2021 — Injection (via crafted ciphertexts).

Remediation centers on ensuring that padding errors do not leak and that Hmac verification occurs in a constant-time, order-safe manner. In Buffalo, always verify the Hmac signature before attempting decryption, and use constant-time comparison for signatures. Also ensure decryption errors (including padding errors) are handled uniformly so no oracle is exposed.

Example: Safe verification flow in Buffalo (Go)

Assume a payload format: base64(ciphertext).base64(hmac). The server must:

1. Split and decode the parts.
2. Compute Hmac over the ciphertext using the shared secret.
3. Compare the computed Hmac with the received Hmac using a constant-time function.
4. Only after verification succeeds, decrypt the ciphertext and handle padding errors uniformly (do not distinguish padding errors from other failures).

import (
	"crypto/hmac"
	"crypto/sha256"
	"encoding/base64"
	"errors"
	"time"

	"github.com/gobuffalo/buffalo"
)

// constantTimeCompare returns true if a and b are equal in constant time.
func constantTimeCompare(a, b []byte) bool {
	return hmac.Equal(a, b)
}

// verifyAndDecrypt handles the full flow safely.
func verifyAndDecrypt(payload string, secret []byte) ([]byte, error) {
	// Expected format: ciphertextB64.hmacB64
	parts := splitTwo(payload, '.')
	if len(parts) != 2 {
		return nil, errors.New("invalid payload format")
	}

	ciphertextB64, hmacB64 := parts[0], parts[1]
	ciphertext, err := base64.StdEncoding.DecodeString(ciphertextB64)
	if err != nil {
		return nil, errors.New("invalid encoding")
	}

	receivedMac, err := base64.StdEncoding.DecodeString(hmacB64)
	if err != nil {
		return nil, errors.New("invalid encoding")
	}

	// Compute Hmac over the ciphertext.
	mac := hmac.New(sha256.New, secret)
	mac.Write(ciphertext)
	expectedMac := mac.Sum(nil)

	// Constant-time comparison to avoid timing leaks.
	if !constantTimeCompare(expectedMac, receivedMac) {
		return nil, errors.New("authentication failed")
	}

	// Only after Hmac verification, attempt decryption.
	// Ensure decryption errors (including padding errors) are handled uniformly.
	plaintext, err := decrypt(ciphertext, secret) // implement decrypt consistently
	if err != nil {
		// Return a generic error to avoid leaking padding vs other issues.
		return nil, errors.New("decryption failed")
	}
	return plaintext, nil
}

func splitTwo(s string, sep byte) []string {
	var parts []string
	start := 0
	for i := 0; i < len(s); i++ {
		if s[i] == sep {
			parts = append(parts, s[start:i])
			start = i + 1
		}
	}
	parts = append(parts, s[start:])
	return parts
}

// Example HTTP handler using the safe flow.
func webhookHandler(c buffalo.Context) error {
	body := c.Request().Body // read once
	payload := string(body)  // or read appropriately for your format
	secret := []byte("super-secret-key") // load from secure config

	plaintext, err := verifyAndDecrypt(payload, secret)
	if err != nil {
		// Always return the same status for auth/verification/padding failures.
		c.Response().WriteHeader(400)
		return c.Render(400, r.JSON(map[string]string{"error": "invalid request"}))
	}

	// Process plaintext safely.
	return c.Render(200, r.JSON(map[string]string{"data": string(plaintext)}))
}

Key practices:

• Verify Hmac over the ciphertext before decryption.
• Use hmac.Equal (constant-time) for signature comparison.
• Do not expose distinct errors for padding failures versus other decryption errors; return a uniform error.
• Ensure no timing side-channels exist in the decryption/padding implementation.

These steps close the oracle by removing the distinguishing behavior that enables an attacker to exploit the combination of Padding Oracle and Hmac Signatures in Buffalo applications.