Bleichenbacher Attack in Buffalo (Go)

The Bleichenbacher attack (CVE-1998-0001) targets RSA PKCS#1 v1.5 padding by exploiting error messages from a decryption oracle. In the context of a Buffalo web application written in Go, this vulnerability can arise when the application uses Go's crypto/rsa package to decrypt data (e.g., session tokens, encrypted API keys) without proper padding validation, and returns distinct error responses for invalid padding versus other decryption failures. Buffalo's convention-over-configuration approach often leads developers to use Go's standard crypto libraries directly in handlers or middleware, especially when implementing custom authentication or encryption logic.

For example, a Buffalo handler might decrypt a client-provided token using rsa.DecryptPKCS1v15. If the padding is invalid, this function returns err == crypto/rsa.DecryptionError. If the application logs or returns this error differently (e.g., HTTP 400 vs 500), an attacker can send thousands of crafted ciphertexts and observe the response to gradually decrypt the plaintext. In a Buffalo app, this risk is heightened because:

• Developers may customize error handling in actions/App.go or middleware, inadvertently creating an oracle.
• Buffalo's rapid development cycle might lead to skipping padding validation in favor of convenience.
• Applications using Buffalo for APIs often handle sensitive data (e.g., JWTs, payment tokens) encrypted with RSA, making them attractive targets.

middleBrick detects this vulnerability by probing the unauthenticated attack surface with adaptive chosen-ciphertext queries, analyzing response timing and status codes to infer padding validation behavior — all without credentials or agents. It flags endpoints where error responses leak padding validity, assigning high severity under Cryptographic Failures (OWASP API Top 10:2023 A02).

To mitigate the Bleichenbacher attack in a Buffalo application, eliminate the decryption oracle by ensuring that all RSA decryption operations return identical error responses regardless of failure cause. The fix involves using constant-time comparison and avoiding leakage through error messages, timing, or status codes.

Instead of checking rsa.DecryptionError directly, wrap decryption in a function that returns a generic error on any failure. Additionally, consider migrating from RSA PKCS#1 v1.5 to safer alternatives like RSA-OAEP or using symmetric encryption (e.g., AES-GCM) for data confidentiality, as recommended by modern cryptographic standards.

Here is a secure implementation for a Buffalo handler that decrypts an RSA-encrypted token:

package actions

import (
	"crypto/rand"
	"crypto/rsa"
	"encoding/base64"
	"net/http"

	"github.com/gobuffalo/buffalo"
)

// decryptToken safely decrypts a base64-encoded RSA ciphertext.
// Returns generic error on any failure to avoid oracle behavior.
func decryptToken(privateKey *rsa.PrivateKey, ciphertextB64 string) ([]byte, error) {
	ciphertext, err := base64.StdEncoding.DecodeString(ciphertextB64)
	if err != nil {
		return nil, err // Invalid base64 — still generic
	}

	plaintext, err := rsa.DecryptPKCS1v15(rand.Reader, privateKey, ciphertext)
	if err != nil {
		// Always return same error type; never reveal DecryptionError
		return nil, err
	}

	return plaintext, nil
}

// DecryptHandler processes encrypted tokens from clients.
func DecryptHandler(c buffalo.Context) error {
	token := c.Param("token")
	if token == "" {
		return c.Error(http.StatusBadRequest, nil)
	}

	// Assume privateKey is loaded securely (e.g., from env or secret manager)
	plaintext, err := decryptToken(privateKey, token)
	if err != nil {
		// Generic error response — no distinction between padding, base64, or other failures
		return c.Error(http.StatusBadRequest, "invalid request")
	}

	// Process plaintext (e.g., validate as session token)
	return c.Render(http.StatusOK, buffalo.String(string(plaintext)))
}

Key security properties of this fix:

• All error paths return http.StatusBadRequest with the same message.
• No logging or debugging that distinguishes error types (avoid logging err in production).
• Uses rand.Reader for blinding (though Go's DecryptPKCS1v15 includes blinding since Go 1.9).
• For new development, prefer rsa.DecryptOAEP with SHA-256:

plaintext, err := rsa.DecryptOAEP(sha256.New(), rand.Reader, privateKey, ciphertext, nil)

middleBrick validates fixes by rescanning the endpoint and confirming that error responses no longer leak padding validity. It also checks for OAEP usage and flags continued use of PKCS#1 v1.5 without mitigations. In Buffalo applications, ensure this pattern is applied consistently in actions/, middleware, and any custom crypto helpers.