Padding Oracle in Buffalo with Basic Auth

A padding oracle attack targets block ciphers in modes such as CBC by exploiting an observable difference between valid and invalid padding after decryption. When HTTP Basic Authentication is used, credentials are transmitted in the Authorization header as base64-encoded username:password. If the backend decrypts this payload (e.g., an encrypted cookie or token derived from credentials) and reveals padding errors differently than other failures, a padding oracle can be constructed even though the credentials themselves are not the cryptographic key.

In Buffalo, a typical pattern is to decode and verify an encrypted session cookie that was generated from user data. If the cookie is encrypted with a symmetric cipher and the server returns distinct HTTP status codes or messages for padding errors versus authentication failures, an attacker can iteratively decrypt the ciphertext without knowing the key. Basic Auth does not prevent this; it supplies a consistent input source that may be incorporated into the encrypted blob. For example, the server might build a structure like { "user": "alice", "iat": 1710000000 }, encrypt it, and store it as a cookie. If padding errors are distinguishable, the attacker can decrypt this blob byte-by-byte by observing whether the server treats a malformed padding block as a recoverable error or an invalid session.

The combination is risky because Basic Auth is often assumed to be low-friction and therefore used in contexts where encryption is applied for confidentiality. If the encryption implementation does not use authenticated encryption (e.g., AES-GCM or AES-CCM) and instead uses raw CBC with a custom padding scheme, the server may inadvertently act as a padding oracle. This is especially dangerous when error messages differ between padding validation and other failure paths, such as missing credentials or invalid credentials, which can be enumerated by an attacker via timing or status-code differences.

Consider a Buffalo handler that decrypts a cookie and then extracts the username to match against Basic Auth:

// WARNING: Simplified example for educational purposes only
func verifySession(c *buffalo.Context) error {
    cookie, err := c.Request().Cookie("session")
    if err != nil {
        c.Response().WriteHeader(http.StatusUnauthorized)
        return nil
    }
    plaintext, err := decryptCBC([]byte(cookie.Value), key, iv)
    if err != nil {
        // Distinguishing padding errors can aid an attacker
        if isPaddingError(err) {
            c.Response().WriteHeader(http.StatusBadRequest)
            return errors.New("invalid padding")
        }
        c.Response().WriteHeader(http.StatusUnauthorized)
        return nil
    }
    // Further authentication logic...
    return nil
}

In this pattern, if isPaddingError returns true and the server responds with a different status code than a missing or malformed cookie, an attacker can use this signal as part of a padding oracle attack. Basic Auth supplies a known credential set that can be tied to the decrypted structure, making it easier to correlate successful decryption with valid user data. Therefore, the presence of Basic Auth does not mitigate the risk; it simply provides a consistent input that may be embedded within the encrypted payload.

Remediation focuses on ensuring that any decryption routine does not leak information via side channels and that authentication is handled uniformly. Use authenticated encryption so that tampering is detected before any padding validation occurs. Avoid differentiating responses based on internal error types; return a generic error status for any invalid input.

First, prefer authenticated encryption such as AES-GCM. This ensures integrity and confidentiality in a single step, making padding oracles infeasible because decryption fails if the ciphertext is modified.

// Example using AES-GCM in Go for Buffalo
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
}

func decryptGCM(ciphertext, 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
    }
    nonceSize := gcm.NonceSize()
    if len(ciphertext) < nonceSize {
        return nil, errors.New("ciphertext too short")
    }
    nonce, ciphertext := ciphertext[:nonceSize], ciphertext[nonceSize:]
    return gcm.Open(nil, nonce, ciphertext, nil)
}

Second, normalize responses so that padding errors, authentication failures, and missing cookies all return the same HTTP status code and generic message. This removes the oracle signal.

func verifySessionSafe(c *buffalo.Context) error {
    cookie, err := c.Request().Cookie("session")
    if err != nil {
        safeRespond(c, http.StatusUnauthorized)
        return nil
    }
    plaintext, err := decryptGCM([]byte(cookie.Value), key)
    if err != nil {
        // Do not distinguish error types to the client
        safeRespond(c, http.StatusUnauthorized)
        return nil
    }
    // Parse plaintext and validate credentials using Basic Auth
    // Example: expect {"user":"..."} and compare with header
    authUser, authPass, ok := c.Request().BasicAuth()
    if !ok || !validUserPass(authUser, authPass, plaintext) {
        safeRespond(c, http.StatusUnauthorized)
        return nil
    }
    // Set secure session...
    return nil
}

func safeRespond(c *buffalo.Context, code int) {
    c.Response().WriteHeader(code)
    c.Response().Write([]byte(`{"error":"unauthorized"}`))
}

Third, if you must continue using CBC, ensure constant-time padding validation and avoid any branching on padding correctness. Combine this with strict transport security and secure handling of the Basic Auth header to reduce the attack surface.

Tools such as middleBrick can help identify whether your API endpoints exhibit differences in error handling or exposure of internal error details. You can scan your Buffalo application with the CLI using middlebrick scan <url> or integrate the GitHub Action to fail builds if security risk scores exceed your chosen threshold. The Dashboard and MCP Server options also support tracking these findings over time.