Bleichenbacher Attack in Echo Go with Hmac Signatures

A Bleichenbacher attack is a practical adaptive chosen-ciphertext attack against RSA-based padding schemes such as PKCS#1 v1.5. When an API endpoint in Echo Go uses Hmac Signatures to protect a token or payload but exposes a padding-oracle-like behavior—typically via distinguishable error responses—an attacker can iteratively query the endpoint to decrypt or forge signed values without knowing the secret key.

In Echo Go, this risk arises when signature verification returns different HTTP statuses or timing differences depending on whether padding is valid. For example, if an endpoint accepts a JWT or a serialized structure with an Hmac signature and reveals 400 Bad Request for bad padding and 401 Unauthorized for a bad signature, an attacker can treat the server as a padding oracle. By carefully crafting ciphertexts and observing which responses change, the attacker performs the classic Bleichenbacher step-by-step byte recovery, eventually reconstructing a valid signature or token.

The specific combination is: Echo Go’s idiomatic routing and middleware chain make it straightforward to build endpoints that verify Hmac Signatures; if developers reuse the same verification logic for both authentication and authorization checks and leak timing or status distinctions, the unauthenticated attack surface includes the signature verification path. Because the scan tests unauthenticated endpoints, middleBrick can detect endpoints where error messages or timing differences hint at a padding-oracle behavior, flagging findings such as BOLA/IDOR or Input Validation that may enable adaptive chosen-ciphertext attacks.

Real-world context: This pattern resembles historic issues in TLS and web frameworks where servers inadvertently expose padding checks. In an API, the equivalent is an endpoint that processes signed tokens with Hmac and provides verbose failure modes. The OWASP API Top 10 category 2023 A01: Broken Object Level Authorization and related input validation weaknesses map to this risk, and middleBrick’s checks for BOLA/IDOR and Input Validation are designed to surface such oracle-like behavior.

Remediation focuses on making signature verification constant-time and avoiding distinguishable error paths. In Echo Go, centralize verification and ensure every failure path responds with the same HTTP status and similar timing characteristics. Do not branch on padding versus signature failures; treat all verification failures as a single invalid signature case.

Example of a vulnerable pattern to avoid:

// Vulnerable: distinguishable responses
func verifyPayload(c echo.Context) error {
    token := c.FormValue("token")
    sig := c.FormValue("sig")
    if !hmacVerified(token, sig) {
        return c.String(http.StatusUnauthorized, "invalid signature")
    }
    if !validPadding(token) {
        return c.String(http.StatusBadRequest, "invalid padding")
    }
    return c.JSON(http.StatusOK, "ok")
}

Example of a hardened, constant-time approach using Hmac Signatures in Echo Go:

import (
    "crypto/hmac"
    "crypto/sha256"
    "net/http"
    "github.com/labstack/echo/v4"
)

func verifyHmacConstantTime(expected, received, secret []byte) bool {
    // Use hmac.Equal which is constant-time
    return hmac.Equal(expected, received)
}

func computeHmac(data, secret []byte) []byte {
    mac := hmac.New(sha256.New, secret)
    mac.Write(data)
    return mac.Sum(nil)
}

// Secure handler: same response for any verification failure
func secureEndpoint(c echo.Context) error {
    token := []byte(c.FormValue("token"))
    sigClient := []byte(c.FormValue("sig"))
    secret := []byte(c.Get("hmacSecret").(string))

    expectedSig := computeHmac(token, secret)
    if !verifyHmacConstantTime(expectedSig, sigClient) {
        // Always return the same status and generic message
        return c.String(http.StatusUnauthorized, "invalid request")
    }

    // Additional business logic here; do not expose padding-specific errors
    return c.JSON(http.StatusOK, map[string]string{"status": "ok"})
}

Additional recommendations:

• Use opaque tokens or encrypted payloads so that structure and padding are not inferable.
• Ensure secret rotation policies and keep secrets out of source code (use environment variables or secure vaults).
• Apply rate limiting at the Echo Go middleware level to reduce oracle query opportunities; middleBrick’s Rate Limiting check can validate this control.
• If you use OpenAPI specs, ensure signature requirements are documented and validated; middleBrick’s OpenAPI/Swagger analysis with full $ref resolution cross-references runtime behavior with spec definitions.

By treating all verification failures uniformly and avoiding branches on padding correctness, the Echo Go service removes the conditions required for a Bleichenbacher attack while preserving the integrity of Hmac Signatures.