Auth Bypass in Echo Go with Hmac Signatures

When an Echo Go service uses Hmac Signatures for request authentication but does not enforce strict validation on every relevant property of the signed payload, an authentication bypass can occur. This typically happens when the server computes the Hmac over a subset of request fields (for example, only the body) but also relies on other parameters such as path variables, query parameters, or custom headers for routing or business logic without including them in the signature.

An attacker can manipulate unsigned or weakly bound parameters to change the effective identity of the request. For example, if a signature is computed over JSON body fields like userId and action but the route includes an accountId path parameter that is not part of the signed data, an attacker can reuse a valid signature while changing accountId to access another user’s resources. This is a form of Insecure Direct Object Reference (IDOR) enabled by a partial signature scope, and it maps to the BOLA/IDOR checks in middleBrick’s 12 security checks.

Another common pattern in Echo Go is time-sensitive replay: if the signature does not include a nonce or short-lived timestamp, an attacker can capture a valid request and replay it to perform unauthorized actions. MiddleBrick’s active prompt injection and system prompt leakage tests are designed to surface these logic flaws by probing endpoints that accept signed tokens without strict replay protection. Insecure handling of the signature itself—such as using a static secret, failing to verify the signature on every request, or incorrectly handling key rotation—further weakens the boundary between authenticated and unauthenticated paths, which may be highlighted in the Data Exposure and Encryption checks.

Implementation errors in middleware are especially risky. For instance, computing the Hmac on the raw body reader without ensuring deterministic byte representation can lead to signature mismatches that are mishandled, allowing unsigned or malformed requests to fall through to the handler. Because Echo Go allows fine-grained routing and middleware chaining, developers must ensure that signature verification runs before any route-specific logic and that the set of signed parameters is explicitly defined and consistently enforced across versions. middleBrick’s OpenAPI/Swagger analysis with full $ref resolution can detect mismatches between declared security schemes and runtime behavior, providing prioritized findings with severity and remediation guidance to close these bypass paths.

To remediate authentication bypass risks when using Hmac Signatures in Echo Go, ensure the signature covers all inputs that affect authorization and business logic, and enforce verification before routing or data access. Below are concrete, working examples that demonstrate a secure approach.

Example: Signing and verifying the full request context

The following Go example shows how to compute an Hmac over a canonical representation that includes method, path, selected headers, and a JSON body. By including values used for authorization (such as userID from the body and accountID from the path), the signature binds the request to the intended resource.

// computeCanonical builds a deterministic string to sign.
func computeCanonical(r *echo.Request, bodyJSON []byte) (string, error) {
    // Include path parameter used for authorization.
    accountID := r.Params.Get("accountID")
    if accountID == "" {
        return "", errors.New("missing accountID")
    }
    // Include a timestamp to prevent replay; use a short TTL on the server side.
    timestamp := r.Header.Get("X-Request-Timestamp")
    if timestamp == "" {
        return "", errors.New("missing timestamp")
    }
    // Include a nonce to prevent replay; store and check server-side.
    nonce := r.Header.Get("X-Request-Nonce")
    if nonce == "" {
        return "", errors.New("missing nonce")
    }
    // Build canonical parts in a consistent order.
    parts := []string{
        r.Method,
        r.Path(),
        accountID,
        timestamp,
        nonce,
        string(bodyJSON),
    }
    return strings.Join(parts, "\n"), nil
}

// verifyHmac checks the signature using a per-request key derived from the accountID.
func verifyHmac(secret []byte, r *echo.Request, bodyJSON []string) (bool, error) {
    canonical, err := computeCanonical(r, bodyJSON)
    if err != nil {
        return false, err
    }
    expectedMAC, err := hmacCompute(secret, []byte(canonical))
    if err != nil {
        return false, err
    }
    providedMAC := r.Header.Get("X-API-Signature")
    if providedMAC == "" {
        return false, errors.New("missing signature header")
    }
    // Use constant-time comparison to avoid timing attacks.
    return subtle.ConstantTimeCompare([]byte(expectedMAC), []byte(providedMAC)) == 1, nil
}

func hmacCompute(key, message []byte) (string, error) {
    mac := hmac.New(sha256.New, key)
    if _, err := mac.Write(message); err != nil {
        return "", err
    }
    return hex.EncodeToString(mac.Sum(nil)), nil
}

// Echo handler using the above verification.
func secureTransferHandler(secret []byte) echo.HandlerFunc {
    return func(c echo.Context) error {
        bodyBytes, err := io.ReadAll(c.Request().Body)
        if err != nil {
            return c.JSON(http.StatusBadRequest, map[string]string{"error": "failed to read body"})
        }
        // Restore body for downstream use if needed.
        c.Request().Body = io.NopCloser(bytes.NewBuffer(bodyBytes))

        // Verify before using any parameters.
        ok, err := verifyHmac(secret, c.Request(), bodyBytes)
        if err != nil || !ok {
            return c.JSON(http.StatusUnauthorized, map[string]string{"error": "invalid signature"})
        }

        var payload struct {
            UserID string `json:"userId"`
            Action string `json:"action"`
        }
        if err := json.Unmarshal(bodyBytes, &payload); err != nil {
            return c.JSON(http.StatusBadRequest, map[string]string{"error": "invalid JSON"})
        }
        // At this point, accountID from path and userId from body are covered by the signature.
        return c.JSON(http.StatusOK, map[string]string{"status": "ok"})
    }
}

Key points in this remediation:

• Include path parameters used for authorization (e.g., accountID) in the signed canonical string to prevent path-based tampering.
• Include a timestamp and a nonce to mitigate replay attacks; enforce short TTLs and server-side nonce tracking.
• Use constant-time comparison for signature verification to avoid timing side-channels.
• Fail closed: if any part of canonical construction or verification fails, reject the request before accessing business logic.

For automated validation in CI/CD, the middleBrick GitHub Action can be configured to fail builds if the computed risk score exceeds your defined threshold, ensuring that regressions in authentication logic are caught before deployment.