Stack Overflow in Echo Go with Hmac Signatures

When building an HTTP API in Go with the Echo framework, using HMAC signatures for request authentication is a common pattern to ensure integrity and origin verification. However, the combination of Echo’s flexible routing and middleware hooks with HMAC verification logic can create a window for request smuggling or body-replay issues when the request body is not handled consistently before signature validation.

In Echo, route handlers are invoked after middleware runs. If a middleware layer reads the request body (for example to compute an HMAC), but does not restore the body for downstream handlers, the actual handler may receive an empty or truncated body. This can lead to a Stack Overflow–style condition where the application logic assumes a parsed payload exists, but the effective parsed data is missing or mismatched, causing panics or incorrect authentication decisions.

More specifically, consider an endpoint that accepts JSON and uses an HMAC signature in a header (e.g., X-Signature) computed over the raw request body plus a shared secret. If the signature is verified after Echo’s middleware has already parsed the body into a struct, the runtime may not expose the raw bytes used for signing. If the client sends a body that triggers edge-case behavior in the framework’s body buffering (for example, very large or chunked payloads), the signature computed on the server may not match the client’s, and inconsistent handling may lead to denial of service or information exposure in error paths.

Additionally, Echo’s default body limit and buffering behavior can interact poorly with HMAC workflows if the request size exceeds configured limits. The framework may terminate the request early and forward an incomplete or nil body to the handler. If the HMAC verification step does not account for this and proceeds to compare signatures over incomplete data, the mismatch can cascade into unhandled panics or stack exhaustion patterns under load, effectively creating a stack exhaustion condition similar to a Stack Overflow exploit in more traditional stack-based languages.

These risks are not theoretical: mismatched body handling and signature scope have led to real-world authentication bypass and denial-of-service findings in APIs using Echo and similar frameworks. For example, consider a route defined as POST /transfer where the HMAC is computed over the JSON payload. If a middleware consumes the body for logging or transformation but does not rewind or replace io.ReadCloser, the handler signature verification may silently validate against an empty body, bypassing authentication while the application proceeds with missing data, triggering downstream faults.

To mitigate these issues in Echo Go, ensure that the raw request body is read once, preserved, and used consistently for both HMAC computation and handler logic. Do not rely on automatic body parsing before signature validation. Below are concrete, working examples demonstrating a secure pattern.

First, read and buffer the raw body in a middleware, compute the HMAC, and then restore the body so subsequent handlers can parse it safely.

import (
    "crypto/hmac"
    "crypto/sha256"
    "io"
    "net/http"

    "github.com/labstack/echo/v4"
)

func HmacMiddleware(secret []byte) echo.MiddlewareFunc {
    return func(next echo.HandlerFunc) echo.HandlerFunc {
        return func(c echo.Context) error {
            // Read raw body without closing the original request body
            body, err := io.ReadAll(c.Request().Body)
            if err != nil {
                return echo.NewHTTPError(http.StatusBadRequest, "failed to read body")
            }

            // Restore body so Echo and downstream handlers can read it
            c.Request().Body = io.NopCloser(io.BytesReader(body))

            // Compute HMAC over raw body
            mac := hmac.New(sha256.New, secret)
            mac.Write(body)
            expected := mac.Sum(nil)

            // Compare with header (use constant-time comparison)
            received := c.Request().Header.Get("X-Signature")
            if !hmac.Equal(expected, []byte(received)) {
                return echo.NewHTTPError(http.StatusUnauthorized, "invalid signature")
            }

            // Store body for later use if needed, or re-assign a reader again
            c.Set("rawBody", body)
            return next(c)
        }
    }
}

In your route handler, you can then safely parse the JSON payload knowing the raw bytes used for the HMAC match what the client sent:

type TransferRequest struct {
    From string `json:"from"`
    To   string `json:"to"`
    Amount int64 `json:"amount"`
}

func TransferHandler(c echo.Context) error {
    raw, exists := c.Get("rawBody")
    if !exists {
        return echo.NewHTTPError(http.StatusInternalServerError, "body not available")
    }

    var req TransferRequest
    if err := json.Unmarshal(raw.([]byte), &req); err != nil {
        return echo.NewHTTPError(http.StatusBadRequest, "invalid json")
    }

    // Proceed with business logic
    return c.JSON(http.StatusOK, map[string]string{"status": "ok"})
}

Key points in this remediation:

• Read the body once in middleware and restore it with io.NopCloser so Echo’s default body parsers work correctly.
• Compute the HMAC on the raw bytes before any JSON unmarshaling to avoid mismatches due to formatting differences (whitespace, field order).
• Use hmac.Equal for constant-time comparison to prevent timing attacks.
• Pass the buffered body to handlers via context rather than re-reading the request, ensuring consistency between verification and processing.

If you use the middleBrick CLI to scan this service, it can surface inconsistencies in how the request body is handled and flag missing body restoration as a finding, helping you align the runtime behavior with your HMAC design.