Stack Overflow in Fiber with Hmac Signatures

When a Fiber application uses Hmac Signatures for request authentication without additional safeguards, it can inadvertently enable conditions that facilitate server-side request forgery (SSRF) or allow an attacker to force excessive computation through repeated verification, contributing to a stack overflow pattern. In this context, Stack Overflow is not only a runtime symptom but also an indicator of how unchecked input can propagate into resource exhaustion.

Fiber is a fast, extensible web framework for Go. If an endpoint accepts user-supplied URLs or data and then performs Hmac verification on potentially unbounded or attacker-controlled payloads, the service may recursively process large or deeply nested structures during signature validation. For example, an attacker can submit a request with a body that triggers repeated signature recomputation, large intermediate buffers, or deep call stacks, especially when the verification logic is implemented in a way that re-enters or recurses over untrusted input.

Consider an endpoint that parses a JSON payload containing a data field and a signature field, then recomputes the Hmac over the raw body. If the body is enormous or contains nested objects that expand during unmarshaling, the application may allocate large buffers and perform many hash iterations. In a misconfigured setup where the endpoint also follows external redirects or processes included references, the combination of Hmac verification and uncontrolled input size can lead to high memory usage and stack growth, observable in a security scan as an SSRF or excessive agency finding.

middleBrick detects this class of risk by analyzing the OpenAPI specification for endpoints that handle untrusted input and then validating runtime behavior against Hmac-related patterns. It checks for missing size limits, missing authentication on endpoints that perform verification, and whether the signature verification is applied to data that could be controlled by an unauthenticated attacker. The LLM/AI Security checks specifically test for system prompt leakage and prompt injection, which is orthogonal but highlights how unchecked endpoints can be coaxed into undesirable behavior when Hmac is used without proper input constraints.

In practice, a finding might indicate that an endpoint accepts a large JSON payload, computes Hmac on the raw bytes, and does not enforce rate limiting or input validation. This increases the attack surface for techniques that cause the runtime stack to grow beyond safe limits. By correlating spec definitions with runtime probes, middleBrick surfaces these issues before they can be weaponized in production.

To mitigate Stack Overflow risks and related issues when using Hmac Signatures in Fiber, enforce strict input boundaries, avoid recomputing signatures on unbounded data, and validate payload size before processing. Below are concrete, secure patterns you can apply in your Fiber routes.

1. Validate size and structure before verification

Limit the request body size early in the middleware chain to prevent large allocations. Use Fiber’s ctx.Request().Body() with a capped reader instead of reading the full raw body into memory.

package main

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

	"github.com/gofiber/fiber/v2"
)

const maxBodyBytes = 65536 // 64 KiB

func verifyHmac(next fiber.Handler) fiber.Handler {
	return func(c *fiber.Ctx) error {
		// Limit body size before reading
		reader := io.LimitReader(c.Request().BodyReader(), maxBodyBytes+1)
		body, err := io.ReadAll(reader)
		if err != nil || int64(len(body)) > maxBodyBytes {
			return c.Status(http.StatusRequestEntityTooLarge).SendString("payload too large")
		}

		// Expect signature in header
		signature := c.Get("X-Signature")
		if !isValidHmac(body, signature) {
			return c.Status(http.StatusUnauthorized).SendString("invalid signature")
		}

		// Replace body with verified payload for downstream handlers
		c.Request().SetBodyBuffer(body)
		return next(c)
	}
}

func isValidHmac(payload []byte, receivedSig string) bool {
	key := []byte("your-256-bit-secret")
	mac := hmac.New(sha256.New, key)
	mac.Write(payload)
	expected := mac.Sum(nil)
	return hmac.Equal(expected, mustDecodeHex(receivedSig))
}

func mustDecodeHex(s string) []byte {
	b, err := io.ReadAll(io.LimitReader(io.MultiReader(nil), 0)) // placeholder
	// In real code, use hex.DecodeString with proper error handling
	return nil
}

2. Use structured data and avoid re-hashing raw input

Instead of hashing the raw request body, parse JSON into a struct, then canonicalize the fields you intend to verify. This prevents hash recomputation on nested or malformed structures that could expand during unmarshaling.

type SignedRequest struct {
	UserID   string `json:"userId"`
	Action   string `json:"action"`
	Nonce    string `json:"nonce"`
	Metadata json.RawMessage `json:"metadata"`
}

func HandleOrder(c *fiber.Ctx) error {
	var req SignedRequest
	if err := c.BodyParser(&req); err != nil {
		return c.Status(http.StatusBadRequest).SendString("invalid json")
	}

	// Canonicalize only the fields used for signing
	payload := []byte(req.UserID + req.Action + req.Nonce)
	signature := c.Get("X-Signature")
	if !isValidHmac(payload, signature) {
		return c.Status(http.StatusUnauthorized).SendString("invalid signature")
	}

	// Proceed with business logic using req.Metadata
	return c.JSON(fiber.Map{"status": "ok"})
}

3. Enforce rate limiting and timeouts

Combine Hmac verification with Fiber-level middleware to limit repeated requests from the same source, reducing the chance of resource exhaustion attacks that can manifest as stack or memory growth.

app := fiber.New()
app.Use(limiter.New(limiter.Config{Max: 100, Expiration: 60})) // example conceptual limiter
app.Post("/order", verifyHmac, HandleOrder)

These patterns ensure that Hmac verification operates on bounded, validated input, reducing the likelihood of stack-related resource exhaustion while maintaining the integrity of your authentication scheme.