Buffer Overflow in Fiber with Hmac Signatures

A buffer overflow in a Fiber-based application that also uses Hmac Signatures can arise when unchecked input sizes are processed before signature validation, or when the code that reads or parses the request body does not respect declared size limits. Even if the Hmac is verified, an oversized payload can still overflow buffers in the HTTP server or in custom parsing logic before the application reaches the signature check, leading to crashes or potentially allowing attacker-controlled data to influence execution flow.

Consider a Fiber route that reads the raw body for Hmac verification without a length guard. An attacker can send a very large request body, causing a stack or heap buffer to be overrun when the application copies bytes into a fixed-size slice or C-based buffer via a CGO call. Because Fiber is a fast, minimalist framework, developers sometimes write low-level read or copy operations for performance or to interface with legacy code, increasing the risk if bounds checks are omitted.

Real-world patterns that can expose this include:

• Reading the entire body into a fixed-size byte array without checking Content-Length or chunk size.
• Using C functions via CGO to compute Hmac on a raw buffer that is not validated for maximum expected length.
• Parsing headers or body with custom byte-level logic that assumes a maximum size but does not enforce it during streaming reads.

An attacker can craft a request with a valid Hmac but an oversized body, causing the overflow before the application rejects the request. While the Hmac ensures integrity for the attacker’s chosen payload, the overflow may corrupt stack variables, overwrite return addresses, or crash the service. This combination is dangerous because the signature appears legitimate, which can mislead developers into focusing only on cryptographic correctness and neglecting input size enforcement.

To assess this risk using middleBrick, you can run a scan against your Fiber endpoint; the tool checks for missing length validation and unsafe consumption patterns across 12 security checks, including Unsafe Consumption and Input Validation, and highlights findings with severity and remediation guidance. middleBrick supports OpenAPI/Swagger 2.0/3.0/3.1 spec analysis with full $ref resolution, cross-referencing spec definitions with runtime behavior to surface inconsistencies such as missing size constraints.

Remediation centers on enforcing strict size limits before processing or verifying Hmac, and using safe abstractions that avoid low-level buffer manipulation. In Fiber, prefer the built-in body parsers with explicit limits, or if you must read raw data, validate Content-Length and cap the number of bytes read.

Example: safe Hmac verification in Fiber with a maximum body size and constant-time comparison.

const maxBodySize = 1024 * 256 // 256 KiB limit

func verifyHmac(next fiber.Handler) fiber.Handler {
    return func(c *fiber.Ctx) error {
        // Enforce maximum body size before reading
        if c.Request().Header.ContentLength() > maxBodySize {
            return c.Status(fiber.StatusRequestEntityTooLarge).SendString("payload too large")
        }

        // Read body safely within the limit
        body := c.Request().Body()
        if len(body) > maxBodySize {
            return c.Status(fiber.StatusRequestEntityTooLarge).SendString("payload too large")
        }

        // Retrieve the provided Hmac header
        provided := c.Get("x-signature")
        if provided == "" {
            return c.Status(fiber.StatusBadRequest).SendString("missing signature")
        }

        // Compute Hmac using a secure key; avoid passing secret as plain string in production
        key := []byte(os.Getenv("HMAC_SECRET"))
        mac := hmac.New(sha256.New, key)
        mac.Write(body)
        expected := hex.EncodeToString(mac.Sum(nil))

        // Constant-time comparison to avoid timing attacks
        if !hmac.Equal([]byte(expected), []byte(provided)) {
            return c.Status(fiber.StatusUnauthorized).SendString("invalid signature")
        }

        return next(c)
    }
}

// Usage in a route
app.Post("/webhook", verifyHmac, func(c *fiber.Ctx) error {
    return c.SendString("verified")
})

Key points:

• Check ContentLength early and reject requests larger than a defined threshold (e.g., 256 KiB).
• Use hmac.Equal for comparison to prevent timing side-channels.
• Avoid reading the body multiple times; store it in a bounded buffer if you need to reuse it.
• If you rely on CGO or custom byte manipulation, validate lengths before passing pointers to C functions.

middleBrick can be integrated into your workflow via the CLI (middlebrick scan <url>) to detect missing length validations and unsafe consumption patterns. For CI/CD, the GitHub Action can fail builds if the score drops below your chosen threshold, and the MCP Server lets you scan APIs directly from your IDE when working on Fiber services.