Heap Overflow in Fiber with Hmac Signatures

A heap overflow in a Fiber application that uses Hmac Signatures can occur when user-controlled data influences memory allocation without proper bounds checking, and the Hmac verification step does not enforce strict length or type constraints. In Go, the Fiber web framework provides a fast, expressive HTTP server, but developers are responsible for validating and limiting input before it is processed further. When Hmac signatures are generated or verified using byte slices derived from request payloads, headers, or query parameters, an attacker can supply oversized or malformed data that leads to excessive memory growth or out-of-bounds writes during signature construction or comparison.

Consider a scenario where a request body is read into a buffer and then used both for Hmac computation and subsequent processing. If the application does not cap the size of the buffer, an attacker can send a very large payload that causes the underlying byte slice to expand on the heap. During Hmac signing, the signature is computed over the entire payload; if the payload is unexpectedly large, the runtime may allocate a larger backing array, and a bug in handling the slice length versus capacity can result in writing beyond allocated memory. This is a heap overflow condition in the language runtime, not a typical buffer overflow in C, but it can corrupt internal data structures and lead to crashes or information disclosure.

Hmac Signatures compound the issue because the verification path may attempt to compare the computed signature with the one provided in headers. If the comparison logic does not use constant-time algorithms and relies on simple byte-by-byte checks that short-circuit on mismatch, an attacker can infer timing differences. More critically, if the signature verification step does not validate the size of the payload before using it to derive a key or context, an oversized payload can cause repeated or large allocations, amplifying the impact of the overflow. Because Fiber encourages middleware chaining, an unchecked payload can propagate through multiple handlers, each potentially interacting with the Hmac logic and increasing the attack surface.

Real-world patterns that expose this include accepting JSON bodies with large nested structures, concatenating multiple header values to form a signing string, or using query parameters to influence key material without enforcing maximum lengths. Even when the Hmac implementation itself is correct, the surrounding Fiber route can pass an oversized or malformed input into the signing function, triggering the heap overflow indirectly. This combination of a high-performance framework, flexible routing, and cryptographic operations requires strict input validation and size limits at the entry point before any Hmac processing occurs.

To remediate heap overflow risks when using Hmac Signatures in Fiber, enforce strict size limits on all inputs that participate in signature generation or verification. Validate payload size before reading the body, and reject requests that exceed a reasonable threshold. Use fixed-size byte arrays where possible, and avoid concatenating unbounded values to form signing strings. The following patterns demonstrate safe handling in a Fiber application.

• Limit request body size globally or per-route and validate Hmac inputs explicitly:

// main.go
package main

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

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

const maxBodySize = 1024 // bytes

func verifyHmac(next fiber.Handler) fiber.Handler {
    return func(c *fiber.Ctx) error {
        // Enforce size limit before reading the body
        if c.Request().Header.ContentLength() > maxBodySize {
            return c.Status(fiber.StatusRequestEntityTooLarge).SendString("payload too large")
        }
        body := c.Body()
        if len(body) == 0 {
            return c.Status(fiber.StatusBadRequest).SendString("missing body")
        }
        providedSignature := c.Get("X-Signature")
        if providedSignature == "" {
            return c.Status(fiber.StatusBadRequest).SendString("missing signature")
        }
        key := []byte("my-32-byte-secret-key-for-hmac-sha256-example")
        mac := hmac.New(sha256.New, key)
        mac.Write(body)
        expected := mac.Sum(nil)
        if !hmac.Equal(expected, []byte(providedSignature)) {
            return c.Status(fiber.StatusUnauthorized).SendString("invalid signature")
        }
        return next(c)
    }
}

func main() {
    app := fiber.New()
    app.Post("/webhook", verifyHmac, func(c *fiber.Ctx) error {
        return c.SendString("ok")
    })
    app.Listen(":3000")
}

• When the signing string is built from multiple sources, enforce length caps on each component and avoid dynamic concatenation of user-controlled data:

// signing.go
package main

import (
    "crypto/hmac"
    "crypto/sha256"
    "fmt"
)

func buildAndVerify(data string, headerValue string, key []byte) (bool, error) {
    const maxComponentLen = 512
    if len(data) > maxComponentLen || len(headerValue) > maxComponentLen {
        return false, fmt.Errorf("input too long")
    }
    // Deterministic concatenation with fixed delimiter
    message := fmt.Sprintf("%s|%s", data, headerValue)
    mac := hmac.New(sha256.New, key)
    mac.Write([]byte(message))
    expected := mac.Sum(nil)
    // In practice, compare against a provided signature using hmac.Equal
    _ = expected // placeholder for real comparison
    return true, nil
}

These examples show how to integrate Hmac validation within Fiber middleware while capping input sizes and using constant-time comparison. By combining request size enforcement, strict validation, and safe cryptographic practices, you reduce the surface for heap overflow conditions related to Hmac Signatures in Fiber.