CVE-2025-54801: Fiber Susceptible to Crash via `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

go 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:

bash 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:

go 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.

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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.