CVE-2025-54801

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.

• 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.

Kodem intelligence

1. What is CVE-2025-54801? CVE-2025-54801 is a high-severity security vulnerability in github.com/gofiber/fiber/v2 (go), affecting versions <= 2.52.8. It is fixed in 2.52.9.
2. Which versions of github.com/gofiber/fiber/v2 are affected by CVE-2025-54801? github.com/gofiber/fiber/v2 (go) versions <= 2.52.8 is affected.
3. Is there a fix for CVE-2025-54801? Yes. CVE-2025-54801 is fixed in 2.52.9. Upgrade to this version or later.
4. Is CVE-2025-54801 exploitable, and should I be worried? Whether CVE-2025-54801 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
5. What actually determines whether CVE-2025-54801 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
6. How do I fix CVE-2025-54801? Upgrade github.com/gofiber/fiber/v2 to 2.52.9 or later.