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Introduction

Fiber is a popular Express-inspired web framework built in the Go language, known for its performance and usability. However, a security vulnerability (CVE-2023-41338) was discovered, affecting certain versions (prior to 2.49.2) of Fiber where it fails to properly restrict access to localhost resources. This article aims to provide a detailed explanation of the vulnerability, its potential impact, and the necessary steps to mitigate the risk associated with this issue.

Vulnerability Details

CVE-2023-41338 specifically affects users who rely on the ctx.IsFromLocal method to restrict access to localhost requests. If exploited, it could potentially enable unauthorized access to resources designed to be exclusive to the localhost. A foreign host setting X-Forwarded-For: 127...1 in the header of a request to an affected server will result in a true value returned by ctx.IsFromLocal, granting the unauthorized access.

Take a look at an example code snippet demonstrating this issue

func isLocalhostRequest(c *fiber.Ctx) error {
  if !c.IsFromLocal() {
    return c.SendStatus(fiber.StatusUnauthorized)
  }
  return c.Next()
}

The function isLocalhostRequest is designed to restrict access only to localhost requests. The affected versions of Fiber would incorrectly return true when a request with X-Forwarded-For: 127...1 header is received from a foreign host.

The impact of this vulnerability is limited in scope to the affected GoFiber process and the specific resources restricted using the ctx.IsFromLocal method.

Solution and Upgrade Path

The Fiber team has addressed this vulnerability in the 2.49.2 version by patching the issue with commit b8c9ede6. You can find further details in the official commit. It is highly recommended that users upgrade to this patched version to secure their applications.

Here's a snippet from the patch to show the code changes

func (ctx *Ctx) IsFromLocal() bool {
  if ip := ctx.IP(); ip == "127...1" || ip == "::1" {
    return true
  }
  // Check X-Forwarded-For header only if app is set to trust header sources
  if ctx.app.Settings.TrustHeaderSources && ctx.get(values.XForwardedFor) == "127...1" {
    return true
  }
  return false
}

The updated version restricts the checks for the X-Forwarded-For header to only trusted header sources, preventing potential abuse of the header.

As of now, there are no known workarounds to remediate this vulnerability without upgrading to the patched 2.49.2 version.

Conclusion

CVE-2023-41338 is a vulnerability affecting some versions of GoFiber web framework. The issue is related to improper validation of requests within the ctx.IsFromLocal method, leading to potential unauthorized access to localhost resources. Upgrading to the latest and secure version 2.49.2 of Fiber is the recommended course of action to mitigate this vulnerability.

Timeline

Published on: 09/08/2023 19:15:00 UTC
Last modified on: 09/12/2023 19:12:00 UTC