Fiber, a powerful Express-inspired web framework built for Go, is widely used to develop blazing fast APIs and web applications. However, a significant vulnerability (CVE-2023-41338) was found in versions before 2.49.2 affecting the ctx.IsFromLocal method — a critical check many developers rely on to restrict certain actions to requests coming from localhost. Let’s break down what went wrong, how attackers can exploit it, and how you can fix it.

What is the Vulnerability?

Normally, when you only want to allow access to sensitive resources (like admin dashboards or debugging endpoints) from the same machine as your server (localhost), you might use code like:

if ctx.IsFromLocal() {
    // Allow sensitive action
} else {
    ctx.Status(403).SendString("Forbidden")
}

ctx.IsFromLocal() is supposed to return true only if the request is from localhost (i.e., the server itself).

The Problem

In all versions of Fiber before 2.49.2, ctx.IsFromLocal() trusts the user-supplied X-Forwarded-For HTTP header without validation. If an attacker sends a request from *anywhere in the world* and sets this header to "127...1", the method mistakenly believes it came from localhost.

That means the following simple curl request could bypass your local-only restrictions

curl -H "X-Forwarded-For: 127...1" http://your-fiber-server.local/sensitive-path

If your app relies on ctx.IsFromLocal() for authorization, this lets anyone access your "localhost-only" endpoints.

Let’s say you have this vulnerable route

app.Get("/only-local", func(ctx *fiber.Ctx) error {
    if ctx.IsFromLocal() {
        return ctx.SendString("Secret data for localhost!")
    }
    return ctx.Status(403).SendString("Go away!")
})

The attacker simply runs

curl -H "X-Forwarded-For: 127...1" http://target:300/only-local

Result: They see Secret data for localhost! — even though they're NOT on localhost!

Technical Details

- Vulnerable Function: ctx.IsFromLocal()

Vulnerable Fiber Versions: < 2.49.2

- Patched Version: 2.49.2 (commit b8c9ede6, see here)
- CVE ID: CVE-2023-41338 MITRE page
- Original Advisory: GitHub Security Advisory

Impact

- Who is affected? Anyone using Fiber <2.49.2 who relies on ctx.IsFromLocal() to protect routes or actions.
- What’s the risk? Unauthorized access to resources intended for localhost only — e.g., admin backdoors, debug endpoints, dangerous controls.
- Scope: Limited to the running Fiber process. (If your process controls something critical, this is very bad.)

Remediation

No workaround exists except upgrading.

`go

require github.com/gofiber/fiber/v2 v2.49.2

`bash

go get github.com/gofiber/fiber/v2@latest

References

- Fiber 2.49.2 Release Notes
- Security Advisory: GHSA-m499-rrc4-w65j
- CVE-2023-41338 (MITRE)
- Fix commit b8c9ede6

Closing Thoughts

Relying on client-controlled headers—like X-Forwarded-For—without validation can be dangerous. This CVE highlights the critical importance of keeping open source software up to date, especially when it comes to security-sensitive functions.
If you use Fiber and depend on ctx.IsFromLocal(), upgrade ASAP! There is no known workaround.

If you want to stay on top of security issues, always watch for new releases and advisories for your third-party libraries.

Timeline

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