CORS localhost requests are considered invalid host and get blocked #5603

Bug Description

If webpack-dev-server gets "no-cors cross-site" requests, localhost is not considered an allowed host and a 403 Forbidden is returned.

By explicitly adding allowedHosts: ['localhost'] to the webpack config, the bug is circumvented.

Link to Minimal Reproduction and step to reproduce

In the following project, an iframe is constructed from a blob HTML string. The iframe contains a script tag which refers to a resource URL which is hosted by webpack dev server.

https://github.com/RedMser/bug-webpack-dev-server-corb

See the readme in the repo for reproduction steps.

Expected Behavior

Requests to localhost should succeed, even if they originate from a "no-cors cross-site" context.

Actual Behavior

The request fails with a 403 Forbidden response.

Environment

System:
    OS: Windows 11 10.0.26100
    CPU: (4) x64 Intel(R) Xeon(R) Platinum 8462Y+
    Memory: 2.84 GB / 19.00 GB
  Binaries:
    Node: 24.11.1 - C:\nvm4w\nodejs\node.EXE
    npm: 11.6.2 - C:\nvm4w\nodejs\npm.CMD
  Browsers:
    Chrome: 141.0.7390.123
    Edge: Chromium (141.0.3537.57)
    Firefox: 140.5.0 - C:\Program Files\Mozilla Firefox\firefox.exe
    Internet Explorer: 11.0.26100.1882
  Packages:
    webpack: ^5.102.1 => 5.102.1
    webpack-cli: ^6.0.1 => 6.0.1
    webpack-dev-server: ^5.2.2 => 5.2.2

Is this a regression?

None

Last Working Version

No response

Additional Context

Responsible is the cross-origin-header-check middleware.
In particular, isValidHost calls isHostAllowed which contains following comment:

According to this outdated comment, localhost should be always allowed, but there is no code that actually ensures this here!
isValidHost does check for localhost among other things, but validateHost == false in this particular instance.

It might be caused by the refactor in 6045b1e but I did not investigate further. This used to work in the past so it's likely a regression, let me know if more info is needed.