It was possible to improperly access the parent directory of an os.Root by opening a filename ending in "../". For example, Root.Open("../") would open the parent directory of the Root. This escape only permits opening the parent directory itself, not ancestors of the parent or files contained within the parent.
https://pkg.go.dev/vuln/GO-2026-4403
https://groups.google.com/g/golang-announce/c/UZoIkUT367A/m/5WDxKizJAQAJ