⚠️ Path traversal in Hex via unvalidated dependency app names (hexpm hex)

Description

Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal') vulnerability in hexpm hex allows a malicious package publisher to delete a directory outside the project's deps directory and extract package files into it when a developer fetches dependencies. A package release's requirements carry an app label that names the dependency's OTP application. Hex takes this label from the repository's signed registry and uses it as the dependency's application name without validating it (Hex.RemoteConverger.deps/2, Hex.Mix.to_lock/1). Mix derives the dependency's checkout directory as deps/<app>, and Hex.SCM.update/1 runs File.rm_rf!/1 on that directory and unpacks the package into it. hex.pm only checks the label's length, so a publisher can set it to a value such as ../../target. When a project depends on such a release directly or transitively, mix deps.get or mix deps.update deletes the directory the label resolves to and writes the labelled package's files there. This happens before any dependency code is compiled. This issue affects hex: from 0.6.0 before 2.5.2.