Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal') vulnerability in gleam-lang gleam allows a malicious package publisher to delete a directory outside the project's build directory when a developer builds a project.

A Hex package release's metadata carries an app field with the release's OTP application name. Gleam reads this field from the hex.pm release API when it downloads dependencies and uses it as the package's build directory name without validating it (hexmanifestpackage in compiler-cli/src/dependencies.rs, ProjectPaths::builddirectoryforpackage). When a dependency fails to compile, Gleam deletes that directory with removedirall. hex.pm didn't restrict the field to a directory name before 2026-10-12, so a publisher could set it to a value such as ../../../../target or an absolute path. When a project depends on such a release directly or transitively and the release fails to compile, gleam build, gleam check, gleam run, gleam test or gleam export deletes the directory the name resolves to. For a pure Gleam package no publisher code runs.

This issue affects gleam: from 0.32.4 before 1.19.2.