Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal') vulnerability in erlang rebar3 allows a malicious package publisher to delete a directory outside the project's _build directory and extract package files into it when a developer fetches dependencies.

A package release's dependencies in a Hex registry carry an app field that names the dependency's OTP application. rebar3 uses this field as the dependency's name without validating it (rebar_packages:parse_deps/1) and builds the dependency's directory from that name (rebar_app_utils:dep_to_app/7). rebar_fetch:download_source_online/2 deletes that directory with rm -rf and moves the fetched package into it. 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. When a project depends on such a release directly or transitively, rebar3 get-deps deletes the directory the name resolves to and writes the dependency's files there. This happens before the dependency's own rebar.config.script is evaluated, and get-deps compiles nothing.

This issue affects rebar3: from 3.7.0-rc1 before 3.27.2.