Path traversal in rebar3 via unvalidated dependency app names

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.

1. hex.pm's registry record lists each release's dependencies as maps with package, requirement, optional, app and repository. The app value comes from the publisher's package metadata and hex.pm doesn't restrict it to a directory name. The record is signed with the repository's key after the publisher has chosen the value, so the signature doesn't constrain it.
2. r3_hex_registry:decode_package/3 decodes each dependency, including app.
3. rebar_packages:parse_deps/1 sets the dependency's name to the app value, falling back to the package name: {maps:get(app, D, Name), {pkg, Name, Constraint, undefined, undefined}}. The name is stored in #package.dependencies and passed through rebar_app_utils:update_source/3 and rebar_prv_install_deps:handle_dep/6.
4. rebar_app_utils:dep_to_app/7 builds the fetch directory with filename:join([DepsDir, Name]) and no check.
5. rebar_fetch:download_source_online/2 runs rebar_file_utils:rm_rf(filename:absname(FetchDir)) and then rebar_file_utils:mv(TmpDir, filename:absname(FetchDir)). filename:absname/1 doesn't collapse .. segments, and on Unix rm_rf/1 runs rm -rf through the shell with rebar_utils:escape_chars/1, which doesn't escape / or ., so the .. segments resolve outside the deps directory.
6. The delete and overwrite happen for the dependency named by app before its own rebar.config.script is consulted (later in rebar_fetch:download_source/2), and get-deps compiles nothing. rebar3 already runs publisher code during get-deps by evaluating a fetched package's rebar.config.script and installing a dependency's plugins; the app name adds a delete and overwrite outside _build for a dependency named by its parent package.

Reproduced with rebar3 3.27.0 on Erlang/OTP 29 against a local Hex repository, with origin and signature verification left on:

1. Build a signed local Hex repository serving parent 1.0.0, whose dependency is #{package => <<"evildep">>, requirement => <<"1.0.0">>, optional => false, app => <<"../../../../CANARY_OUTSIDE">>}, and evildep 1.0.0.
2. Create a project with {deps, [{parent, "1.0.0"}]} that uses the local repository, and a file CANARY_OUTSIDE/precious.txt one level above the project root.
3. Run rebar3 get-deps. It prints Fetching parent v1.0.0 and then Fetching ../../../../CANARY_OUTSIDE v1.0.0.
4. CANARY_OUTSIDE/precious.txt is gone and CANARY_OUTSIDE/ holds evildep's extracted files (hex_metadata.config, src/evildep.app.src). No scripts, plugins or hooks are in either package.

Affects projects whose dependency tree, directly or transitively, contains a package release whose registry metadata gives a dependency a crafted app name, when dependencies are fetched with rebar3 get-deps or another command that fetches dependencies. hex.pm rejects such names when a release is published since 2026-10-12, and every requirement app name in hex.pm's public registry on 2026-10-06 was a letter followed by letters, digits, _ and -. Packages on other Hex repositories are affected when the repository operator signs such metadata.

Upgrade rebar3 to 3.27.2 or later, for example with rebar3 local upgrade. rebar3 then fails with an error when a dependency's name, including the app name a package release gives a dependency, isn't a letter followed by letters, digits, _ and -.