CVE-2021-39135: UNIX Symbolic Link (Symlink) Following in @npmcli/arborist

Impact

Arbitrary File Creation, Arbitrary File Overwrite, Arbitrary Code Execution

@npmcli/arborist, the library that calculates dependency trees and manages the nodemodules folder hierarchy for the npm command line interface, aims to guarantee that package dependency contracts will be met, and the extraction of package contents will always be performed into the expected folder.

This is accomplished by extracting package contents into a project's nodemodules folder.

If the nodemodules folder of the root project or any of its dependencies is somehow replaced with a symbolic link, it could allow Arborist to write package dependencies to any arbitrary location on the file system.

Note that symbolic links contained within package artifact contents are filtered out, so another means of creating a nodemodules symbolic link would have to be employed.

1. A preinstall script could replace nodemodules with a symlink. (This is prevented by using --ignore-scripts.) 2. An attacker could supply the target with a git repository, instructing them to run npm install --ignore-scripts in the root. This may be successful, because npm install --ignore-scripts is typically not capable of making changes outside of the project directory, so it may be deemed safe.

Patches

2.8.2 (included in npm v7.20.7 and above)

Workarounds

Do not run npm install on untrusted codebases, without first ensuring that the nodemodules directory in the project is not a symbolic link.

Fix

Prior to extracting any package contents, the nodemodules folder into which it is extracted is verified to be a real directory. If it is not, then it is removed.

Caveat: if you are currently relying on creating a symbolic link to the nodemodules folder in order to share dependencies between projects, then that will no longer be possible. Please use the npm link command, explicit file:... dependencies, and/or workspaces to share dependencies in a development environment.