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Keeping your dependencies close

Background

When deploying an application or service, many projects run npm install which can cause problems. James Shore discusses the problem and several solutions, none of which are ideal.

  • Network trouble reaching registry.npmjs.org becomes a single point of failure.
  • An extra npm shrinkwrap step is necessary to ensure that the versions used during testing are the same as the versions deployed (Shore's analysis predates package locks), or
  • Developers check node_modules into revision control which may include architecture-specific binaries.
  • Local changes may be silently lost when re-installed on a dev machine or on upgrade.

Many organizations use tools to manage a local replica.

  • Artifactory is a language agnostic dependency manager that supports Node.
  • Sinopia is a Node specific repository server.
  • Verdaccio is fork of Sinopia.
  • Yarn is a package manager backed by the same registry.npmjs but which can be pointed at an offline mirror. The offline mirror can have multiple tarballs per module to deal with architecture specific builds. Its --offline mode prevents falling back to central, though does not prevent network fetches by module scripts.

Node's security working group has a process for managing vulnerabilities in third-party code.

Problem

Threats: 0DY MTP

Security teams needs to match vulnerability reports with projects that use affected modules so that they can respond to zero days. Centralizing module installation allows them to figure out whether a report affects a module.

Large organizations with dedicated security specialists need to be able to locally patch security issues or critical bugs and push to production without waiting for upstream to push a new version. When someone in the organization discovers a vulnerability in a third-party module, they should disclose it to the third-party maintainer, but they should not wait before protecting end users who would be at risk if an attacker independently discovered the same vulnerability.

Success Criteria

We can have a reliable pipeline from the central repository, through local repositories and to deployed services if:

  • A failure in registry.npmjs.org does not lead to compromise or denial of service by npm install during deployment, and/or
  • npm install is not necessary for deployment.

and

  • access to registry.npmjs.org is not necessary to publish a patch to an open source module as seen within an organization.

and

  • installing or deploying a module locally cannot abuse publish privileges, and/or
  • an organization can limit its exposure to compromise of registry.npmjs.org, and ideally vice-versa.

and

  • installation scripts only affect node_modules so cannot compromise local repositories, abuse commit privileges, or plant trojans.

Existing solutions

Having a local replica simplifies deploying targeted patches to affected projects. When responding, security specialists might develop a patch before upstream. They may be able to take into account how their products use the module to produce a targeted patch faster than upstream maintainers who have greater or less-well-understood backwards compatibility constraints.

Keeping a local replica narrows the window for MTP attacks. Someone trying to inject malicious code has to have it up and available from registry.npmjs.org at the time the install script pulls it down which is hard for an attacker to predict. There is a monoculture tradeoff — having a smaller number of versions across all projects increases the potential reach of such an attack once successfully executed. Centralized monitoring and reporting tilts in the defenders' favor though.

Incident Response

There is one piece that isn't provided directly by the local replica providers aboce; security responders need a way to relate vulnerability reports to affected projects when a zero day clock starts ticking so they can figure out whom to notify.

  • If an organization shares revision control across all projects, then responders can find all package.jsons and use git commit logs to identify likely points of contact. Much of this is scriptable.
  • If an organization archives all production bundles before deployment, then tools can similarly scan archived bundles for package.json.
  • If an organization has an up-to-date database of projects with up-to-date links to revision control systems, then security teams may be able to automate scanning as above. Some managers like to have "skunkworks" projects that they keep out of project databases. Managers should be free to use codenames, but security teams need to ensure that "unlisted" doesn't mean "not supportable by incident response."
  • If none of the above work, security teams will need to maintain a database so that they have it when they need it. If the local replica is on a shared file system mount, then access logs may be sufficient. If not, instrumenting yarn, may be the only option.

Managing a Local Replica

If you don't have access to a commercial solution, some tooling can make it easier to transition to and maintain a local replica. We assume yarn below, but there are free versions of others which may do some of this out of the box.

  • Developers' muscle memory may cause them to invoke npm instead of yarn so on a developer machine $(which npm) run in an interactive shell should halt and remind the developer to use yarn instead. Presubmit checks should scan scripts for invocations of npm to remind developers to use yarn. It may be possible to use a project specific .npmrc with flags that cause it to dry-run or dump usage and exit, but this would affect non-interactive scripts so tread carefully.
  • A script can aid installing new modules into the local replica. It should:

    1. Run yarn install --ignore-scripts to fetch the module content into a revision controlled repository
    2. Build the module tarballs. (See below)
    3. Check the revision controlled portion and any organization-specific metadata into revision control
    4. File a tracking issue for review of the new module, so that code quality checks can happen in parallel with the developers test-driving the module and figuring out whether it really solves their problem.
    5. Optionally, yarn adds the module to the developer's package.json.
  • Developers shouldn't have direct write access to the local replica so that malicious code running on a single developer's workstation cannot compromise other developers via the local replica.

Finally, all Node.js projects need to have a symlink to the organization's .yarnrc at their root that points to the local replica.

Running install script safely

Running {pre-,,post-}install scripts without developer privileges prevents malicious code (see MTP) from:

  • Modifying code in a local repository.
  • Committing code as the developer possibly signing commits with keys available to ssh-agent.
  • Adding scripts to directories on a developer's $PATH.
  • Abusing npm login or yarn login credentials.

Ideally one would run these on a separate sandboxed machine. Many organizations have access to banks of machines that test client-side JavaScript apps by running instrumented browsers and include Windows boxes for testing IE, and MacOS boxes for testing Safari. These banks might also run install scripts without any developer privileges and with an airgap between the install scripts and source code files.

If that doesn't work, running install scripts via sudo -uguest where guest is a low-privilege account makes it harder for the install script to piggyback on the developer's private keys.

Proposed Solutions

A local replica manager should make it easy to:

  • Locally cache npm packages so that an interruption in service by registry.npmjs doesn't affect the ability to deploy a security update to existing products.
  • Cherrypick versions from registry.npmjs so that reviewers can exercise oversight, and remove versions with known, security-relevant regressions.
  • Publish one's own local patches to packages in the global namespace, so that incident responders can workaround zero-days without waiting for upstream.
  • Associate organization specific metadata with packages and versions so that the organization can aggregate lessons learned about specific dependencies.
  • Cross-compile binaries so that developers do not have to run installation scripts on their own machines.

The local repository providers mentioned above address many of these, but we have not comprehensively evalated any of them.

Cherrypicking a version should not require using a tool other than npm or yarn. Cherrypicking a version when npm communicates directly with registry.npmjs should be a no-op, so the npm interface could support cherrypicking.

Existing tools do not prevent abuse of developer privileges by install scripts. The first tool to do so should be preferred by security conscious organizations.

Ideally npm and yarn would be configurable so that they could delegate running installation script to a local replica manager. We would like to see local replica managers compete on their ability to do so securely. We realize that this is no small change, but abuse of developer privileges can directly affect source base integrity.

If an npm configuration could opt into sending the project name from package.json then local replica managers could make it easier for incident responders to find projects affected by a security alert for a specific module.

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