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angular-cn/aio/aio-builds-setup/docs/overview--security-model.md

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Overview - Security model

Whenever a PR job is run on Travis, we want to build angular.io and upload the build artifacts to a publicly accessible server so that collaborators (developers, designers, authors, etc) can preview the changes without having to checkout and build the app locally.

This document discusses the security considerations associated with uploading build artifacts as part of the CI setup and serving them publicly.

Security objectives

  • Prevent uploading arbitrary content to our servers. Since there is no restriction on who can submit a PR, we cannot allow any PR's build artifacts to be uploaded.

  • Prevent overwriting other peoples uploaded content. There needs to be a mechanism in place to ensure that the uploaded content does indeed correspond to the PR indicated by its URL.

  • Prevent arbitrary access on the server. Since the PR author has full access over the build artifacts that would be uploaded, we must ensure that the uploaded files will not enable arbitrary access to the server or expose sensitive info.

Issues / Caveats

  • Because the PR author can change the scripts run on CI, any security mechanisms must be immune to such changes.

  • For security reasons, encrypted Travis variables are not available to PRs, so we can't rely on them to implement security.

Implemented approach

In a nutshell

The implemented approach can be broken up to the following sub-tasks:

  1. Verify which PR the uploaded artifacts correspond to.
  2. Determine the author of the PR.
  3. Check whether the PR author is a member of some whitelisted GitHub team.
  4. Deploy the artifacts to the corresponding PR's directory.
  5. Prevent overwriting previously deployed artifacts (which ensures that the guarantees established during deployment will remain valid until the artifacts are removed).
  6. Prevent uploaded files from accessing anything outside their directory.

Implementation details

This section describes how each of the aforementioned sub-tasks is accomplished:

  1. Verify which PR the uploaded artifacts correspond to.

    We are taking advantage of Travis' JWT addon. By sharing a secret between Travis (which keeps it private but uses it to sign a JWT) and the server (which uses it to verify the authenticity of the JWT), we can accomplish the following: a. Verify that the upload request comes from Travis. b. Determine the PR that these artifacts correspond to (since Travis puts that information into the JWT, without the PR author being able to modify it).

    Note: There are currently certain limitation in the implementation of the JWT addon. See the next section for more details.

  2. Determine the author of the PR.

    Once we have securely associated the uploaded artifaacts to a PR, we retrieve the PR's metadata - including the author's username - using the GitHub API. To avoid rate-limit restrictions, we use a Personal Access Token (issued by @mary-poppins).

  3. Check whether the PR author is a member of some whitelisted GitHub team.

    Again using the GitHub API, we can verify the author's membership in one of the whitelisted/trusted GitHub teams. For this operation, we need a PErsonal Access Token with the read:org scope issued by a user that can "see" the specified GitHub organization. Here too, we use token by @mary-poppins.

  4. Deploy the artifacts to the corresponding PR's directory.

    With the preceeding steps, we have verified that the uploaded artifacts have been uploaded by Travis and correspond to a PR whose author is a member of a trusted team. Essentially, as long as sub-tasks 1, 2 and 3 can be securely accomplished, it is possible to "project" the trust we have in a team's members through the PR and Travis to the build artifacts.

  5. Prevent overwriting previously deployed artifacts.

    In order to enforce this restriction (and ensure that the deployed artifacts validity is preserved throughout their "lifetime"), the server that handles the upload (currently a Node.js Express server) rejects uploads that target an existing directory. Note: A PR can contain multiple uploads; one for each SHA that was built on Travis.

  6. Prevent uploaded files from accessing anything outside their directory.

    Nginx (which is used to serve the uploaded artifacts) has been configured to not follow symlinks outside of the directory where the build artifacts are stored.

Assumptions / Things to keep in mind

  • Each trusted PR author has full control over the content that is uploaded for their PRs. Part of the security model relies on the trustworthiness of these authors.

  • If anyone gets access to the PREVIEW_DEPLOYMENT_TOKEN (a.k.a. NGBUILDS_IO_KEY on angular/angular) variable generated for each Travis job, they will be able to impersonate the corresponding PR's author on the preview server for as long as the token is valid (currently 90 mins). Because of this, the value of the PREVIEW_DEPLOYMENT_TOKEN should not be made publicly accessible (e.g. by printing it on the Travis job log).

  • Travis does only allow specific whitelisted property names to be used with the JWT addon. The only known such property at the time is SAUCE_ACCESS_KEY (used for integration with SauceLabs). In order to be able to actually use the JWT addon we had to name the encrypted variable SAUCE_ACCESS_KEY (which we later re-assign to NGBUILDS_IO_KEY).