Exposure of trigger tokens on project exports

HackerOne report #497144 by mishre on 2019-02-16, assigned to asaba:

Summary:
Any user can configure a trigger token for himself so that pipelines will be used to create CI/CD jobs on his own user context. When using the trigger token, Jobs created on the user's behalf will have access to all the resources(repositories) the user have (as explained here: https://docs.gitlab.com/ee/ci/triggers/#taking-ownership-of-a-trigger). Maintainers of the repository cannot usually see the trigger tokens for other users (only for themselves) as it will give them complete access (via ci-cd jobs) to perform actions on their behalf.

Description:
All maintainers can perform an export of the repository, which in turn will leak all the trigger tokens created for all the other maintainers of the repository through a file called project.json which contains different information for later restoring the project.

Steps To Reproduce:

1. Create a new repository.
2. Go to the Members section of the repository and add another maintainer (the attacker).
3. Go to the CI/CD settings of the project and create a trigger token.
4. Login as the attacker and go to the CI/CD settings of the project - you should not be able to see the trigger token.
5. Now create an export of the project.
6. Download the exported .tar.gz file and open a file called project.json
7. Look for the word "trigger" in the json file -> you now should be able to see the token at plaintext and use it to impersonate the victim.

Supporting Material/References:

• List any additional material (e.g. screenshots, logs, etc.)

Impact

An attacker can impersonate the victim and have access to all the repositories the victims has access to.

Documentation

• Update the user docs to explicitly call out the fact that trigger tokens are not exported for security reasons.

Implementation plan

• backend Remove :triggers from https://gitlab.com/gitlab-org/gitlab/blob/master/lib/gitlab/import_export/project/import_export.yml#L91,
• documentation Mark Pipeline Triggers as unsupported for GitLab in https://gitlab.com/gitlab-com/customer-success/professional-services-group/global-practice-development/migration/congregate/-/blob/master/customer/gitlab-migration-features-matrix.md#L42, Congregate tool should be able to still migrate triggers as it is using GitLab API and not File Export
• documentation Add Pipeline triggers to list in docs https://gitlab.com/gitlab-org/gitlab/blob/master/doc/user/project/settings/import_export.md#L133
• test Verify if exported project does not contain that token,