Remote Command Execution (RCE) in project import

This is from the Recurity whitebox testing. I've verified that this works. You can even use:

ssh://-oProxyCommand=[snip]/a

To execute arbitrary shell commands using backticks.

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Via the Import Repository functionality it is possible to execute commands on the GitLab server under the git user.

Reproduction steps:

1. Click New Project, fill out a project name.
2. Click git Repo by URL.
3. Paste a URL like ssh://-oProxyCommand=[snip]/a.
4. Click Create project

Now the ruby -esleep 1000 can be observed running on the GitLab server under the git user.

As a short term mitigation gitlab-ee/app/validators/addressable_url_validator.rb should exclude the ssh entry in DEFAULT_OPTIONS = { protocols: %w(http https ssh git) }.freeze.

However the main problem lies in git clone, we've investigated this issue a bit further, and figured that it's possible to create git repositories which will execute arbitrary commands upon recursive checkout. If you like to we can report this issue to the git maintainers.

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