cmd/go: remote command execution during "go get -u"

go get -u downloads, updates, and builds source code. It is not supposed to execute arbitrary code.

The go get command is vulnerable to remote code execution when executed with the -u flag and the import path of a malicious Go package, or a package that imports it directly or indirectly. Specifically, it is only vulnerable in GOPATH mode, but not in module mode (the distinction is documented at https://golang.org/cmd/go/#hdr-Module_aware_go_get). Using custom domains, it’s possible to arrange things so that a Git repository is cloned to a folder named .git by using a vanity import path that ends with /.git. If the Git repository root contains a HEAD file, a config file, an objects directory, a refs directory, with some work to ensure the proper ordering of operations, go get -u can be tricked into considering the parent directory as a repository root, and running Git commands on it. That will use the config file in the original Git repository root for its configuration, and if that config file contains malicious commands, they will execute on the system running go get -u.

Note that forbidding import paths with a .git element might not be sufficient to mitigate this issue, as on certain systems there can be other aliases for VCS state folders.

Thanks to Etienne Stalmans from the Heroku platform security team for discovering and reporting this issue.

This issue is CVE-2018-16873.