GHSA-rwj8-pgh3-r573: GitPython: Environment-variable exfiltration via os.path.expandvars() on Repo.clone_from() URL

Repo.clone_from() passes the caller-supplied remote URL through Git.polish_url(), which on every non-Cygwin platform calls os.path.expandvars() on the URL before handing it to git clone. An attacker who controls the URL argument — the documented use case for clone_from() in “import repository from URL” features of CI servers, git-hosting mirrors, and dependency scanners — can embed $NAME / ${NAME} tokens that are expanded server-side to the values of the hosting process’s environment variables. The resulting URL, now containing the secret, is transmitted over the network to the attacker-named host. This crosses the trust boundary between an untrusted remote URL and the server’s process environment, disclosing secrets such as AWS_SECRET_ACCESS_KEY or GITHUB_TOKEN with no precondition beyond the ability to submit a clone URL.