GitPython before 3.1.52 Environment Variable Exfiltration via clone_from

GitPython before 3.1.52 is vulnerable to environment-variable exfiltration in Repo.clone_from(). The caller-supplied remote URL is passed through Git.polish_url(), which on non-Cygwin platforms calls os.path.expandvars() on the URL before invoking git clone. An attacker who controls the clone URL can embed $NAME or ${NAME} tokens that are expanded to the values of the hosting process's environment variables (e.g., AWS_SECRET_ACCESS_KEY or GITHUB_TOKEN). The resulting URL, now containing the secret, is transmitted over the network to an attacker-controlled host during the clone attempt, disclosing the secret.