CVE-2026-67322: GitPython before 3.1.52 Environment Variable Exfiltration via clone_from

GitPython before 3.1.52 is vulnerable to environment-variable exfiltration in Repo.clonefrom(). The caller-supplied remote URL is passed through Git.polishurl(), 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., AWSSECRETACCESSKEY or GITHUBTOKEN). The resulting URL, now containing the secret, is transmitted over the network to an attacker-controlled host during the clone attempt, disclosing the secret.