The vulnerability lies in the salt.auth.pki module's authentication mechanism. Specifically, the auth function within this module is responsible for the flawed authentication process. The provided description clearly states that the module (and thus its auth function) performs improper PKI authentication by only checking a public certificate against a CA, without requiring proof of private key possession. The patches in commits 5ff18fd0ececdfd083ddce693c3ccef30e44f155 and d7cb64e44db5f82fd615373f5dca2eb1fb29bbab directly modify the salt.auth.pki.auth function to add a deprecation warning, explicitly stating that the module is insecure. This directly points to salt.auth.pki.auth as the function containing the vulnerable logic. While commit d7cb64e44db5f82fd615373f5dca2eb1fb29bbab includes many other changes, they appear to be broader security hardening measures rather than a direct fix for this specific PKI authentication flaw, which is addressed by deprecating the module.