I've actually seen cases where, once in the knickers, intruders have patched all the exploits, including the one that got them inside in the first place so they could operate without worrying about other hacks.
“The Russians, Chinese, and North Koreans are scanning billions of ports per second 24/7/365. Once they realized what they were into, they’d have taken active measures to freeze the amateurs out.”
Ah, but we know Guccifer got in anyway, and he was not a sophisticated hacker. So they obviously didn’t seal the thing up very tightly if they were able to get in first.