Washington (CNN) "Nearly all" the weapons systems that were being developed by the US military from 2012 to 2017 are vulnerable to cyber-attack, according to a new report by the Government Accountability Office.

The watchdog's report says the GAO "found that from 2012 to 2017, (Department of Defense) testers routinely found mission-critical cyber vulnerabilities in nearly all weapon systems that were under development."

During some of the tests, testers were able to hack into some of these complex weapons systems and take control over them "using relatively simple tools and techniques."

"In one case, it took a two-person test team just one hour to gain initial access to a weapon system and one day to gain full control of the system they were testing," the report said.

In some cases, the "weapon systems used commercial or open source software, but did not change the default password when the software was installed, which allowed test teams to look up the password on the internet and gain administrator privileges."

Read More