Hackers have been infiltrating Israeli military networks by using emails with attack software, researchers said Friday. They said the effort had been going on for about four months and was most likely carried out by Arabic-speaking computer programmers, Reuters reported.

Security researchers at Blue Coat Systems said that the effort appeared to be on a budget. Waylon Grange, the researcher who discovered the hacking effort, said much of the software used was taken from resources that already were available, rather than the hackers writing their own particular code. In one of the programs, Arabic was the default language.

Social engineering also played a role, with some of the emails the hackers sent including important news or a video clip of girls. Whether important information had been leaked or data stolen in the breach remained unclear, as Grange told Reuters he did not know whether this had happened.

The infiltration came days after an Israeli security expert warned that the next major terror attack would be carried out through hacking rather than physical attacks. Dr. Gabi Siboni, who heads the cybersecurity program at the Institute for National Security Studies in Israel, told the Times of Israel, "The next 9/11 will happen without suicide bombers aboard the plane with box-cutters, but will occur because of a cyber incident perpetrated by a terror organization."

The Israeli military said it was not aware of any hacking.

In a prior incident, in 2013, Arab hackers infiltrated Israeli government websites, including that of the parliament. The hacking came shortly after a group called Anonymous Arab posted a YouTube video warning of the attack. The head of Israel's National Cyber Directorate said at the time that "there has no been real damage, and the hackers don't have the tools to cause damage to the vital infrastructures."