Citing US official sources, American media last week reported that the Army Cyber Command had crippled the Iranian Revolutionary Guard's air defence units that shot down a sophisticated drone on June 20.

Military computing security is usually "hardened" to defend against attack but highly-skilled computer scientists in cyber units of modern armies are always working to find a way in.

"The simplest way would be for a special forces member to plug in a USB (carrying a virus) to the right place," Loic Guezo of the French Information Security Club told AFP.

This is almost certainly how the well-known US-Israeli Stexnet virus was introduced in 2010, into the computers of Iran's nuclear complex, according to experts.

Iran at the time accused the US and Israel of using the virus to target its centrifuges used for uranium enrichment.

The Revolutionary Guards are believed to have since bolstered precautionary measures in a bid to isolate their military computer networks from the internet.

- 'Nothing is impenetrable' -

However, according to a military expert who requested anonymity, an "anti-aircraft defence system requires radars, control and command centres and ground-to-air missile sites to be inter-connected".

Those components are connected by intranet networks that "at one time or another" must be connected to the internet.

"In the past, there was no way to connect to a weapon system," said Guezo.

Today, however, he said most computer operating systems are commercial and vulnerable to attacks even if "everything is done to make them impenetrable".

"Nothing is impenetrable," said Guezo.

Modern cyber crime units, especially American and Israeli, have massive resources at their disposal and recruit high-level experts.

The Cyber Command became a fully-fledged combat unit within the US army in May with a budget running into billions of dollars.

In Israel, its renowned 8200 electronic warfare unit attracts the country's best talent.

When cyber attackers "have identified an entry point,", they "intrude the network with messages carrying hyper-aggressive malware which will at least partially cripple the air defence network", said an anonymous military specialist.

- Operation Orchard -

In March 2017, researcher Remy Hemez of the French Institute for International Relations (IFRI) told of how the Israeli army had used a programme called SUTER in 2007 to temporarily blind Syrian air defence radars.

Operation Orchard, according to Hemez, showed how cyber weapons can successfully be used in battle.

Israeli fighter jets had penetrated deep into Syrian air space to destroy a suspected nuclear facility after having disabled Syrian air defences.

Last week's US claim of an attack on Iranian missile launching systems would have required months or even years of preparation, said Guezo.

"You have to study the architecture of the equipment and then create attack plans," Guezo said.