They sign as Cybercaliphate. Last January, this name appeared on the YouTube and Twitter accounts of the United States military's central command, with Isis (Islamic State) propaganda. And, last week, Cybercaliphate resurfaced when hackers seized the Paris television network TV5Monde.
Cyber coups have emerged - with on-line videos of grisly executions - as a potent weapon in asymmetrical warfare, where small groups take on military giants like the US.
The new face of the cyber terror isn't jihadis brandishing AK-47 rifles and using suicide bombs, but someone with a laptop, an internet connection and hacking skills.
The identity of Cybercaliphate is hotly debated, but it is a high-tech cadre. The Syrian Electronic Army and the Iran-based Ajax Security Team, helped trail blaze this David-versus-Goliath battlefield. The SEA, which backs President Bashar al-Assad, has hacked the New York Times, Financial Times and other media websites critical of his regime. Word is that Isis is keen to attract more zealous young hackers.
Finding people with top cyber security skills - hard enough in the corporate world - is quickly becoming a vital element in this secretive, high-tech war.
"You're going to see the cyber skills set become the most pressing issue in cyber security," says Bryce Boland, a top technology expert for the Asia-Pacific region for FireEye, an American cyber security business.
Faced by individual hackers or shadowy state surrogates like APT (Advanced Persistent Threat) 30, believed to have operated from China for a decade, the US military has lowered its fitness standards to recruit geeks in the cyber race.
Three years ago US Defence Secretary Leon Panetta warned of a "cyber Pearl Harbour", after large US financial institutions were hacked and the Shamoon virus wrought havoc with Saudi Arabia and Qatar's energy infrastructure. The most serious scenario, he said, was a mass cyber attack coupled with a physical assault.
Cyber attacks by states or state surrogates are now a routine part of state espionage and an offensive military tool.
The US, backed by Israel, is believed to have launched the Stuxnet virus in 2010 against Iran's nuclear programme. A People Liberation Army unit has been accused of facilitating Chinese cyber attacks on the US. And North Korea was fingered for last year's Sony Studios hack, allegedly drawing US retaliation.
High on the list of concerns is a cyber attack that targets important infrastructure, such as energy grids, telecommunications or "air-gapped" military networks that are not hooked up to the internet.
These networks can be infiltrated by, say, infecting a USB stick with malware. In 2008 a US Defence Department hack jumped the air gap, as has APT 30, which FireEye says has been engaged in cyber espionage throughout Asia.
Last year's Global Economic Crime Survey found 39 per cent of financial service respondents were hacked.
Inadequate security and an increasing global reliance on internet-connected computers create low-hanging fruit for hackers, who now include Isis and other jihadi extremists.
This week, the US Government Accountability Office warned that suicidal hackers could crash commercial jets by breaching firewalls that separate in-flight Wi-Fi from aircraft avionics systems
Fighting jihadis with laptops can require a disproportionate response. A small army of security experts countered the hacks cited by Panetta, reportedly by Iranian Government surrogates like the Qassam Cyber Fighters.
So far, Isis has focused on propaganda. The TV5Monde attack hacked 11 channels, blacked out broadcasts and hijacked social media.
It was a sophisticated operation, and Boland says that if it was done by Isis, it shows the Islamists are honing "better tools and better trade craft".
The hack was payback for French carrier-launched air strikes against targets in Iraq and Syria, as part of the US-lead, anti-Isis coalition. Documents, said to be identity cards of relatives of French troops in Iraq, were posted on TV5Monde's Facebook page with threats.
"Soldiers of France, stay away from Islamic State," warned one message. "You have a chance to save your families, take advantage of it."
As with assaults in France by gunmen - most dramatically the Charlie Hebdo attacks in January - the TV5Monde hack was a chilling reminder that 21st century warfare has a global reach, far into any home front. No one is beyond the range of determined foes whose message is blunt: we know who, and where, you are.
Whereas al-Qaeda focused on physical terror attacks - 9/11 was the group's apogee - the tech-savvy warriors of Isis, operating from a nascent caliphate that is effectively a rogue state, have chillingly upped the ante.
Last month, they threatened Twitter co-founder Jack Dorsey and his staff. Posted on a site in Poland, the threat was written in Arabic and included a graphic with Dorsey in a rifle's crosshairs.
"We told you from the beginning it's not your war, but you didn't get it and kept closing our accounts on Twitter, but we always come back," it said. "But when our lions come and take your breath, you will never come back to life."
Could Isis murder a powerful Silicon Valley executive? Who knows? But as the US pounds Isis in Iraq, this new twist on asymmetrical warfare, vowing retaliation in America's high-tech heartland, means non-combatants half a world away from Middle East carnage may start looking over their shoulders, a propaganda coup for extremists who appear to have the edge.