From a new article by Robert N. Charette in IEEE Spectrum, Open-Source Warfare:
To understand open-source warfare, it's instructive to revisit Eric S. Raymond's 1997 manifesto, The Cathedral and the Bazaar, in which he describes how a large community of open-source software hackers created the operating system Linux.
“Linux is subversive,” Raymond wrote. “Who would have thought even five years ago  that a world-class operating system could coalesce as if by magic out of part-time hacking by several thousand developers scattered all over the planet, connected only by the tenuous strands of the Internet?” He likened the rise of Linux to the public marketplace of the bazaar. The programmers agreed to observe a few simple principles but were otherwise free to innovate and create. Raymond contrasted that style with the “cathedral” approach to software, in which a single organization, using highly planned, sequentially structured steps, maintained tight managerial control over every aspect of the process.
Eventually, the open-source culture would triumph over the proprietary world, Raymond argued, not because it was morally right “but simply because the closed-source world cannot win an evolutionary arms race with open-source communities that can put orders of magnitude more skilled time into a problem.”
In studying the behaviors of insurgencies in Iraq and elsewhere, as well as organized-crime syndicates and other groups, [John Robb, a counterterrorism expert and author of the book Brave New War] noticed the many parallels to the open-source model in software. In addition to working in counterterrorism, he has also had a successful career as a software entrepreneur.
Groups like al-Qaeda resemble in some ways the classic insurgents of the past, such as the Palestine Liberation Organization, but several factors distinguish them from their predecessors, Robb says. For one, they aren't state-sponsored, which makes them harder to track down and eradicate. Being self-financed, they generate significant income from donations as well as from black-market commerce. Also, members of the group don't report to a central authority; they operate relatively autonomously, and they tend to be well educated, media-savvy, and comfortable operating in a globalized, high-tech world. And the use of information technology has given modern terrorists an operational edge their predecessors lacked.
Mimicking open-source developers, insurgent groups “hack at the source code of warfare,” Robb says. By that, he means they aren't bound by the traditional rules of military engagement; they use whatever works, with their tactics, techniques, and procedures all open to scrutiny and improvement by the community. Although such groups are weak by conventional military benchmarks—they'd clearly be outgunned and outmanned on an open battlefield—they can still threaten strong national militaries. That's because they don't aim to invade, hold, or govern territory, but rather to exert political influence by exhausting an adversary's capacity to fight back. Their preferred method of attack is to disrupt infrastructure, whether physical, financial, or political. “System disruption is going to be the main thrust of warfare for quite a long time,” Robb predicts.