The firm's new report uncovered evidence that in 2013 opposition forces were on the receiving end of a simple Skype-led cyberattack that laid bare past battle plans, lists of fighters, weapons consignments and even details of sympathetic refugees.
Normally, cyberattacks are assumed to about gaining an edge to aid long-term strategic goals but these attacks would have given the attackers actionable intelligence on the battle configuration of forces opposing the Syrian Army in some areas of the country.
The campaign, which ran across 2013, was based on Remote Access Trojan (RAT) malware that infected targets set up using bogus but appealing female Skype avatars. Chat conversations would winkle out the devices being used by opposition fighters, their names, what they looked like, as well as directing them to fake opposition sites hosting malware. Photographs of women would be sent in return which when opened would initiate infection by the DarkComet RAT.
It does appear to have been that easy.
The stolen data amounted to 7.7GB, comprising 12,356 contacts, over 240,000 messages, 64 Skype account databases, and 31,107 conversations held on Skype itself.
If gaining access to that volume of data siunds unexpected, the campaign's success was helped by the opposition's limited infrastructure in which numerous individuals would use the same PCs to communicate via satellite, FireEye said.
Most seriously, the firm uncovered evidence that the attackers had gained advance knowledge of a meticulously-planned military offensive against Khirbet Ghazaleh near Deraa sometime between November 2013 and January 2014 involving 800 men, tanks and other assets.
From FireEye's slightly convoluted account it appears this data wasn't stolen until after the attack had occurred but it remains possible that other attacks were revealed in advance.
The technique of targeting opposition fighters using Skype and RAT malware has been well documented from early 2012 onwards and is not a revelation. However, the success and possible wider effects of this disarmingly simple technique is more of a jolt.
"In the course of our threat research, we found the activity focused on the Syrian opposition that shows another innovative way threat groups have found to gain the advantage they seek," said FireEye senior threat intelligence researcher, Nart Villeneuve.
"While we cannot positively identify who is behind these attacks, we know that they used social media to infiltrate victims' machines and steal military information that would provide an advantage to President Assad's forces on the battlefield."
The analysis goes some way to explaining why Assad's once-crumbling army has performed beyond expectations since the early days of the insurgency in 2011, holding its opponents at way with apparent ease.
On that topic, the issue of who was behind the cyberattack is intriguing. FireEye found numerous references to and knowledge of neighbouring Lebanon. This implies that the group is connected to the Iranian-backed Hezbollah paramilitary organisation that provided men and weapons to help stabilise Assad's regime in its darkest hour.
The well-known Syrian Electronic Army (SEA), which has carried out more public cyberattacks on Western targets, is rumoured to be controlled by foreign-based activists, similarly trained and connected to Iran.