WASHINGTON — To the young Syrian rebel fighter, the Skype message in early December 2013 appeared to come from a woman in Lebanon, named Iman Almasri, interested in his cause. Her picture, in a small icon alongside her name, showed a fair-skinned 20-something in a black head covering, wearing sunglasses.

They chatted online for nearly two hours, seemingly united in their opposition to the rule of Bashar al-Assad, the Syrian leader still in power after a civil war that has taken more than 200,000 lives. Eventually saying she worked “in a programing company in Beirut,” the woman asked the fighter whether he was talking from his computer or his smartphone. He sent her a photo of himself and asked for another of her in return. She sent one immediately, apologizing that it was a few years old.

“Angel like,” he responded. “You drive me crazy.”

What the fighter did not know was that buried in the code of the second photo was a particularly potent piece of malware that copied files from his computer, including tactical battle plans and troves of information about him, his friends and fellow fighters. The woman was not a friendly chat partner, but a pro-Assad hacker — the photos all appear to have been plucked from the web.

The Syrian conflict has been marked by a very active, if only sporadically visible, cyberbattle that has engulfed all sides, one that is less dramatic than the barrel bombs, snipers and chemical weapons — but perhaps just as effective. The United States had deeply penetrated the web and phone systems in Syria a year before the Arab Spring uprisings spread throughout the country. And once it began, Mr. Assad’s digital warriors have been out in force, looking for any advantage that could keep him in power.