Steve Coll worries about them:
Iran is one of two-dozen-plus countries believed to possess an explicit cyber-warfare capability, akin to America’s Cyber Command. Russia is highly effective; China is active and capable. Specialists do not rate the United States as especially dominant on offense, but the country looks strikingly weak on defense.
"Because of its greater dependence on cyber-controlled systems and its inability thus far to create national cyber defenses, the United States is currently far more vulnerable to cyber war than Russia or China," write Richard A. Clarke and Robert K. Knake in their book, "Cyber War: The Next Threat to National Security and What to Do About It."