A sophisticated form of malware known as Project Sauron went undetected for five years at a string of organisations, according to security researchers.
The malware may have been designed by a state-sponsored group.
It can disguise itself as benign files and does not operate in predictable ways, making it harder to detect.
Experts from Kaspersky Lab and Symantec said it allows the attacker to spy on infected computers.
In September last year, Kaspersky first detected the malware on an unspecified “government organisation” network.