Talos researcher Asheer Malhotra said in a detailed blog post that the theme of the documents indicated that South Asian militaries and governments were being targeted.
As it usual with serious research, Cisco did not make any attempt at attribution.
Cobalt Strike is software that is used for adversary simulations and red team operations – security assessments that duplicate the tactics and techniques used by adversaries in a network. These assessment help in security operations and incident response.
The attack came through a modular dropper executable that Talos christened IndigoDrop; it was delivered to a victim's endpoint using the aforementioned malicious Microsoft Office documents.
A sample malicious document used as a lure found by the Talos Group.
"IndigoDrop is responsible for obtaining the final payload from a download URL for deployment," Malhotra said. "The final payloads currently observed by Talos are Cobalt Strike beacons."
There were two techniques of infection identified: one through malicious macros which were embedded and ready to be executed and the second through malicious macros that were downloaded as part of a template that was linked externally and injected into the malicious document that acted as the initial lure.
Malhotra said that while many targeted attacks observed by Talos consisted of a document that ran to a couple of pages to make it look legitimate, the IndigoDrop documents had legitimate content running to as much as 64 pages, which made them seem genuine.
He outlined the attacks in great detail, also pointing out how they had evolved from the first discovery in April 2018 to the last in October 2019.
Malhotra said that the attackers used a combination of public and private servers to host payloads with a growing trend towards using public servers.
"The attack variants discovered over time show us that the threat actor can evolve their TTPs (tactics, techniques and procedures) in a short period of time," Malhotra concluded.
"The earliest observable campaigns of this actor date back to April 2018 and continue to operate today along with the most recent evolutions of the attacks. Evidence of rapid ideation, testing and production of new and diversified modules and IndigoDrop iterations indicates highly motivated and agile adversaries.
"The use of adversarial frameworks like Cobalt Strike suggests that the attackers are looking to expand their malicious arsenal at a significant rate with self-authored and customisable artifacts."