The exploitation of a recently fixed Windows zero-day vulnerability was attributed to known ransomware operators, according to Microsoft and threat intelligence firm RiskIQ.

The existence of the zero-day, called CVE-2021-40444, was revealed on September 7, when Microsoft released countermeasures and cautioned that the vulnerability had been exploited in targeted attacks using specially designed Office documents. 

The vulnerability connected to Office's MSHTML browser engine can and has been misused for remote code execution. As part of its Patch Tuesday updates, Microsoft delivered upgrades on September 14th. 

Microsoft announced the acquisition of RiskIQ in July and posted separate blog posts detailing the attacks exploiting CVE-2021-40444. 

The first exploitation efforts were discovered in mid-August. But Microsoft observed a massive spike in exploitation attempts when the proof-of-concept (PoC) code and other details were made public after the initial announcement. 

As per the company, several threat actors, including ransomware-as-a-service affiliates, have used the public PoC code, but some of the exploitation attempts are part of testing rather than criminal operations. 

The company initially saw less than ten exploitation attempts and leveraged CVE-2021-40444 to deliver custom Cobalt Strike Beacon loaders. Microsoft has identified the attackers as DEV-0413 — DEV is allotted to emerging threat groups or unusual activity. To deliver the malware, they apparently used emails referencing contracts and legal agreements to get the targets to open documents formatted to abuse the MSHTML vulnerability.

Surprisingly, the Cobalt Strike infrastructure utilised in the assaults has earlier been linked to cybercrime organisations known for targeting big corporations with ransomware like Conti and Ryuk. These threat actors are tracked as Wizard Spider (CrowdStrike), UNC1878 (FireEye), DEV-0193, and DEV-0365 (Microsoft).

RiskIQ stated in its blog post, “Despite the historical connections, we cannot say with confidence that the threat actor behind the zero-day campaign is part of WIZARD SPIDER or its affiliates, or is even a criminal actor at all, though it is possible. If the threat actors were part of these groups, it means they almost surely purchased the zero-day exploit from a third party because they have not previously shown the ability to develop exploit chains of this complexity.” 

The company added, “Instead, we assess with medium confidence that the goal of the operators behind the zero-day may, in fact, be traditional espionage. This goal could easily be obscured by a ransomware deployment and blend into the current wave of targeted ransomware attacks.” 

RiskIQ states that the cyberspies could have gained access to the ransomware infrastructure, or they may have been allowed by the ransomware operators to utilise their infrastructure. Only one group might be involved in espionage and cybercrime, or the two groups use the same bulletproof hosting provider. 

According to Microsoft, the initial malicious document in attacks abusing CVE-2021-40444 emerges from the internet, and it should be labelled as the "mark of the web." 

Microsoft Office should open the document in Protected Mode unless the user specifically allows modification, limiting the misuse. However, if the attackers figure out a means to keep the document from being a “mark of the web,” they may utilise the vulnerability to execute the payload on the page without requiring user input.