McAfee Enterprise Defender Blog | MSHTML CVE-2021-40444

Microsoft is warning its users of a zero-day vulnerability in Windows 10 and versions of Windows Server that is being leveraged by remote, unauthenticated attackers to execute code on the target system using specifically crafted office documents. Tracked as CVE-2021-40444 (CVSS score: 8.8), the remote code execution flaw is rooted in MSHTML (aka Trident), a proprietary browser engine for the now-discontinued Internet Explorer and which is used in Microsoft Office to render web content inside Word, Excel, and PowerPoint documents. This vulnerability is being actively exploited and protections should be put into place to prevent that. Microsoft has released guidance on a workaround, as well as updates to prevent exploitation, but below are additional McAfee Enterprise countermeasures you can use to protect your business.

MVISION Insights Campaign – “CVE-2021-40444 – Microsoft MSHTML Remote Code Execution Vulnerability”

Since originally reported, vulnerability exploitation has grown worldwide.

Figure 1. Latest MITRE ATT&CK framework for Exploitation of CVE-2021-40444. Source: MVISION Insights

Additional MITRE ATT&CK techniques have been identified since our original report. MVISION Insights will be regularly updated with the latest IOCs and hunting rules for proactive detection in your environment.

Figure 2. Latest MITRE ATT&CK framework for Exploitation of CVE-2021-40444. Source: MVISION Insights

McAfee Enterprise Product Protections

The following McAfee Enterprise products can protect you against this threat.

Figure 3. Protection by ENS Module

For ENS, it’s important to have both Threat Protection (TP) and Adaptive Threat Protection (ATP) with GTI enabled. We are seeing 50% of detections based on ATP behavior analysis rules.

Figure 4. Protection by ENS Module

More details on Endpoint protection including MVISION EDR are included below.

Preventing Exploit with McAfee ENS

McAfee Global Threat Intelligence (GTI) is currently detecting the analyzed IOCs for this exploitation. GTI will be continually updated as new indicators are observed in the wild.

ENS Threat Prevention module can provide added protections against exploitation of CVE-2021-40444 until a patch is deployed. The following signature in Exploit Prevention has shown coverage in testing of observed exploits; this signature could cause false positives, so it is highly advised to test in Report Mode or in sandbox environments before blocking in production environments.

Signature 2844: Microsoft Word WordPerfect5 Converter Module Buffer Overflow Vulnerability

Several custom Expert Rules can be implemented to prevent or detect potential exploitation attempts. As with all Expert Rules, please test them in your environment before deploying widely to all endpoints. Recommended to implement this rule in a log only mode to start.

Figure 5. Expert Rule to block or log exploitation attempts

Figure 6. Expert Rule to block or log exploitation attempts

ATP Rules

Adaptive Threat Protection module provides behavior-blocking capability through threat intelligence, rules destined to detect abnormal application activity or system changes and cloud-based machine-learning. To exploit this vulnerability, the attacker must gain access to a vulnerable system, most likely through Spearphishing with malicious attachments. These rules may also be effective in preventing initial access and execution. It is recommended to have the following rules in Observe mode at least and monitor for threat events in ePO.

Rule 2: Use Enterprise Reputations to identify malicious files.
Rule 4: Use GTI file reputation to identify trusted or malicious files
Rule 5: Use GTI file reputation to identify trusted or malicious URLs
Rule 300: Prevent office applications from being abused to deliver malicious payloads
Rule 309: Prevent office applications from being abused to deliver malicious payloads
Rule 312: Prevent email applications from spawning potentially malicious tools

As with all ATP Rules, please test them in your environment before deploying widely to all endpoints or turning on blocking mode.

Utilizing MVISION EDR for Hunting of Threat Activity

The Real-Time Search feature in MVISION EDR provides the ability to search across your environment for behavior associated with the exploitation of this Microsoft vulnerability. Please see the queries to locate the “mshtml” loaded module associated with various application processes.

EDR Query One

Processes where Processes parentimagepath matches “winword|excel|powerpnt” and Processes cmdline matches “AppData/Local/Temp/|.inf|.dll” and Processes imagepath ends with “control.exe”

EDR Query Two

HostInfo hostname and LoadedModules where LoadedModules process_name matches “winword|excel|powerpnt” and LoadedModules module_name contains “mshtml” and LoadedModules module_name contains “urlmon” and LoadedModules module_name contains “wininet“

Additionally, the Historical Search feature within MVISION EDR will allow for the searching of IOCs even if a system is currently offline.

Figure 7. Using Historical Search to locate IOCs across all devices. Source: MVISION EDR

McAfee Enterprise has published the following KB article that will be updated as more information and coverage is released.

McAfee Enterprise coverage for CVE-2021-40444 – MSHTML Remote Code Execution

Further Protection for Threat Actor Behavior After Exploitation

Since public disclosure of the vulnerability, it has been observed from successful exploitation of CVE-2021-40444 in the wild that threat actors are utilizing a Cobalt Strike payload to then drop ransomware later in the compromised environment. The association between this vulnerability and ransomware point to the possibility that the exploit has been added to the tools utilized in the ransomware-as-a-service (RaaS) ecosystem.

Figure 8. CVE-2021-40444-attack-chain (Microsoft)

The Ransomware Gangs that have been observed in these attacks have in the past been known to utilize the Ryuk and Conti variants of ransomware.

Please see below additional mitigations that can be utilized in the event your environment is compromised and added protections are needed to prevent further TTPs.

Cobalt Strike BEACON

MVISION Insights Campaign – Threat Profile: CobaltStrike C2s

Endpoint Security – Advanced Threat Protection:

Rule 2: Use Enterprise Reputations to identify malicious files.

Rule 4: Use GTI file reputation to identify trusted or malicious files

Rule 517: Prevent actor process with unknown reputations from launching processes in common system folders

Ryuk Ransomware Protection

MVISION Insights Campaign – Threat Profile: Ryuk Ransomware

Endpoint Security – Advanced Threat Protection:

Rule 2: Use Enterprise Reputations to identify malicious files.

Rule 4: Use GTI file reputation to identify trusted or malicious files

Rule 5: Use GTI file reputation to identify trusted or malicious URLs