MSHTML Remote Code Execution via Malicious Office Document

CVE-2021-40444 is a remote-code-execution vulnerability in MSHTML, the legacy browser-rendering engine that Windows and Microsoft Office components still use for certain document-rendering tasks. Microsoft disclosed it on September 7, 2021, alongside confirmation that it was already being exploited in the wild as a zero-day, via malicious Office documents that loaded a specially crafted ActiveX control to achieve code execution.

Why it mattered

The attack required only that a victim open a malicious Word document — no macros, no additional prompts beyond normal document handling in many configurations — making it an effective initial-access vector for phishing campaigns. Microsoft’s initial mitigation, published before the full patch, involved disabling ActiveX control installation in Internet Explorer via registry changes, since MSHTML underpinned the vulnerable rendering path even inside Office.

Because a full, tested patch took roughly two weeks after public disclosure, threat intelligence teams observed a window of active exploitation by multiple actors delivering commodity malware loaders through the flaw before most organizations could patch. The vulnerability was added to CISA’s KEV catalog. Full mitigation and patch guidance is in Microsoft’s security advisory and at the NVD link above.