APT28 Exploitation of MSHTML Zero-Day CVE-2026-21513 via Malicious HTML/LNK Files
Akamai reported that Russia-linked APT28 likely exploited CVE-2026-21513 (CVSS 8.8), a high-severity MSHTML security feature bypass, prior to Microsoft’s fix in the February 2026 Patch Tuesday release. Microsoft confirmed the vulnerability was exploited as a zero-day in real-world attacks and credited MSTIC, MSRC, the Office Product Group Security Team, and Google Threat Intelligence Group (GTIG) for reporting it. The issue is described as an Internet Explorer/MSHTML security control bypass that can be triggered when a victim opens a malicious HTML page or LNK (shortcut) file, potentially enabling code execution by causing content to be handled by Windows shell mechanisms.
Technical analysis from Akamai tied the root cause to hyperlink navigation logic in ieframe.dll, where insufficient URL validation can allow attacker-controlled input to reach ShellExecuteExW, enabling execution outside the browser sandbox. Akamai identified an exploit-related artifact (described as document.doc.LnK.download) uploaded to VirusTotal on January 30, 2026, and associated it with infrastructure linked to APT28; reporting also noted the sample had been flagged by CERT-UA in the context of APT28 activity. Overall, the reporting indicates active pre-patch exploitation and reinforces the need to prioritize patching and to monitor for delivery vectors involving HTML and LNK attachments/links consistent with APT28 tradecraft.
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