A high-severity Microsoft vulnerability (CVE-2026-21513) in the MSHTML Framework, which allows security feature bypass, was exploited as a zero-day. The Russia-linked threat actor APT28 is suspected in these attacks, using malicious LNK files to bypass security contexts and execute code. The main topics covered are the specific software vulnerability, its exploitation by a state-sponsored group, and the technical mechanism of the attack.
A recently disclosed security flaw patched by Microsoft may have been exploited by the Russia-linked state-sponsored threat actor known as APT28, according to new findings from Akamai.
The vulnerability in question is CVE-2026-21513 (CVSS score: 8.8), a high-severity security feature bypass affecting the MSHTML Framework.
"Protection mechanism failure in MSHTML Framework allows an unauthorized attacker to bypass a security feature over a network," Microsoft noted in its advisory for the flaw. It was fixed by the Windows maker as part of its February 2026 Patch Tuesday update.
However, the tech giant also noted that the vulnerability had been exploited as a zero-day in real-world attacks, crediting the Microsoft Threat Intelligence Center (MSTIC), Microsoft Security Response Center (MSRC), and Office Product Group Security Team, along with Google Threat Intelligence Group (GTIG), for reporting it.
In a hypothetical attack scenario, a threat actor could weaponize the vulnerability by persuading a victim to open a malicious HTML file or shortcut (LNK) file delivered through a link or as an email attachment.
Once the crafted file is opened, it manipulates browser and Windows Shell handling, causing the content to be executed by the operating system, Microsoft noted. This, in turn, allows the attacker to bypass security features and potentially achieve code execution.
While the company has not officially shared any details about the zero-day exploitation effort, Akamai said it identified a malicious artifact that was uploaded to VirusTotal on January 30, 2026, and is associated with infrastructure linked to APT28.
It's worth noting that the sample was flagged by the Computer Emergency Response Team of Ukraine (CERT-UA) early last month in connection with APT28's attacks exploiting another security flaw in Microsoft Office (CVE-2026-21509, CVSS score: 7.8).
The web infrastructure company said CVE-2026-21513 is rooted in the logic within "ieframe.dll" that handles hyperlink navigation, and that it's the result of insufficient validation of the target URL, which allows attacker-controlled input to reach code paths that invoke ShellExecuteExW. This, in turn, enables execution of local or remote resources outside the intended browser security context.
"This payload involves a specially crafted Windows Shortcut (LNK) that embeds an HTML file immediately after the standard LNK structure," security researcher Maor Dahan said. "The LNK file initiates communication with the domain wellnesscaremed[.]com, which is attributed to APT28 and has been in extensive use for the campaign's multistage payloads. The exploit leverages nested iframes and multiple DOM contexts to manipulate trust boundaries."
Akamai noted that the technique makes it possible for an attacker to bypass Mark-of-the-Web (MotW) and Internet Explorer Enhanced Security Configuration (IE ESC), leading to a downgrade of the security context and ultimately facilitating the execution of malicious code outside of the browser sandbox via ShellExecuteExW.
"While the observed campaign leverages malicious LNK files, the vulnerable code path can be triggered through any component embedding MSHTML," the company added. "Therefore, additional delivery mechanisms beyond LNK-based phishing should be expected."
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