CVE-2021-27085: Microsoft Internet Explorer Remote Code Execution Vulnerability

CVE-2021-27085 is a remote code execution vulnerability in Microsoft Internet Explorer. An attacker who successfully exploits it could run code on a vulnerable system, which makes prompt attention important for any environment still running the browser.

Public detail on the exact weakness is limited. Treat it as a high-priority browser RCE and confirm all version, configuration, and fix information directly against the Microsoft advisory.

How it works

The CWE is not specified in the available record. CISA describes the issue only as an unspecified vulnerability in Microsoft Internet Explorer that allows remote code execution.

In general, browser RCE flaws are abused when a user is convinced to open or render attacker-controlled content (for example a malicious web page or crafted file that the browser processes). Successful exploitation can let the attacker execute code in the context of the browser process or the logged-on user. Exact trigger conditions, memory-corruption details, or exploit primitives are not provided in the public summary; do not assume a particular technique and verify behavior against the vendor advisory.

Am I affected? How to find it in your systems

Internet Explorer has historically been present on Windows desktops and servers, sometimes as a legacy component, compatibility mode, or embedded WebBrowser control even when another browser is the default.

• Inventory Windows endpoints and servers for the presence of Internet Explorer or IE-based rendering components (installed features, browser helper objects, or applications that host the IE engine).
• Identify systems that still launch iexplore.exe, use IE mode, or process untrusted content through IE components.
• Compare installed browser and cumulative update levels with the versions listed as affected in the Microsoft security advisory for CVE-2021-27085; do not rely on assumed version ranges.
• Review web proxy, DNS, and endpoint logs for unusual IE process launches, crashes, or connections to untrusted sites around the time of suspected activity. Specific exploitation telemetry signatures are not detailed in the given facts, so treat anomalous IE behavior as worth investigating.

How to remediate

Apply the updates Microsoft released for this vulnerability, following the vendor instructions referenced by CISA. Patching is the primary remediation.

• Deploy the relevant Microsoft security update through your normal patch management channel (WSUS, ConfigMgr, Intune, or equivalent) and verify installation on all inventoried systems that include Internet Explorer components.
• After patching, confirm that IE or IE-mode usage is reduced or eliminated where business needs allow; prefer modern, supported browsers for general browsing.
• Harden remaining legacy use: run with least privilege, restrict the ability of users to install add-ons, and keep the broader Windows baseline current so related components are also updated.

If you can't patch immediately

Until the vendor update can be applied, reduce exposure with compensating controls appropriate to a browser RCE class of issue.

• Segment and limit which systems are allowed to run Internet Explorer or process untrusted web content; prefer isolated VDI or kiosk instances for any unavoidable legacy requirement.
• Use application control or software restriction policies to block or constrain iexplore.exe where it is not required.
• Apply web filtering, proxy rules, or virtual-patching/WAF controls that block known-malicious destinations and suspicious content types commonly used against browsers.
• Disable or tightly restrict IE features and zones that increase attack surface (for example, active scripting or less-trusted zones) if operationally feasible; confirm any setting changes against Microsoft guidance so you do not break required line-of-business apps.
• Increase monitoring on endpoints still running IE: alert on unexpected child processes of the browser, unusual network connections, and crash or exploit-mitigation events.

If your data may have been exposed

Actively exploited remote code execution vulnerabilities can lead to system compromise and data theft. Known ransomware use is not documented for this CVE in the provided facts, but any confirmed exploitation should be handled through your incident-response process: isolate affected hosts, preserve evidence, and assess what credentials or data the browser process could reach. You can run a free exposure scan of your email addresses against known breach data as one quick check for previously leaked credentials that attackers might reuse.