CVE-2020-1380: Microsoft Internet Explorer Scripting Engine Memory Corruption Vulnerability

CVE-2020-1380 is a memory corruption vulnerability in the scripting engine of Microsoft Internet Explorer. It can allow an attacker to achieve remote code execution in the context of the current user, which means a successful exploit could run code with whatever rights that user already has on the system.

For IT and security teams, this matters because browsers and their scripting components are common entry points. Even where Internet Explorer is no longer the primary browser, it may still be present, invoked by legacy apps, or reachable via crafted content. Confirm exact scope and fixed builds against the vendor advisory.

How it works

This issue is classed as CWE-787 (out-of-bounds write), a form of memory corruption. In a scripting engine, untrusted script or related content is parsed and executed; a flaw that lets an attacker write outside the intended memory bounds can corrupt internal structures the engine relies on.

In practical terms, an attacker who can get a user to process malicious content in the affected Internet Explorer scripting path may trigger that corruption and then attempt to run arbitrary code as the logged-on user. The CISA summary describes remote code execution in the current user context; it does not require inventing specific trigger pages, heap layouts, or shellcode. Exact exploitation conditions and any prerequisites must be taken from the vendor advisory rather than assumed.

Am I affected? How to find it in your systems

Internet Explorer has historically shipped with Windows and may still appear on endpoints even when another browser is default. It can be launched directly, embedded via legacy controls, or used by internal line-of-business applications that still depend on the Trident/MSHTML stack.

• Inventory Windows endpoints and note where Internet Explorer or the IE scripting/MSHTML components are installed or enabled.
• Check software and patch inventory against the versions and updates listed in Microsoft’s advisory for CVE-2020-1380; do not rely on guessed build numbers.
• Identify systems that still allow IE as a handler for web content, ActiveX, or intranet zones, and any automated processes that drive IE.
• Review whether group policy or application compatibility settings keep IE available for specific sites or apps.

Telemetry signs of exploitation are not uniquely defined in the provided facts. In general for this class, look for unexpected IE or scripting-host process crashes, unusual child processes spawned from IE-related binaries, and anomalous outbound connections shortly after browsing activity. Correlate with EDR/process creation logs and confirm any indicators against vendor or trusted threat intel rather than assuming a fixed signature set.

How to remediate

Patch first. Apply the updates Microsoft released for this vulnerability, following the vendor instructions as CISA requires. Use your standard patch deployment channel, verify installation on representative hosts, and confirm the advisory’s superseded or cumulative update guidance so you are not left on a partial fix.

• Prioritize endpoints that still run or expose Internet Explorer to untrusted content.
• After patching, validate that IE/scripting components report the expected updated state via your inventory tools.
• Where IE is no longer required, reduce attack surface by disabling or restricting it through supported Windows policies and by moving legacy apps to modern, supported browsers.
• Harden remaining use: limit intranet/trusted zone relaxations, restrict legacy scripting/ActiveX where business-feasible, and keep least-privilege user accounts so RCE in the user context has less impact.

If you can't patch immediately

Until the vendor update is installed, reduce exposure with compensating controls aimed at this browser/scripting class:

• Segment and restrict: limit which users and networks can reach untrusted web content from hosts that still have IE enabled; prefer isolated VDI or kiosk patterns for unavoidable legacy IE use.
• Disable or block the affected path where possible: use software restriction / application control to prevent launch of Internet Explorer for users who do not need it; tighten zone settings and disable unnecessary scripting features if policy allows without breaking required apps.
• Virtual patching / content filtering: if you operate a web proxy or secure web gateway, block or sandbox categories and URLs associated with drive-by or exploit kit activity; any WAF-style controls apply mainly to web apps you host, not to client-side IE, so focus on egress and browsing controls.
• Heightened monitoring: alert on IE process crashes, unusual child processes (e.g., cmd, powershell, or office apps spawned unexpectedly from IE), and new persistence or lateral movement shortly after browsing. Retain and review endpoint logs until patches are complete.

These steps lower risk; they do not replace the vendor update.

If your data may have been exposed

Actively exploited browser RCE vulnerabilities can lead to account takeover, malware installation, or follow-on data theft in the user context. Known ransomware use is not documented for this CVE in the provided facts, but any confirmed compromise should still be handled through your incident process: isolate hosts, reset credentials, and scope for lateral movement.

If you believe credentials or personal data may have been involved, check whether those identities appear in known breach collections. You can run a free exposure scan of your email against known breach data to see whether it has shown up in prior incidents and then proceed with password changes and monitoring as appropriate.