CVE-2020-0878

What is Edge and Internet Explorer?

At the time of this vulnerability, Internet Explorer and the legacy (EdgeHTML-based) Microsoft Edge were still widely deployed browsers on Windows, either as the default browser or kept installed for compatibility with older intranet applications. Browsers are a prime target for memory-corruption research because they parse enormous amounts of untrusted, attacker-controlled content — HTML, JavaScript, images, fonts — from any website a user visits, making a single malicious page a potential vector for full code execution.

Overview

CVE-2020-0878 is a memory corruption vulnerability affecting how Edge and Internet Explorer handle certain objects in memory. If successfully exploited, an attacker who convinces a user to visit a malicious or compromised website — or view a specially crafted document — can gain the ability to execute arbitrary code in the context of the current user. Microsoft rated it MEDIUM severity in this instance largely because CVSS scoring reflects high attack complexity and required user interaction, though the underlying bug class (memory corruption in a browser rendering engine) is the same category historically used for drive-by browser exploitation.

Technical Details

The vulnerability is classified as CWE-787 (Out-of-Bounds Write), meaning the browser's rendering or scripting engine writes data past the boundary of an allocated memory buffer. In browser exploitation, out-of-bounds writes are typically used to corrupt adjacent heap objects — such as function pointers or object metadata — providing attackers a primitive that can be escalated into arbitrary code execution, often chained with an information leak to defeat ASLR. The CVSS vector (AC:H/UI:R) indicates that reliably triggering the bug requires a somewhat complex exploitation chain and does require the victim to interact with malicious content, consistent with a browser rendering-engine bug rather than a trivially-scriptable network exploit.

Discovery

Addressed through Microsoft's routine security update process as part of the September 2020 Patch Tuesday cycle; no independent researcher is publicly credited in the record available for this CVE.

Exploitation Context

CISA's KEV catalog listing confirms this vulnerability has been exploited in the wild, and the frontmatter marks confirmed ransomware association — consistent with browser memory-corruption bugs being used as an initial-access vector, often via malicious ad networks, compromised websites, or spearphishing links, before pivoting to deploy follow-on payloads such as ransomware.

Remediation

1. Apply the September 2020 cumulative Windows/IE update (or any later update) across all affected systems.
2. Migrate away from legacy Internet Explorer and EdgeHTML-based Edge where possible in favor of actively maintained, Chromium-based Edge.
3. Restrict IE/legacy Edge usage to only the specific intranet applications that require it, via Enterprise Mode Site List policies, reducing general web-browsing exposure.
4. Deploy web content filtering and browser exploit-mitigation tooling to reduce the chance of drive-by exploitation reaching end users.
5. Review endpoint logs for signs of browser-based exploitation predating patch deployment, particularly unexpected child processes spawned from browser processes.