Threats / Microsoft / CVE-2021-33742
CVE-2021-33742
· EUVD no mirror located
· GCVE no mirror located
Verified 2026-09-19
Microsoft Windows vulnerability
Microsoft Windows MSHTML Platform remote code execution vulnerability exploited in the wild. Attackers can execute arbitrary code through unspecified vectors in the HTML rendering engine.
Verdict
Today item — known-exploited.
Active exploitation in the wild with high EPSS score (0.72) indicates imminent risk. Buffer overflow and out-of-bounds write vulnerabilities in MSHTML enable unauthenticated remote code execution, likely through malicious web content or email attachments.
CISA KEV Yes · 2021-11-035EPSS 0.59407 (verify live)6CVSS 7.53CISA SSVC Exploitation: active · Automatable: no · Impact: total4
01
Is it exploitable?
— the evidence, ranked above the scoreReported exploitation
8 independent public reports of in-the-wild exploitation are cataloged.Distinct reporting sources (vendor, incident response, government); open them for the underlying claims.
Exploited in the wild
Listed in the CISA Known Exploited Vulnerabilities catalog (added 2021-11-03).
Probability (EPSS)
EPSS 0.59407 — modeled likelihood of exploitation activity.EPSS is a daily-changing model output — open the source for today's value.
Severity / affected
CVSS 7.5 (CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:H/I:H/A:H/E:F/RL:O/RC:C) · Affected: Microsoft, Windows. Confirm exact fixed builds in the vendor advisory.CVSS — CNA-assigned (CVE.org), version 3.1, set at disclosure.
Weakness (CWE)
Mapped to CWE-787 Out-of-bounds Write, CWE-823 CWE-823 — weakness family: Memory safety.CWE assignment from the public NVD record (may be stale); the weakness class drives how the flaw is exploited.
CISA SSVC (triage disposition)
Exploitation: active · Automatable: no · Technical Impact: totalCISA's own stakeholder-specific triage inputs, published in the CVE record (CISA Coordinator role). NOTE: this is CISA's three decision points; the final Act/Attend/Track decision also requires a Mission & Well-being input not published here, so no overall decision is asserted.
Currency of the record
CISA added this to its KEV catalog 148 days after the CVE record was published.KEV lag = CISA-KEV dateAdded − CVE.org datePublished; a floor on how long the known-exploited flaw sat in the public record before the federal catalog listed it. Severity/weakness here is CNA- or CISA-assigned (current at disclosure), independent of NVD's analysis backlog.
02
Who’s exploiting it?
— attribution turns risk into urgencyAttribution not established
No confirmed (advisory-backed) threat-actor attribution is established for this record. Absence of a named actor is not absence of compromise — see Coverage & confidence.
03
Why it matters
— the attack path, told twice: adversary, then board1
Front door — unauthenticated access narrative 1
Attacker
I craft malicious HTML or web content that triggers a buffer overflow in the MSHTML rendering engine when processed by a Windows user.
Business
Attackers gain arbitrary code execution on user endpoints, enabling data theft, lateral movement, and persistent compromise of corporate networks.
2
Keys to the kingdom — privilege/identity takeover narrative 2
Attacker
I distribute the malicious content via email, phishing links, or compromised websites to maximize victim exposure.
Business
Widespread endpoint compromise across the organization without requiring user interaction beyond opening email or visiting a website.
3
Lateral reach — past segmentation narrative 3
Attacker
I establish command and control channels and move laterally through the network using compromised credentials and system access.
Business
Critical infrastructure, intellectual property, and sensitive data become accessible to threat actors with minimal detection.
04
What to do
— defensible action- Remediate per the vendor advisory — confirm the fixed build for your version and verify exposure.1
Say it to the boardA vulnerability with this evidence profile is a defensible budget line, not a backlog ticket — fund the change against the proof above.
05
Coverage & confidence
— what we know, and what we don’tEstablished (cited)
Coverage gaps — stated, not hidden
Disclosure & credit2
Catalogued by microsoftCNA
Credited with finding itClément Lecigne of Google’s Threat Analysis Groupfinder
Found an error? Propose a correction.
A correction is a cited counter-claim — it must cite a source,
gets reviewed by a second human, and is never auto-applied. See the
correction convention and our identity
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