Threats / Microsoft / CVE-2021-26411
CVE-2021-26411
· EUVD no mirror located
· GCVE no mirror located
Verified 2026-08-04
Microsoft Internet Explorer vulnerability
Microsoft Internet Explorer use-after-free vulnerability enabling memory corruption. Actively exploited in ransomware campaigns with high exploitation likelihood.
Verdict
Today item, not a backlog item.
Use-after-free flaw in Internet Explorer permits memory corruption attacks. High EPSS score and confirmed wild exploitation tied to ransomware operations indicate imminent threat requiring immediate patching.
CISA KEV Yes · 2021-11-035Ransomware use Flagged5EPSS 0.80667 (verify live)6CVSS 8.83CISA SSVC Exploitation: active · Automatable: no · Impact: total4
01
Is it exploitable?
— the evidence, ranked above the scoreReported exploitation
15 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), flagged for known ransomware use.
Probability (EPSS)
EPSS 0.80667 — modeled likelihood of exploitation activity.EPSS is a daily-changing model output — open the source for today's value.
Severity / affected
CVSS 8.8 (CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:H/A:L/E:P/RL:O/RC:C) · Affected: Microsoft, Internet Explorer. Confirm exact fixed builds in the vendor advisory.CVSS — CNA-assigned (CVE.org), version 3.1, set at disclosure.
Weakness (CWE)
Mapped to CWE-416 Use After Free — 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 237 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 a malicious webpage or email attachment that triggers the use-after-free condition in Internet Explorer's memory management.
Business
Employee browsing compromised or attacker-controlled web content becomes initial infection vector for ransomware deployment.
2
Keys to the kingdom — privilege/identity takeover narrative 2
Attacker
I achieve arbitrary code execution through memory corruption, establishing foothold on the victim's system with user privileges.
Business
Attacker gains persistent access to corporate network, enabling lateral movement and reconnaissance.
3
Lateral reach — past segmentation narrative 3
Attacker
I escalate privileges and deploy ransomware payload across networked systems.
Business
Critical business systems encrypted, operations halted, and ransom demanded with threat of data exfiltration.
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