Threats / Microsoft / CVE-2026-21509
CVE-2026-21509
· EUVD no mirror located
· GCVE no mirror located
Verified 2026-08-04
Microsoft Office vulnerability
Microsoft Office security feature bypass via untrusted input reliance allows local attackers to circumvent security controls.
Verdict
Today item — known-exploited.
A local attacker can exploit improper validation of untrusted inputs in Office security decisions to bypass protective mechanisms. Exploitation requires local access and affects some end-of-life product versions.
CISA KEV Yes · 2026-01-265EPSS 0.72152 (verify live)6Exploit Public PoC7CVSS 7.83CISA SSVC Exploitation: active · Automatable: no · Impact: total4
01
Is it exploitable?
— the evidence, ranked above the scoreExploit available
Public proof-of-concept exploit code is cataloged for this vulnerability.We link the existence of the exploit; we do not host or redistribute payloads.
Reported exploitation
28 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 2026-01-26).
Probability (EPSS)
EPSS 0.72152 — modeled likelihood of exploitation activity.EPSS is a daily-changing model output — open the source for today's value.
Severity / affected
CVSS 7.8 (CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H/E:F/RL:O/RC:C) · Affected: Microsoft, Office. Confirm exact fixed builds in the vendor advisory.CVSS — CNA-assigned (CVE.org), version 3.1, set at disclosure.
Weakness (CWE)
Mapped to CWE-807 CWE-807.CWE — CNA-assigned (CVE.org); 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 the same day 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.
WeaknessCWE-807 · CWE-807
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 input that Office processes without proper validation in its security decision logic.
Business
Security controls fail to prevent unauthorized local actions, reducing defense effectiveness.
2
Keys to the kingdom — privilege/identity takeover narrative 2
Attacker
I supply untrusted data that the application trusts when making security policy enforcement decisions.
Business
Attackers gain capability to perform restricted operations that should be blocked by security features.
3
Lateral reach — past segmentation narrative 3
Attacker
I leverage the bypassed security feature to execute restricted functionality or access protected resources locally.
Business
Confidentiality or integrity of user data and systems is compromised through circumvented protections.
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