What is CVE-2026-21509?

CVE-2026-21509 is Microsoft Office Security Feature Bypass Vulnerability. The supplied enrichment data classifies it as Unclassified mapped to CWE-807 CWE-807.

Which versions are affected?

Affected product and version evidence is listed in the product table above. Current public advisories should be checked for exact version scope before patch targeting is finalized.

Is there active exploitation?

Yes: CISA KEV signal present. If exploitation is not confirmed from current sources, teams should document that status and continue monitoring KEV, vendor, and telemetry updates.

Is there a public exploit?

24 public PoC repositories reported; 2 marked weaponized in current dataset. Public exploit or PoC availability should raise triage priority, especially for exposed or business-critical systems.

How can defenders detect it?

Start with Not reported where available. Look for vulnerability-class behavior, product errors, suspicious authentication events, exploit probes, and abnormal process activity based only on evidence that applies to the affected product.

How should teams prioritize it?

Prioritize CVE-2026-21509 using CVSS 7.8, EPSS 73%, KEV status, public PoC status, and asset exposure. Internet-facing, privileged, or business-critical deployments should be handled first.

Are workarounds available?

The supplied mitigation guidance says to apply vendor patches and restrict external exposure until fixed. Fixed-version evidence is listed in the mitigation section above.

Which threat actors are linked publicly?

Publicly linked actor or ransomware signals in the supplied enrichment data: White Bear (), Roaming Tiger (), TAG-28 (), APT 28 (), TA2552 (). Do not attribute exploitation to a threat actor unless the source evidence explicitly supports that linkage.

What should be done in the next 24 hours?

Identify exposed assets running affected vendor/product/version combinations. Treat as emergency remediation because CISA KEV status is present. Search available logs for exploit probes, errors, authentication anomalies, or suspicious child processes matching the vulnerability class. Open remediation tickets with affected assets, owners, evidence status, and detection coverage so decisions are traceable.

CISA KEVhighpublic exploit

CVE-2026-21509

CVE-2026-21509 — Microsoft Office Security Feature Bypass Vulnerability

Q: Is it in CISA KEV?

Yes. The supplied enrichment data lists CVE-2026-21509 in CISA KEV with required action: Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable.. KEV status should be monitored because it can change after initial disclosure or after exploitation becomes publicly confirmed.

Reliance on untrusted inputs in a security decision in Microsoft Office allows an unauthorized attacker to bypass a security feature locally.

Published Jan 26, 2026Updated Jun 25, 2026Sources: NVD, Microsoft (CNA), CISA, FIRST EPSS, SOCRadar CTI

Triage

Is it exploited, how likely is exploitation, what does it touch, and how severe do the scoring sources call it.

Exploitation

Confirmed

CISA KEV, listed Jan 26, 2026

EPSS

73%

ahead of 99.4% of scored CVEs

Affects

Microsoft

5 products listed

CVSS base

7.8

high · 2 sources

CISA SSVC assessment

Three decision points CISA publishes for the CVEs it assesses · SSVC 2.0.3. A stakeholder decision, not a severity score.

CISA

Exploitation

Active

none · proof-of-concept · active

Automatable

can an attacker script all four kill-chain steps

Technical impact

Total

partial · total control of the vulnerable component

Remediation

The vendor's own words where we have them.

Upgrade Microsoft 365 Apps for Enterprise to a fixed release. Apply vendor patches per advisory and restrict external exposure of the affected component until patched.

CISA required action

Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable.

Federal deadline Feb 16, 2026 — passed 217 days ago

First 24 hours

Ordered from the record's own fields — exposure first, because you cannot patch what you have not found.

Identify exposed assets running affected vendor/product/version combinations.
Treat as emergency remediation because CISA KEV status is present.
Search available logs for exploit probes, errors, authentication anomalies, or suspicious child processes matching the vulnerability class.

Affected scope

Vendor, product and version as the advisories word them.

Vendor	Product	Versions	Status
Microsoft	Microsoft 365 Apps for Enterprise	16.0.1	Vulnerable
Microsoft	Microsoft Office 2016	16.0.0 to < 16.0.5539.1001	Vulnerable
Microsoft	Microsoft Office 2019	19.0.0 to < 16.0.10417.20095	Vulnerable
Microsoft	Microsoft Office LTSC 2021	16.0.1	Vulnerable
Microsoft	Microsoft Office LTSC 2024	16.0.0	Vulnerable

Attack characteristics

The CVSS vector, decoded. It describes the attack, not your exposure to it.

Availability

High

Confidentiality

High

Integrity

High

Scope

Unchanged

Attack Complexity

Low

Attack Vector

Local

Privileges Req

None

User Interaction

Required

Every base score collected

Sources score independently and disagree; each row says who scored it and under which version.

Score	Version	Severity	Expl.	Impact	Source
7.8	CVSS 3.1	high	1.8	5.9	Microsoft (CNA)
7.8	CVSS 3.1	high	—	—	Microsoft (CNA)

CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H

What this weakness leads to

MITRE's own consequences and mitigations for the weakness class — the authority's wording, not guidance derived from the CVSS vector.

MITRE

CWE-807 · Reliance on Untrusted Inputs in a Security Decision

Bypass Protection Mechanism
Gain Privileges or Assume Identity
Varies by Context

Mitigation: Store state information and sensitive data on the server side only. Ensure that the system definitively and unambiguously keeps track of its own state and user state and has rules defined for legitimate state transitions. Do not allow any application user to affect state directly in any way other than through legitimate actions leading to state transitions. If information must be stored on the client, do not do so without encryption and integrity checking, or otherwise having a mechanism on the server side to catch tampering. Use a message authentication code (MAC) algorithm, such as Hash Message Authentication Code (HMAC) [REF-529]. Apply this against the state or sensitive data that you has to be exposed, which can guarantee the integrity of the data - i.e., that the data has not been modified. Ensure that a strong hash function is used (CWE-328).

Weakness & attack patterns

CWE-807

CISA KEV details

CISA lists a CVE here once it has evidence of exploitation against real targets.

CISAThat month →

Listed

Jan 26, 2026

Federal deadline

Feb 16, 2026

passed 217 days ago

Ransomware use

Unknown

CISA records ransomware use only for campaigns it has confirmed, so “Unknown” means unrecorded, not ruled out. The deadline binds US federal agencies; for everyone else it is a date to argue against.

Public exploit

Capability, not use: code existing is a different claim from anyone running it.

Repositories

2 of the 5 sampled are weaponized

Exploit entries

Exploit-DB and CTI exploit indexes

Social mentions

Posts naming this CVE. Attention, not evidence of exploitation — it spikes on disclosure and decays whether or not anything is exploited.

35 more posts on this CVEShowing 5 of 40 · Who is discussing this CVE, and how loudlyUnlock free

Detection

Read off the CVSS vector and the weakness class. Starting points, not rules we have tested.

Monitor for scanner or exploit-pattern traffic after 24 public PoC repositories were reported.

Groups associated with this CVE

Groups reported in connection with this vulnerability. A reported association, not a confirmed observation of that group using it — and not scoped to any sector or campaign.

White BearRoaming TigerTAG-28APT 28TA2552

11 more threat groups on this CVEShowing 5 of 16 · Attribution, campaigns and the malware each one runsUnlock free

ip120.28.199.2473
AlienVault OTX Feeds · Dec 4, 2025
ip110.37.89.21044
AbuseIPDB Hourly Update · Jan 28, 2026
ip110.39.248.11756
GreyList · Nov 26, 2025
ip110.36.86.058
AbuseIPDB Hourly Update · Jan 30, 2026
ip180.191.230.6180
Abuse.ch-Urlhaus-C&Cs · Feb 5, 2026

5 more indicators on this CVEShowing 5 of 10 · Hashes, addresses and domains from a live threat exchangeUnlock free

Timeline

What happened to this CVE, newest first — with the readings a source repeats on a schedule counted underneath rather than listed.

2026
Added · CVSS 3.1 7.8 (AV:L)
Jun 25, 2026 · NVD
Added to CISA KEV catalog
Jan 26, 2026 · CISA
Initial · CVE published
Jan 26, 2026 · NVD

Source activity

Readings a source repeats on a schedule, counted rather than listed.

23×SVRS was changedMay 24, 2026 – Aug 25, 2026 · SOCRadar CTI
13×TweetsMay 28, 2026 – Aug 14, 2026 · SOCRadar CTI
2×Github ReposJun 30, 2026 – Jul 23, 2026 · SOCRadar CTI
1×CVE modified by MITRE: CVSS metrics updated.Jun 25, 2026 · SOCRadar CTI

References

5 on the record

msrc.microsoft.com/update-guide/vulnerability/CVE-2026-21509
Vendor Advisory, Mitre, NIST
www.vicarius.io/vsociety/posts/cve-2026-21509-detection-script-microsoft-office-security-feature-bypass-vulnerability
Third Party Advisory, NIST, GITHUB
www.vicarius.io/vsociety/posts/cve-2026-21509-mitigation-script-microsoft-office-security-feature-bypass-vulnerability
Mitigation, Third Party Advisory, NIST, GITHUB
www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2026-21509
US Government Resource, NIST
nvd.nist.gov/vuln/detail/CVE-2026-21509
Vendor Advisory

Coverage

19 stories from the feeds we poll

14 more reports on this CVEShowing 5 of 19 · Vendor research and security press coverageUnlock free

Elsewhere on this site

Microsoftevery CVE for this vendor
January 2026what else CISA listed
Unclassifiedsame class
CWE-807other pages naming this weakness

Not in any source we poll

Listed rather than left blank: an empty field and an unmeasured one look identical on screen, and only one is a reason to look elsewhere.

Only the 5 indicator(s) reported for this CVE are available; no packet-level or host-forensic artifacts supplied.
No organization-specific asset inventory, compensating-control status, or patch deployment evidence supplied.
No exploit packet captures, log samples, or incident case IDs supplied.

Answered from this record1

What should defenders know first?

CVE-2026-21509 is Microsoft Office Security Feature Bypass Vulnerability, a high vulnerability affecting Microsoft 365 Apps for Enterprise, Microsoft Office 2016, Microsoft Office 2019, Microsoft Office LTSC 2021, Microsoft Office LTSC 2024 from Microsoft. The current evidence lists it in CISA KEV, and the exploit status is: Yes: CISA KEV signal present. Public exploit evidence is: 24 public PoC repositories reported; 2 marked weaponized in current dataset. The affected-version evidence is listed in the key facts and affected products tables. Defenders should first verify whether exposed or business-critical assets run those versions, then apply vendor patches or mitigations, restrict reachable attack surface, and preserve logs for detection review. CVSS 7.8 describes technical severity, while EPSS 73% helps estimate near-term exploit likelihood; neither replaces asset context. Unknown fields should remain explicit in tickets, and threat actor, IOC, victimology, or payload claims should not be added unless a cited source supports them. Monitor CISA KEV, vendor advisories, NVD changes, public PoC repositories, and internal telemetry for update triggers.