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

Triage

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

CISA SSVC assessment CISA

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

Remediation

Fixes and mitigations documented by the sources — linked to the original advisory.

• Documented mitigation — www.vicarius.io (NIST National Vulnerability Database)

CISA required action CISA

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

US federal deadline: Feb 16, 2026 — passed 242 day(s) ago

First 24 hours

Ordered from the record’s own fields — exposure first, because you cannot patch what you have not found. A rule-generated starting point.

1. Identify exposed assets running the affected vendor/product/version combinations (Microsoft 365 Apps, Microsoft Microsoft 365 Apps for Enterprise, Microsoft Microsoft Office 2016…).
2. Treat as emergency remediation: the CVE is in CISA KEV (confirmed exploitation).
3. Apply the documented mitigations until a definitive fix is available.
4. Exploitation requires user interaction: tighten email/web filtering and warn affected users.
5. EPSS ahead of 99% of scored CVEs: prioritize it in the patch queue.
6. Search available logs for exploit probes, errors and authentication anomalies.

Affected scope

Vendor, product and versions as the sources word them.

Attack characteristics

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

Every base score collected

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

Weakness MITRE

Weakness class according to MITRE’s CWE catalog.

CWE-807

Other CVEs with this weakness →

CISA KEV details CISA

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

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 reference date.

Public exploits, PoCs and detection templates

Public code that demonstrates or tests the flaw raises the odds of mass exploitation — and Nuclei templates help find vulnerable assets in your environment.

Warning: public PoCs are sometimes disguised malware. Use them only for authorized testing in an isolated environment. This portal does not host exploit code — it only points to the sources.

EPSS over time FIRST

Oct 12, 2026 → Oct 15, 2026 · 4 reading(s) · max 70.80%

Detection

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

• Local/physical vector: focus on endpoint telemetry (EDR) from affected hosts.
• Hunt retroactively in logs from before Jan 26, 2026 (KEV listing): exploitation may have started earlier.

Timeline

What happened to this CVE, newest first.

• 2026
• Affected products added · microsoft/microsoft 365 apps for enterprise, microsoft/microsoft office 2016, microsoft/microsoft office 2019, microsoft
  Oct 13, 2026 · CVE Program (cve.org) e CISA ADP
• SSVC assessment updated
  Oct 13, 2026 · CVE Program (cve.org) e CISA ADP
• Record published
  Oct 13, 2026
• Fix documented
  Oct 13, 2026 · NIST National Vulnerability Database
• Affected products added · microsoft/365 apps, microsoft/office, microsoft/office long term servicing channel
  Oct 13, 2026 · NIST National Vulnerability Database
• CVSS changed · 7.8
  Oct 13, 2026 · NIST National Vulnerability Database
• Source status changed · → Analyzed
  Oct 13, 2026 · NIST National Vulnerability Database
• Added to CISA KEV (detected)
  Oct 13, 2026 · CISA Known Exploited Vulnerabilities
• Record created in this database
  Oct 13, 2026 · CISA Known Exploited Vulnerabilities
• CVE published
  Jan 26, 2026 · microsoft
• Added to CISA KEV catalog
  Jan 25, 2026 · CISA

Source activity

Repeated readings are counted rather than listed.

• 2× References added · Oct 13, 2026 – Oct 13, 2026
• 1× Official description updated · Oct 13, 2026 – Oct 13, 2026

References

6 on the record

• msrc.microsoft.com/update-guide/vulnerability/CVE-2026-21509
  Vendor Advisory, vendor-advisory, patch, NIST National Vulnerability Database
• www.vicarius.io/vsociety/posts/cve-2026-21509-mitigation-script-microsoft-office-secu
  Mitigation, Third Party Advisory, NIST National Vulnerability Database
• www.cisa.gov/known-exploited-vulnerabilities-catalog
  US Government Resource, NIST National Vulnerability Database
• nvd.nist.gov/vuln/detail/CVE-2026-21509
  GitHub Security Advisories
• www.vicarius.io/vsociety/posts/cve-2026-21509-detection-script-microsoft-office-secur
  Third Party Advisory, NIST National Vulnerability Database
• github.com/advisories/GHSA-ch84-h92g-mw93
  GitHub Security Advisories

Elsewhere on this site

Related CVEs (same products)

• Medium CVE-2026-108455 Microsoft Office PowerPoint Information Disclosure Vulnerability 7h
• Medium CVE-2026-108453 Microsoft Office PowerPoint Information Disclosure Vulnerability 7h
• Medium CVE-2026-108454 Microsoft Office PowerPoint Information Disclosure Vulnerability 7h
• High CVE-2026-108448 Microsoft Office Outlook Remote Code Execution Vulnerability 7h
• High CVE-2026-108447 Microsoft Office Word Remote Code Execution Vulnerability 7h
• High CVE-2026-108451 Microsoft Office Word Remote Code Execution Vulnerability 7h

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.

• No confirmed IOCs, IP addresses, domains, file hashes, or malware 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 record

Text assembled only from the fields above — no additional claims.

What should defenders know first?

CVE-2026-21509 is "Microsoft Office Security Feature Bypass Vulnerability", rated High. It affects 365 Apps (Microsoft); versions are in the scope table. It is in CISA KEV: there is evidence of real-world exploitation. CVSS 7.8 describes technical severity. EPSS 70.80% estimates the probability of exploitation in 30 days; neither replaces your asset context. First verify whether exposed or business-critical assets run the affected versions, apply vendor fixes or mitigations, and preserve logs for review.

Comments

Moderated technical discussion. Read the community guidelines.

No comments yet.

Sign in or create an account to comment and suggest corrections.