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Known exploited vulnerability

CVE-2026-55040

criticalCVSS 3.1 9.1In CISA KEVEPSS 70%Published · Microsoft

The short answer

CVE-2026-55040 affects Microsoft Microsoft SharePoint Enterprise Server 2016 and is rated critical (CVSS 9.1). CISA added it to the Known Exploited Vulnerabilities catalog on 2026-08-18: attackers are using it, so patch it first. The vendor has linked a patch.

What it is

Weak authentication in Microsoft Office SharePoint allows an unauthorized attacker to bypass a security feature over a network.

Weakness: CWE-1390.

Severity

9.1 critical

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

Exploited (CISA KEV)

Yes, since Aug 18, 2026

Federal due date Aug 21, 2026

EPSS, next 30 days

70%

Higher than 99% of scored CVEs

How it can be exploited

Attack vector
Network
Attack complexity
Low
Privileges required
None
User interaction
None
Scope
Unchanged
Confidentiality impact
High
Integrity impact
High
Availability impact
None

What CISA says to do

Apply mitigations in accordance with vendor instructions, ensuring compliance with CISA’s BOD 26-04 Prioritizing Security Updates Based on Risk (see URL in Notes) guidance and CISA’s “Forensics Triage Requirements” (see URL in Notes). Follow applicable BOD 26-04 guidance for cloud services or discontinue use of the product if mitigations are unavailable. Stakeholders are responsible for…

Affected products

Microsoft SharePoint Enterprise Server 2016, Microsoft SharePoint Server 2019, Microsoft SharePoint Server Subscription Edition. NVD's record lists the exact affected versions.

References

Questions

Is CVE-2026-55040 being exploited?

Yes. CISA added CVE-2026-55040 to its Known Exploited Vulnerabilities catalog on 2026-08-18, with a federal remediation due date of 2026-08-21.

How do I fix CVE-2026-55040?

CISA's required action: Apply mitigations in accordance with vendor instructions, ensuring compliance with CISA’s BOD 26-04 Prioritizing Security Updates Based on Risk (see URL in Notes) guidance and CISA’s “Forensics Triage Requirements” (see URL in Notes). Follow applicable BOD 26-04 guidance for cloud services or discontinue use of the product if mitigations are unavailable. Stakeholders are responsible for…

From NVD, CISA KEV, FIRST EPSS and OSV.dev, checked . Status on NVD: Analyzed. The description is NVD's text, quoted as published.

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