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Exploited vulnerability · CISA KEV

Is CVE-2026-55040 exploited? Yes: in CISA KEV since 18 Aug 2026

Gociux · Microsoft SharePoint · data: CISA KEV, CVE record, FIRST EPSS

Yes. CVE-2026-55040, Microsoft SharePoint Weak Authentication Vulnerability, is being exploited in the wild. CISA added it to its Known Exploited Vulnerabilities catalog on 18 Aug 2026, 35 days after it was published. It affects Microsoft SharePoint. If you run it, CISA's required action is: 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 evaluating each asset's internet exposure and ensuring adherence to BOD 26-04 patching guidelines.

exploitedYesCISA KEV · 18 Aug 2026
ransomwareNot knownno confirmed use
EPSStop 1%among the most likely CVEs to be exploited
severity9.1CVSS 3.1 · critical
US federal due21 Aug 2026BOD 22-01 deadline

What it is

Microsoft SharePoint contains a weak authentication vulnerability which allows an unauthorized attacker to bypass a security feature over a network.

VendorMicrosoft (391 exploited CVEs in KEV)
ProductSharePoint
VulnerabilityMicrosoft SharePoint Weak Authentication Vulnerability
WeaknessCWE-1390
Published14 Jul 2026
Added to KEV18 Aug 2026
Reported by (CNA)microsoft

What to do

  1. Check whether you run it. Look for SharePoint in your asset inventory, including appliances and cloud images. Stack check lists every exploited Microsoft CVE at once.
  2. Apply the fix. 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 evaluating each asset's internet exposure and ensuring adherence to BOD 26-04 patching guidelines.
  3. If you can't patch today, take the affected service off the internet or restrict who can reach it, and check its logs for signs of compromise since 14 Jul 2026.
  4. Get told next time. Email alerts when CISA adds a CVE for Microsoft.

Other exploited CVEs in SharePoint

References

Questions

Is CVE-2026-55040 being exploited?

Yes. CISA added CVE-2026-55040 to the Known Exploited Vulnerabilities catalog on 18 Aug 2026, which means there is reliable evidence of exploitation in the wild.

Is CVE-2026-55040 used in ransomware attacks?

CISA does not list known ransomware use for it. That can change, and exploitation alone is reason to fix it first.

How do I fix CVE-2026-55040?

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 evaluating each asset's internet exposure and ensuring adherence to BOD 26-04 patching guidelines. Check Microsoft's advisory for the fixed versions, and if you can't patch quickly, restrict access to the affected SharePoint and watch it for signs of compromise.

Sources: CISA KEV (exploitation, ransomware use, required action, due date), the CVE record (publication date, CVSS, CNA), NVD and FIRST EPSS (live score). More: all KEV additions in August 2026 · Microsoft scorecard · act-now radar.

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