CVEs / CVE-2026-55040
CVE-2026-55040: Microsoft SharePoint Weak Authentication Vulnerability
- Known exploited in the CISA KEV catalog since 18 Aug 2026
- critical 9.1 CVSS 3.1 (from the CNA)
- EPSS 69.5% chance of exploitation in the next 30 days, higher than 99% of CVEs (30 Oct 2026)
- Last changed at its sources
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Summary
Microsoft SharePoint contains a weak authentication vulnerability which allows an unauthorized attacker to bypass a security feature over a network.
Description
Weak authentication in Microsoft Office SharePoint allows an unauthorized attacker to bypass a security feature over a network.
Affected and fixed versions
| Product | Affected | Fixed in |
|---|---|---|
| Microsoft Microsoft SharePoint Enterprise Server 2016 | from 16.0.0, before 16.0.5561.1001 | 16.0.5561.1001 |
| Microsoft Microsoft SharePoint Server 2019 | from 16.0.0, before 16.0.10417.20175 | 16.0.10417.20175 |
| Microsoft Microsoft SharePoint Server Subscription Edition | from 16.0.0, before 16.0.19725.20434 | 16.0.19725.20434 |
Exploitation
| In CISA KEV since | 18 Aug 2026 |
|---|---|
| Due date for US federal agencies | 21 Aug 2026 |
| Used in ransomware campaigns | Unknown |
| 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 evaluating each asset's internet exposure and ensuring adherence to BOD 26-04 patching guidelines. |
Severity
| CVSS 3.1 | 9.1 criticalCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:NScored by the CNA ([email protected]) |
|---|---|
| Weakness | CWE-1390 |
References
- CVE-2026-55040 on NVD
- CVE-2026-55040 in the CISA KEV catalog
- https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-55040 (cited by CISA)
- https://www.cisa.gov/news-events/directives/bod-26-04-prioritizing-security-updates-based-risk (cited by CISA)
- https://www.cisa.gov/news-events/directives/bod-26-04-implementation-guidance-prioritizing-security-updates-based-risk (cited by CISA)
- https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-55040 Patch, Vendor Advisory
- https://github.com/sfewer-r7/CVE-2026-55040 Exploit
- https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2026-55040 Exploit, Third Party Advisory
- https://www.rapid7.com/blog/post/ra-microsoft-sharepoint-jwt-token-authentication-bypass-cve-2026-55040/ US Government Resource
Sources
Exploitation status from the CISA Known Exploited Vulnerabilities catalog. EPSS scores from FIRST. Descriptions, severity and references from the National Vulnerability Database. Open-source package versions from OSV (CC-BY 4.0). Text is shown as each source states it; Stackurity does not edit it. Check the vendor's advisory before acting.
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