Microsoft SharePoint Weak Authentication
CVE-2026-55040 is a critical-severity weak authentication vulnerability in Microsoft SharePoint Enterprise Server 2016 from Microsoft, with a CVSS 3.1 base score of 9.1 out of 10. It can be exploited remotely over the network by an unauthenticated attacker, without any action from a user. The record lists version 16.0.5561.1001 as the first without the flaw, so updating Microsoft SharePoint Enterprise Server 2016 to that release or later removes the exposure. CISA added it to the Known Exploited Vulnerabilities catalogue on August 18, 2026, which means attacks have been observed in the wild.
Attackers are already exploiting this vulnerability, according to CISA.
What to do
- Update Microsoft SharePoint Enterprise Server 2016 to 16.0.5561.1001 or a later version.
- Update Microsoft SharePoint Server 2019 to 16.0.10417.20175 or a later version.
- Update Microsoft SharePoint Server Subscription Edition to 16.0.19725.20434 or a later version.
- 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 evaluating each asset's internet exposure and ensuring adherence to BOD 26-04 patching guidelines.
- Read the advisory on msrc.microsoft.com for workarounds and exact fixed builds.
- Until you can update, restrict network access to the affected service so it is not reachable from untrusted networks.
Affected products and versions
| Vendor | Product | Affected versions | Fixed in |
|---|---|---|---|
| Microsoft | Microsoft SharePoint Enterprise Server 2016 | 16.0.0 up to, but not including, 16.0.5561.1001 | 16.0.5561.1001 |
| Microsoft | Microsoft SharePoint Server 2019 | 16.0.0 up to, but not including, 16.0.10417.20175 | 16.0.10417.20175 |
| Microsoft | Microsoft SharePoint Server Subscription Edition | 16.0.0 up to, but not including, 16.0.19725.20434 | 16.0.19725.20434 |
| Microsoft | Sharepoint Server | before 16.0.19725.20434; 2016; 2019 | 16.0.19725.20434 |
Version data comes from the organisation that assigned the CVE. Confirm against the vendor advisory before deciding you are unaffected.
How the severity score breaks down
- Attack vector
- NetworkCan be exploited remotely over a network, including the internet.
- Attack complexity
- LowNo special conditions are needed; the attack works reliably.
- Privileges required
- NoneNo account or login is needed.
- User interaction
- NoneNo action by a victim is required.
- Scope
- UnchangedImpact stays within the vulnerable component.
- Confidentiality
- HighAll data handled by the component can be read.
- Integrity
- HighData or code can be modified at will.
- Availability
- NoneAvailability is not affected.
Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N. Scored by CNA (microsoft.com).
Official description
Weak authentication in Microsoft Office SharePoint allows an unauthorized attacker to bypass a security feature over a network.
Source: the CVE record as published by its numbering authority, via NVD and CVE.org.
The underlying weakness: Weak Authentication
The product uses an authentication mechanism to restrict access to specific users or identities, but the mechanism does not sufficiently prove that the claimed identity is correct.
Timeline
- CISA published its SSVC assessment (exploitation: active, automatable: yes, technical impact: total).
- CVE record published.
- Added to CISA's Known Exploited Vulnerabilities catalogue.
- Record last changed materially (score, affected products, status or references).
- Deadline for US federal agencies to remediate, set by CISA.
References
Third-party and government advisories
Technical write-ups (may include proof of concept)
- github.com/sfewer-r7/CVE-2026-55040Exploit
- cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2026-55040Exploit, Third Party Advisory
Other vulnerabilities in Microsoft SharePoint Enterprise Server 2016
- 9.3 Critical severity
- 8.8 High severity
Microsoft SharePoint Code Injection Exploited
Recent Weak Authentication vulnerabilities
- 8.8 High severity
- 9.1 Critical severity
- 8.1 High severity
- 9.8 Critical severity
- 8.2 High severity
- 8.2 High severity
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