---
id: "CVE-2026-55040"
kind: "vulnerability"
aliases: ["CVE-2026-55040"]
title: "microsoft sharepoint server: Weak authentication in Microsoft Office SharePoint allows an unauthorized attacker to bypass a security feature over\u2026"
severity: "critical"
cvss: 9.1
epss: null
exploited: true
status: "open"
category: "vendor"
products: [{"name": "Microsoft SharePoint", "fixed": [], "affected": "", "ecosystem": "vendor"}, {"name": "microsoft sharepoint server", "fixed": ["16.0.19725.20434"], "affected": "< 16.0.19725.20434", "ecosystem": "vendor"}, {"name": "microsoft sharepoint server", "fixed": [], "affected": "= 2016", "ecosystem": "vendor"}, {"name": "microsoft sharepoint server", "fixed": [], "affected": "= 2019", "ecosystem": "vendor"}]
ecosystems: ["vendor"]
tags: ["exploited", "microsoft-365", "self-hosted", "sharepoint", "vendor"]
not_affected_if: ["no-sharepoint-deployment"]
published_at: "2026-07-14T18:18:15+00:00"
first_seen_at: "2026-10-23T19:11:51+00:00"
updated_at: "2026-10-23T23:44:52+00:00"
sources: [{"source": "kev", "url": "https://www.cisa.gov/known-exploited-vulnerabilities-catalog?search_api_fulltext=CVE-2026-55040", "first_seen_at": "2026-10-23T19:11:51+00:00"}, {"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-55040", "first_seen_at": "2026-10-23T23:44:52+00:00"}]
---

# microsoft sharepoint server: Weak authentication in Microsoft Office SharePoint allows an unauthorized attacker to bypass a security feature over…

Microsoft SharePoint has a weak authentication flaw that lets an unauthenticated remote attacker bypass a security feature over the network. CISA lists it as actively exploited in the wild.

_Summary, check and fix written by a model from the sources; labelled as such._

## Affected

- vendor:Microsoft SharePoint affected ? fixed ?
- vendor:microsoft sharepoint server affected < 16.0.19725.20434 fixed 16.0.19725.20434
- vendor:microsoft sharepoint server affected = 2016 fixed ?
- vendor:microsoft sharepoint server affected = 2019 fixed ?

## How to check

Confirm whether you run Microsoft SharePoint (on-premises or hosted) and identify its exposure to the internet.
Check Microsoft's security advisory for CVE-2026-55040 for affected builds and whether your build is patched.

## Fix

Apply the mitigations and patch from Microsoft's vendor instructions for CVE-2026-55040; for cloud-hosted SharePoint follow the vendor-side guidance.
If no mitigation is available, discontinue use of the affected SharePoint deployment per CISA BOD 26-04 guidance.

## Timeline

- 2026-10-23T19:11:51+00:00 kev https://www.cisa.gov/known-exploited-vulnerabilities-catalog?search_api_fulltext=CVE-2026-55040
- 2026-10-23T23:44:52+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2026-55040

## From the sources

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

CISA 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.

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_Free to read at a human pace. An API with keys and an MCP server are in preparation; early access: hello@infrastack.ai_