CVERadar
CRITICAL · CVSS 3.1 9.1CISA KEV · added 2026-08-18Public PoCEPSS 69.54% (p99)Threat level 81

CVE-2026-55040: Microsoft SharePoint Weak Authentication Vulnerability

Microsoft · SharePoint · NVD published 2026-07-14 · posted 2026-08-18 · last modified 2026-08-19

What it is

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

WeaknessCWE-1390
CVSS vectorCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N (source: [email protected])
CISA due date2026-08-21 (for US federal agencies under BOD; a good deadline for everyone)
ATT&CK tacticsn/a

Exploitation evidence

First exploit evidence seen: 2026-07-28 (earliest date across the sources below).

Exploited in the wild

  • CISA KEV: listed since 2026-08-18. Source

Public exploit code

GitHub proof-of-concept repositories

5 public repositories from 5 accounts, first seen 2026-07-28. Most starred: sfewer-r7/CVE-2026-55040 (59 stars), view repository.

Verify before running: public PoCs can be malicious. Read the code and only run it in an isolated lab. A PoC means exploit code is public, not that this CVE is exploited in the wild.

Sources: CISA KEV · nomi-sec/PoC-in-GitHub.

No ATT&CK techniques could be mapped or inferred for this CVE yet.

How to fix it

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. (source: CISA KEV)

Fixed versions

  • microsoft sharepoint server: upgrade to 16.0.19725.20434 or later

Derived from the NVD CPE configuration (versionEndExcluding / versionEndIncluding). Always confirm with the vendor advisory.

Vendor advisories and patches

Detection Generic guidance

    Response checklist Generic guidance

    1. Inventory: find every instance of the affected product and version (include internet-facing and forgotten ones).
    2. Patch or mitigate per the vendor advisory; if you cannot patch, remove internet exposure or apply the vendor workaround.
    3. Assume compromise if the system was exposed before patching: hunt for the indicators in the advisory and review logs back to the disclosure date.
    4. Rotate credentials and secrets stored on or used by the affected system if compromise cannot be ruled out.

    Sources