Security readout for executives and security teams
Plain-English summary
CVE-2026-55040 lets an unauthenticated network attacker bypass a SharePoint security control. Successful exploitation can expose or alter sensitive information without user interaction. Because CISA lists it as known exploited and public exploit material is referenced, affected organizations should treat remediation as urgent.
Executive priority
Immediate. Assign accountable owners, identify affected servers, patch urgently, and investigate possible compromise. KEV inclusion means this is not merely theoretical; delayed remediation creates material risk to SharePoint-hosted information and business workflows.
Technical view
This is a weak-authentication security-feature bypass in Microsoft SharePoint Enterprise Server 2016, SharePoint Server 2019, and SharePoint Server Subscription Edition. CVSS 3.1 rates it 9.1: network-accessible, low complexity, no privileges, and no user interaction, with high confidentiality and integrity impact but no stated availability impact.
Likely exposure
Organizations running the listed SharePoint Server products are potentially exposed, especially where SharePoint is reachable from untrusted networks. The bundle identifies version 16.0.0 for each product but does not provide deployment-specific build ranges, configurations, or evidence concerning unlisted products.
Exploitation context
CISA KEV status supports active exploitation. The bundle also references public exploit material and a Rapid7 technical analysis. It does not provide campaign scope, victim counts, observed indicators, or attribution, so those details should not be assumed.
Researcher notes
The supplied vector is AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N, with CWE-1390. Evidence supports authentication bypass and active exploitation, but the bundle does not establish root-cause details, reliable indicators, campaign prevalence, or complete affected-build boundaries. Validate those points through the cited vendor and technical sources.
Mitigation direction
- Apply the Microsoft security update identified in the vendor advisory.
- Prioritize externally reachable and business-critical SharePoint servers.
- Restrict untrusted network access until remediation is verified.
- Review Microsoft guidance for any required post-update actions or mitigations.
- Preserve relevant logs before investigation or remediation changes.
Validation and detection
- Inventory SharePoint 2016, 2019, and Subscription Edition deployments.
- Compare installed builds and updates against Microsoft’s CVE advisory.
- Confirm the update succeeded on every SharePoint server in each farm.
- Review authentication and access logs for unexplained unauthorized activity.
- Retest network exposure and access controls after remediation.
Public sources used
Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.
Conservative CVE-to-ATT&CK context
These mappings and lookup hints may be relevant to the vulnerability behavior, CWE, affected product, or exposure path. Glexia-inferred context is not an official MITRE, ATT&CK, CWE, or CVE Program mapping.
ATT&CK lookup starting points
Use these exact CWE pages and searches to review the Glexia ATT&CK library from this CVE's weakness and description context.
CWE-1390: Exact CWE lookup
Use the exact CWE identifier as the starting point before reviewing related ATT&CK behavior. Open the exact CWE lookup page first, then review the ATT&CK searches from that MITRE weakness context. This is a Glexia lookup hint, not an official ATT&CK mapping.
Open ATT&CK lookupCVE-2026-55040 mapping review
Open the CVE-to-ATT&CK bridge for reviewed, inferred, or future official mappings tied to this CVE.
Open ATT&CK lookup- Severity
- Critical
- CVSS
- 9.1 (3.1)
- Known Exploited
- Yes
- Published
Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N/E:U/RL:O/RC:C
CNA and ADP enrichment extracted from CVE v5
These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.
CISA KEV status
SSVC decision data
CVSS vector scores
1 official scoreWe collect every scored CVSS vector available in the official CNA and ADP containers. When more than one version is present, the table keeps the source vectors side by side instead of collapsing them into the highest score.
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N/E:U/RL:O/RC:C3.95.2microsoftVulnerability scoring details
Base CVSS 3.1 score
9.1CriticalVector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N/E:U/RL:O/RC:C
Vulnerability timeline
Timeline events are normalized from CVE metadata, CNA source timelines, ADP timelines, and KEV metadata when present.
- CVE reservedCVE Program
The CVE ID was reserved by the assigning CNA.
- CVE publishedCVE Program
The CVE record was published.
- Added to KEVCISA-ADP
CISA Known Exploited Vulnerabilities metadata lists this CVE as known exploited.
- ADP timelineCISA-ADP
CVE-2026-55040 added to CISA KEV
- CVE updatedCVE Program
The CVE record metadata indicates this as the latest update time.
ADP provider summaries
- 2026-08-18T00:00:00.000Z: CVE-2026-55040 added to CISA KEV
Source materials
- CVE List V5 sourceCVE List V5
- Microsoft SharePoint Server Security Feature Bypass VulnerabilityCVE reference, microsoft · vendor-advisory, patch
- https://www.rapid7.com/blog/post/ra-microsoft-sharepoint-jwt-token-authentication-bypass-cve-2026-55040/CVE reference, CISA-ADP · technical-description
- https://github.com/sfewer-r7/CVE-2026-55040CVE reference, CISA-ADP · exploit
- https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2026-55040CVE reference, CISA-ADP · government-resource
Products and packages named in the record
CWE details
CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.
Weak Authentication
Weak Authentication represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.