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CVE-2026-55040

CRITICAL 9.1 KNOWN EXPLOITED PoC CATALOGUED DETECTION TEMPLATE ALL-YEARS

Published 2026-07-14 · Last modified 2026-10-08 · microsoft · NVD: Analyzed

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

ACTIVELY EXPLOITED

Confirmed exploited in the wild — in CISA KEV since 2026-08-18. Patch or mitigate now.

Exploitation likelihood

69.5%chance of exploitation in 30 days · 99th percentile

● In CISA KEV (2026-08-18)● Catalogued PoC◆ SSVC: exploitation active, automatable yes

Impact if exploited

9.1CVSS 3.1 · CRITICAL · CNA

  • ConfidentialityHigh
  • IntegrityHigh
  • AvailabilityNone

What an attacker needs

  • ✓Access: Reachable over the network — no local access needed
  • ✓Privileges: No account or privileges required
  • ✓User interaction: No user interaction needed
  • ✓Complexity: No special conditions — reliably repeatable

✓ lowers the bar for an attacker · ⚠ raises it

Proof of concept

Public PoC or exploit resources are catalogued below.

Listed by the named source; Vulnpedia has not tested these resources.

PoC data: JSON

Detection templates

Detection checks are listed separately from PoCs.

Resource commands

nuclei -id CVE-2026-55040 -u https://TARGETDetection template — review its checks and affected versions before use.
curl -s https://vulnpedia.com/cve/CVE-2026-55040/poc.jsonMachine-readable PoC index for this CVE (for automation).

Search for new PoCs

Search GitHub now — results may be empty or unverified.

Tracked sources are checked weekly for the full archive, including PoCs published after the original disclosure. A later release appears after a tracked source lists it and a refresh succeeds.

Data snapshot:

Affected

Vendors Microsoft

Products Microsoft Sharepoint Enterprise Server 2016 Microsoft Sharepoint Server 2019 Microsoft Sharepoint Server Subscription Edition Sharepoint Server

Weakness (CWE)

  • CWE-1390: : Weak Authentication

CVSS 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

References and research data (JSON)

Unverified PoC candidates

Catalogue matches for further research. These repositories have not been validated as working exploits.

5 candidates

Additional exploitation reports

Source-specific reports, separate from CISA KEV. Repeated observations from one source are not independent confirmations.

  • VulnCheck KEV · 2026-08-12T00:00:00Z

    Source guidance: 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 ransomware assessment: Unknown

    VulnCheck KEV · CVE-2026-55040 · Updated 2026-08-12T00:00:00Z
  • VulnCheck KEV · 2026-08-12T00:00:00ZVulnCheck KEV · CVE-2026-55040
    VulnCheck KEV · CVE-2026-55040
    VulnCheck KEV · CVE-2026-55040
  • VulnCheck KEV · 2026-08-14T00:00:00ZVulnCheck KEV · CVE-2026-55040
  • VulnCheck KEV · 2026-08-18T00:00:00ZVulnCheck KEV · CVE-2026-55040
  • VulnCheck KEV · 2026-08-25T00:00:00ZVulnCheck KEV · CVE-2026-55040
  • VulnCheck KEV · 2026-09-08T00:00:00ZVulnCheck KEV · CVE-2026-55040

Known Affected Software Configurations

VendorProductVersion range
MicrosoftSharepoint Server< 16.0.19725.20434
MicrosoftSharepoint Server2016
MicrosoftSharepoint Server2019

All CVSS metrics

  • CRITICAL 9.1 v3.1 · CNA Primary
    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
  • CRITICAL 9.1 v3.1 · NVD Secondary
    CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N

References

Exploit-tagged references (not proof of a PoC)

Patches & mitigations

Advisories

Technical & other

Sources: NVD · CVE.org · All sources and attribution · EPSS