CVE-2026-55040
KEVMicrosoft SharePoint Weak Authentication Vulnerability
Description
Microsoft SharePoint contains a weak authentication vulnerability which allows an unauthorized attacker to bypass a security feature over a network.
Weak authentication in Microsoft Office SharePoint allows an unauthorized attacker to bypass a security feature over a network.
CVSS 3.1 vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:NPublishedJul 14, 2026
Last modifiedAug 19, 2026
Added to KEVAug 18, 2026
Remediation dueAug 21, 2026
References
- msrc.microsoft.com/update-guide/vulnerability/CVE-2026-55040PatchVendor Advisory
- github.com/sfewer-r7/CVE-2026-55040Exploit
- cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2026-55040ExploitThird Party Advisory
- rapid7.com/blog/post/ra-microsoft-sharepoint-jwt-token-authentication-bypass-cve-2026-55040/US Government Resource
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82
Risk assessment · Critical
Actively exploited. Patch immediately.
- Exploitation
- Confirmed in the wild (CISA KEV)
- Federal remediation deadline
- Overdue by 66 days
- CVSS
- 9.1 CRITICAL
- EPSS (30-day exploit probability)
- 69.5% · top 1% of all CVEs
- Weakness
- CWE-1390
- Attack vector
- Network, remotely reachable
- Preconditions
- No privileges and no user interaction
Recommended actions
- 1.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.
- 2.Treat as emergency change: patch or isolate internet-facing instances first.
- 3.Review exposure: confirm which instances are reachable from the internet and restrict access until patched.
- 4.Hunt for compromise: review logs for the disclosure window, not just from the patch date.
Assessment is derived automatically from the live signals above; verify against your own asset inventory.