CVE-2026-55040: Microsoft SharePoint Weak Authentication Vulnerability

Should you act?

Exploited: patch now if you run it

CISA has confirmed exploitation in the wild. This signal is computed from CISA KEV, EPSS and CVSS data; it is not an editorial verdict.

• Yes, if you run Microsoft SharePoint. CISA added this CVE to its Known Exploited Vulnerabilities catalog on 2026-08-18, which means there is reliable evidence of exploitation in the wild. US federal agencies must remediate by 2026-08-21; treat that as the outer limit, not the target.
• EPSS 69.5%, percentile 99: FIRST’s model estimates a 69.5% chance of exploitation activity in the next 30 days, higher than 99% of all scored CVEs (very high). EPSS is a probability, not a verdict.
• Remotely reachable with no login and no user interaction. If the vulnerable service is exposed to the internet, prioritize it.
• Editorial verdict: Act now. SharePoint: Four July-August 2026 Flaws Now Exploited — Any on-premises SharePoint farm not yet on the August 2026 builds is exposed to exploited bugs with public exploit code; patch, rotate machine keys and check for compromise now.

What it is

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

CISA’s summary

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

Weakness type: CWE-1390.

Product: Microsoft SharePoint · Assigned by: microsoft · Published: 2026-07-14

Affected and fixed versions

Severity scores

• Attack vector: network (remotely reachable)
• Attack complexity: low
• Privileges required: none (no login)
• User interaction: none
• Scope: unchanged

CVSS measures how bad a flaw is if exploited, not how likely exploitation is. Read it next to the exploitation data below.

Exploitation and urgency

• CISA KEV: added 2026-08-18; federal due date 2026-08-21. Known ransomware use: Unknown.
• 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.
• CISA SSVC assessment: exploitation active, automatable yes, technical impact total.
• FIRST EPSS (model of 2026-10-21): 69.54% probability of exploitation activity in the next 30 days; percentile 99.

Timeline

• 2026-07-14 — CVE record published by microsoft
• 2026-07-14 — erroneo.us brief: SharePoint: Four July-August 2026 Flaws Now Exploited
• 2026-08-18 — Added to CISA’s Known Exploited Vulnerabilities catalog
• 2026-08-21 — CISA remediation deadline for US federal agencies
• 2026-10-08 — CVE record last updated

Sources and references

Original sources first. Version and fix details should always be confirmed against the vendor or project advisory.

Vendor and project advisories, patches

1. Microsoft SharePoint Server Security Feature Bypass Vulnerability msrc.microsoft.com

Vulnerability records

1. CVE.org record CVE-2026-55040 cve.org
2. NVD entry CVE-2026-55040 nvd.nist.gov

Government

1. CISA Known Exploited Vulnerabilities catalog entry cisa.gov
2. Listed in CISA KEV notes (cisa.gov) cisa.gov
3. Listed in CISA KEV notes (cisa.gov) cisa.gov

Other references from the CVE record and advisory

1. rapid7.com rapid7.com
2. github.com github.com

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