CVE-2026-45659: Microsoft SharePoint Server Deserialization of Untrusted Data 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 Server. CISA added this CVE to its Known Exploited Vulnerabilities catalog on 2026-07-01, which means there is reliable evidence of exploitation in the wild. US federal agencies must remediate by 2026-07-04; treat that as the outer limit, not the target. CISA also lists it as known to be used in ransomware campaigns.
• EPSS 2.7%, percentile 86: FIRST’s model estimates a 2.7% chance of exploitation activity in the next 30 days, higher than 86% of all scored CVEs (elevated). EPSS is a probability, not a verdict.
• Needs an account (low (any account)). Exposure depends on who can log in.
• Editorial verdict: Act now. SharePoint CVE-2026-56164: Exploited Unauthenticated 0-Day — Ignore the modest vendor score: this was exploited as a zero-day against on-prem SharePoint, so internet-facing farms need the July 2026 updates, a machine-key rotation and a compromise check now.

What it is

Deserialization of untrusted data in Microsoft Office SharePoint allows an authorized attacker to execute code over a network.

CISA’s summary

Microsoft SharePoint Server contains a deserialization of untrusted data vulnerability which allows an authorized attacker to execute code over a network.

Weakness type: CWE-502.

Product: Microsoft SharePoint Server · Assigned by: microsoft · Published: 2026-05-22

Affected and fixed versions

Severity scores

• Attack vector: network (remotely reachable)
• Attack complexity: low
• Privileges required: low (any account)
• 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 2026-07-01; federal due date 2026-07-04. Known ransomware use: Known.
• 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 no, technical impact total.
• FIRST EPSS (model of 2026-10-21): 2.70% probability of exploitation activity in the next 30 days; percentile 86.

Timeline

• 2026-05-22 — CVE record published by microsoft
• 2026-07-01 — Added to CISA’s Known Exploited Vulnerabilities catalog
• 2026-07-04 — CISA remediation deadline for US federal agencies
• 2026-07-14 — erroneo.us brief: SharePoint CVE-2026-56164: Exploited Unauthenticated 0-Day
• 2026-08-11 — 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 Remote Code Execution Vulnerability msrc.microsoft.com

Vulnerability records

1. CVE.org record CVE-2026-45659 cve.org
2. NVD entry CVE-2026-45659 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

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