CVE-2026-45659: Microsoft SharePoint Server Deserialization of Untrusted Data Vulnerability

In short

Microsoft SharePoint Server has a weakness classified as deserialization of untrusted data (CWE-502). CISA added it to its Known Exploited Vulnerabilities catalog on 2026-07-01. NVD lists CVSS 3.1 at 8.8 HIGH. CISA reports known ransomware campaign use.

What it is

“Microsoft SharePoint Server contains a deserialization of untrusted data vulnerability which allows an authorized attacker to execute code over a network.” — CISA KEV, retrieved 2026-11-01

“Deserialization of untrusted data in Microsoft Office SharePoint allows an authorized attacker to execute code over a network.” — NVD, retrieved 2026-11-01

Who is affected

Microsoft SharePoint Server. NVD lists the following versions as affected:

• Versions before 16.0.19725.20280
• Versions version 2016
• Versions version 2019

Exploitation status

CISA KEV added: 2026-07-01. Known ransomware campaign use: Known (reported by CISA). Due date CISA set for US federal civilian agencies: 2026-07-04.

Scores

NVD CVSS 3.1: 8.8 HIGH; vector CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H; source [email protected].

EPSS by FIRST: 80.24%; percentile 99.62; score date 2026-10-31. EPSS estimates the probability of exploitation in the next 30 days.

CWE: CWE-502.

Timeline

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 KEV

What to do

• Confirm whether you run Microsoft SharePoint Server and which version.
• Apply the vendor update or mitigation in the advisory.
• Follow CISA’s required action.
• Review the vendor guidance on signs of compromise.
• Track the fix through completion.

References

• Vendor Advisory: https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-45659
• NVD detail
• CVE.org record
• CISA KEV catalog

Sources and dates

CISA Known Exploited Vulnerabilities Catalog, version 2026.10.30; retrieved 2026-11-01. CISA KEV data: CC0 1.0. No endorsement by CISA.

NVD retrieved: 2026-11-01. This product uses the NVD API but is not endorsed or certified by the NVD.

EPSS scores by FIRST; score date 2026-10-31.