---
title: "CVE-2026-45659: Microsoft SharePoint Server Deserialization of Untrusted Data Vulnerability"
url: https://erroneo.us/security/cve/CVE-2026-45659/
description: "CVE-2026-45659 (Microsoft SharePoint Server): Exploited in the wild (CISA KEV). Deserialization of untrusted data in Microsoft Office SharePoint allows an…"
type: vulnerability-record
section: "Security Digest"
date_published: 2026-05-22
date_modified: 2026-10-21
publisher: erroneo.us
author: erroneo.us Security Desk
cve: "CVE-2026-45659"
kev: true
cvss: 8.8
epss: 0.02704
license: All rights reserved. Quote with attribution and a link.
---

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

**Signal:** Exploited: patch now if you run it. CISA has confirmed exploitation in the wild. (Computed from CISA KEV, EPSS and CVSS; not an editorial verdict.)

## Should you act?

- 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.

## Affected products (CVE record)

- Microsoft Microsoft SharePoint Enterprise Server 2016: 16.0.0 < 16.0.5552.1002
- Microsoft Microsoft SharePoint Server 2019: 16.0.0 < 16.0.10417.20128
- Microsoft Microsoft SharePoint Server Subscription Edition: 16.0.0 < 16.0.19725.20280

## Severity

- microsoft CVSS 3.1: 8.8 `CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H/E:U/RL:O/RC:C`

## Exploitation

- 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.

## Sources

- [Microsoft SharePoint Remote Code Execution Vulnerability](https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-45659)
- [CVE.org record CVE-2026-45659](https://www.cve.org/CVERecord?id=CVE-2026-45659)
- [NVD entry CVE-2026-45659](https://nvd.nist.gov/vuln/detail/CVE-2026-45659)
- [CISA Known Exploited Vulnerabilities catalog entry](https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2026-45659)
- [Listed in CISA KEV notes (cisa.gov)](https://www.cisa.gov/news-events/directives/bod-26-04-prioritizing-security-updates-based-risk)
- [Listed in CISA KEV notes (cisa.gov)](https://www.cisa.gov/news-events/directives/bod-26-04-implementation-guidance-prioritizing-security-updates-based-risk)

---
Source page: https://erroneo.us/security/cve/CVE-2026-45659/ · Security Digest: https://erroneo.us/security/ · More: https://erroneo.us/llms.txt