Microsoft SharePoint Server: Code execution via deserialization of untrusted data
Summary
A deserialization flaw in Microsoft Office SharePoint allows an authorized attacker to execute code over the network. The CVSS 3.1 base score is 8.8 (High). The attacker needs valid low-privilege credentials, but no user interaction is required. Exposure of the server, especially from the internet, increases the risk.
Affected
- Microsoft SharePoint Server Subscription Edition: 16.0.0 to before 16.0.20326.20136 (per CNA data mirrored by Strix; not confirmed by MSRC)
- SharePoint Server 2019 and 2016: Unknown. Not assumed to be affected or unaffected.
Fixed in
- Subscription Edition: Likely fixed in the November 2026 security update (KB 5002945, build 16.0.20326.20278 or later). Not confirmed; verify the fix for CVE-2026-123390 in the MSRC advisory.
- SharePoint Server 2019 and 2016: Unknown.
Workaround
- No workaround specific to this CVE is known.
- Do not expose SharePoint Servers directly to the internet.
- Block external access to SharePoint Central Administration and restrict farm and database communication to required systems.
- Check that AMSI is enabled for each web application, with Full Mode request body scanning where feasible.
Exploitation
- Not listed in the CISA Known Exploited Vulnerabilities catalogue at publication.
- No public proof of concept found in the sources reviewed.
- Related SharePoint deserialization flaw CVE-2026-45659 is listed in KEV; this does not confirm exploitation of CVE-2026-123390.
Our recommendation
- Confirm whether the organisation runs Subscription Edition, 2019 or 2016.
- Patch Subscription Edition to the November 2026 CU or later once MSRC confirms the fix.
- Check internet exposure and AMSI configuration now, and prioritise internet-facing servers.
- Monitor for a KEV addition and review account and access logs for SharePoint.
Information from Microsoft (Patch Tuesday November 2026)
- Severity (Microsoft)
- Important
- Impact
- Remote code execution – Microsoft Office SharePoint
- Status on Patch Tuesday
- Neither exploited nor publicly disclosed
- Product family
- Microsoft Office
- Updates (KB)
- KB5002908
Public exploits
PoC-in-GitHub, Metasploit and Exploit-DB list nothing for this CVE.
Research sources
- Microsoft MSRC Security Update Guide (CVE-2026-123390) msrc.microsoft.com
- Strix CVE record CVE-2026-123390 www.strix.ai
- Stefan Goßner: SharePoint security fixes November 2026 blog.stefan-gossner.com
- Stefan Goßner: November 2026 CU for SharePoint Subscription Edition blog.stefan-gossner.com
- CISA: SharePoint hardening after new exploitations www.cisa.gov
- SecurityWeek: CISA warns of actively exploited SharePoint vulnerability www.securityweek.com
- Helix ThreatWatch: SharePoint CVE-2026-45659 helix.ml
Researched and written up in English and German by an AI (Claude by Anthropic) based on NVD, CISA, EPSS and the sources listed. The vendor information is authoritative. Researched on 10 Nov 2026, 21:12.
Original description (NVD)
Deserialization of untrusted data in Microsoft Office SharePoint allows an authorized attacker to execute code over a network.
NVD references (1)
Sources
The tracker combines official, freely usable data sources – updated continuously:
- NVD (NIST) – CVEs, rating (CVSS), affected products
- CISA KEV – vulnerabilities confirmed by CISA as actively exploited
- CISA Vulnrichment – CISA assessment (SSVC) in the official CVE record
- FIRST EPSS – probability of exploitation
- Microsoft MSRC – Patch Tuesday: vulnerabilities, severity, updates
- PoC-in-GitHub – public proof-of-concept repositories
- Metasploit (Rapid7) – modules of the Metasploit Framework
- Exploit-DB (OffSec) – archive of public exploits (links only)
- Patch days: official vendor pages (linked per vendor) (Vendor patch days)
This product uses data from the NVD API but is not endorsed or certified by the NVD. EPSS data © FIRST.org.
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