CVE-2026-20963
CVE-2026-20963 is a security flaw in Microsoft SharePoint, Microsoft’s software for company intranets and shared documents. Attackers can run their own code on Microsoft SharePoint. The US Cybersecurity and Infrastructure Security Agency (CISA) added it to its catalog of flaws known to be used in attacks on 18 Mar 2026.[1]
Microsoft SharePoint is business software or equipment. Most families won’t have it, but attackers break into it to get inside companies, and from there to what those companies hold about their customers, patients and staff. Fixed in version 16.0.5535.1001, 16.0.10417.20083 and 16.0.19127.20442.
Description
CISA calls it a deserialization of untrusted data in Microsoft SharePoint. In its words: “Microsoft SharePoint contains a deserialization of untrusted data vulnerability that allows an unauthorized attacker to execute code over a network.”[1]
The underlying weakness is deserialization of untrusted data (CWE-502), in MITRE’s list of common software weaknesses.[2]
It scores 9.8 out of 10 on the Common Vulnerability Scoring System, which rates it critical, scored by the organisation that published the CVE using CVSS 3.1.[2]
Affected versions
NVD’s record lists these fixed versions: in each line, everything before the version shown is affected.[2]
| Product | Fixed in |
|---|---|
| Microsoft SharePoint Enterprise Server 2016 | 16.0.5535.1001 |
| Microsoft SharePoint Server 2019 | 16.0.10417.20083 |
| Microsoft SharePoint Server Subscription Edition | 16.0.19127.20442 |
Most families won’t have Microsoft SharePoint; it’s for whoever looks after IT at work.
Exploitation
CISA added it to its catalog of known exploited vulnerabilities on 18 Mar 2026, 64 days after it was made public. CISA lists a flaw only when it has reliable evidence of it being exploited, and gave US federal agencies until 21 Mar 2026 to fix it.[1]
FIRST’s Exploit Prediction Scoring System, which estimates how likely a flaw is to be used in attacks in the next 30 days from what it sees across the internet, puts it at 29%. That’s in the top 2% of the flaws it scores.[4]
CISA assesses it as automatable: attackers can run it against many targets at once.
CISA doesn’t say who was attacked, or how.
Fix
Microsoft has published a fix or advice.[3] CISA’s required action: “Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable.”[1]
- Check whether you use it.Microsoft SharePoint is business software or equipment, so most families won’t have it. If you run a business, or work from home on company equipment, ask whoever looks after your IT.
- Send this page to your IT person.Ask them to confirm it’s fixed (fixed in version 16.0.5535.1001, 16.0.10417.20083 and 16.0.19127.20442), or not in use. CISA gave US government agencies until 21 Mar 2026 to fix it.
- If it can’t be fixed, take it offline.CISA’s advice when there’s no fix is to stop using the product, or take it off the internet, until there is one.
You can’t fix someone else’s server, but you can watch for what follows a break-in like this: a breach notice, a password reset you didn’t ask for, or a scam message that knows things it shouldn’t. HARD watches for those for you and your family.
Timeline
| Date | Event |
|---|---|
| The flaw is made public in the National Vulnerability Database, with a severity score of 9.8.[2] | |
| CISA adds it to its catalog of flaws known to be used in attacks.[1] | |
| CISA’s deadline for US government agencies to fix it.[1] | |
| FIRST puts the odds of it being used in attacks in the next 30 days at 29%.[4] |
See also
Other Microsoft SharePoint flaws
- CVE-2026-65660: code injection ·
- CVE-2026-55040: weak authentication ·
- CVE-2026-50522: deserialization of untrusted data ·
- CVE-2026-58644: deserialization of untrusted data ·
- CVE-2025-49706: improper authentication ·
- CVE-2025-49704: code injection ·
Other Microsoft flaws
- CVE-2026-96951, Microsoft Windows: command injection ·
- CVE-2026-85880, Microsoft Windows: heap-based buffer overflow ·
- CVE-2026-81963, Microsoft Windows: link following ·
- CVE-2019-1068, Microsoft SQL Server: remote code execution ·
- CVE-2026-33824, Microsoft Internet Key Exchange (IKE) Service Extensions: double free ·
Other deserialization of untrusted data flaws
- CVE-2021-23758, Ajax.NET Professional: deserialization of untrusted data ·
- CVE-2026-63077, JetBrains TeamCity: deserialization of untrusted data ·
- CVE-2026-45247, Mirasvit Full Page Cache Warmer: full page cache warmer deserialization of untrusted data ·
- CVE-2026-20131, Cisco Secure Firewall Management Center (FMC): software and cisco security cloud control (SCC) firewall management deserialization of untrusted data ·
- CVE-2025-26399, SolarWinds Web Help Desk: deserialization of untrusted data ·
Added the same day
- CVE-2025-66376, Synacor Zimbra Collaboration Suite (ZCS): cross-site scripting ·
Every Microsoft flaw on CISA’s list · Everything added in March 2026
References
- ^ Cybersecurity and Infrastructure Security Agency. “Microsoft SharePoint Deserialization of Untrusted Data Vulnerability”. Known Exploited Vulnerabilities Catalog, added 18 Mar 2026.
- ^ National Institute of Standards and Technology. “CVE-2026-20963 Detail”. National Vulnerability Database, published 13 Jan 2026.
- ^ Microsoft. Advisories and fixes: msrc.microsoft.com.
- ^ FIRST. “Exploit Prediction Scoring System”. EPSS 0.292, percentile 98.2, 31 Oct 2026.
- CVE Program. “CVE-2026-20963”. The official CVE record.
This article is built from CISA’s catalog (version 2026.10.30), NVD’s record and FIRST’s scores, kept in HARD’s database and checked every morning. The plain English is HARD’s.