CVE-2026-55040
CVE-2026-55040 is a security flaw in Microsoft SharePoint, Microsoft’s software for company intranets and shared documents. Attackers can get past a security protection in 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 Aug 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 16.0.5561.1001, 16.0.10417.20175 and 16.0.19725.20434.
Description
CISA calls it a weak authentication in Microsoft SharePoint. In its words: “Microsoft SharePoint contains a weak authentication vulnerability which allows an unauthorized attacker to bypass a security feature over a network.”[1]
The underlying weakness is CWE-1390, in MITRE’s list of common software weaknesses.[2]
It scores 9.1 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 | 16.0.5561.1001 |
| Microsoft SharePoint Server 2019 | 16.0.10417.20175 |
| Microsoft SharePoint Server Subscription Edition | 16.0.19725.20434 |
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 Aug 2026, 35 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 Aug 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 70%. That’s in the top 1% 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 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).”[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 16.0.5561.1001, 16.0.10417.20175 and 16.0.19725.20434), or not in use. CISA gave US government agencies until 21 Aug 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 9.1.[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 70%.[4] |
See also
Other Microsoft SharePoint flaws
- CVE-2026-65660: code injection ·
- CVE-2026-50522: deserialization of untrusted data ·
- CVE-2026-58644: deserialization of untrusted data ·
- CVE-2026-20963: 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 ·
Added the same day
- CVE-2026-65400, Apple macOS: improper authentication ·
- CVE-2026-59310, Broadcom VMware vCenter: path traversal ·
- CVE-2026-33824, Microsoft Internet Key Exchange (IKE) Service Extensions: double free ·
Every Microsoft flaw on CISA’s list · Everything added in August 2026
References
- ^ Cybersecurity and Infrastructure Security Agency. “Microsoft SharePoint Weak Authentication Vulnerability”. Known Exploited Vulnerabilities Catalog, added 18 Aug 2026.
- ^ National Institute of Standards and Technology. “CVE-2026-55040 Detail”. National Vulnerability Database, published 14 Jul 2026.
- ^ Microsoft. Advisories and fixes: msrc.microsoft.com.
- ^ FIRST. “Exploit Prediction Scoring System”. EPSS 0.695, percentile 99.4, 31 Oct 2026.
- CVE Program. “CVE-2026-55040”. 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.