CVE-2026-55040: Microsoft SharePoint Weak Authentication Vulnerability
Actively exploited. At least one CVE in this advisory is listed in the CISA Known Exploited Vulnerabilities catalog — exploitation has been observed in the wild. Treat remediation as urgent.
Microsoft SharePoint contains a weak authentication vulnerability which allows an unauthorized attacker to bypass a security feature over a network.
CSIRTS triage
- What
- SharePoint contains a weak authentication vulnerability that allows unauthorized attackers to bypass security features over the network.
- Who is affected
- All SharePoint deployments exposed to network access without version-specific restrictions stated.
- Urgency
- Critical; actively exploited vulnerability with high impact on authentication integrity.
- Action
- Apply the latest security update from Microsoft for SharePoint immediately.
AI-assisted analysis generated from the source advisory — verify against the original.
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Details
Original advisory: https://nvd.nist.gov/vuln/detail/CVE-2026-55040
Exploitation outlook
EPSS (FIRST.org) estimates each CVE’s probability of exploitation in the next 30 days — here is the CSIRTS.com read on those numbers.
- Exploitation confirmedCVE-2026-55040Already exploited in the wild (CISA KEV) — the prediction phase is over. Patch now. Riskier than 92% of all EPSS-scored CVEs.
Referenced CVEs
| CVE | CSIRTS overview | External |
|---|---|---|
| CVE-2026-55040 | coverage & exploitation status | NVD · CVE.org |
Same CVEs, other sources
How other CERTs, PSIRTs and databases cover the vulnerabilities in this advisory.
- unknownexploitedMicrosoft security advisory – August 2026 monthly rollup (AV26-804) – Update 1cccs
- highexploitedCISA Adds Four Known Exploited Vulnerabilities to Catalogcisa
- unknownexploitedNCSC-2026-0237 [1.02] [H/H] Vulnerabilities Fixed in Microsoft Officencsc-nl
- unknownexploitedNCSC-2026-0237 [1.01] [H/H] Vulnerabilities fixed in Microsoft Officencsc-nl
- criticalexploitedAL26-017 - Critical vulnerabilities impacting Microsoft SharePoint Server – CVE-2026-56164, CVE-2026-55040 and…cccs
- unknownexploitedNCSC-2026-0237 [1.00] [M/H] Vulnerabilities fixed in Microsoft Officencsc-nl
- criticalCVE-2026-55040: Weak authentication in Microsoft Office SharePoint allows an unauthorized attacker to bypass a…nvd
- unknownexploitedCISA Urges SharePoint Hardening After New Exploitationscisa
- criticalCVE-2026-55040: Microsoft SharePoint Server Security Feature Bypass Vulnerabilitymsrc
Recent advisories for Microsoft SharePoint Weak
A cluster of recent advisories against the same product widens the attack surface — attackers routinely chain freshly published CVEs on one product, so review these together.
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