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CVE-2026-55040 is a security feature bypass vulnerability in Microsoft SharePoint Server caused by weak authentication in the JWT token validation pipeline. Multiple validation weaknesses allow a remote unauthenticated attacker to bypass authentication and impersonate a SharePoint site user or administrator. The issue affects on-premises SharePoint deployments and enables the attacker to perform operations in the security context of the impersonated account. Research associated with the disclosure showed that the flaw can be used as the initial access component in a broader exploit chain and was successfully chained with a separate authenticated SharePoint remote code execution vulnerability to achieve unauthenticated remote code execution against a vulnerable server.
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No public exploit code observed for this vulnerability.
15 sources tracked across advisories, community write-ups, and news. New activity surfaces here as Mallory finds it.
A newly disclosed Microsoft SharePoint Server vulnerability identified by Microsoft as posing potential risk if left unpatched; not yet known to be exploited according to the content.
A SharePoint Server JWT authentication bypass vulnerability that can be chained with a separate unpatched RCE bug to achieve unauthenticated remote code execution.
An authentication bypass vulnerability in Microsoft SharePoint caused by issues in the JWT token validation pipeline. It allows a remote unauthenticated attacker to impersonate a SharePoint site user or administrator and can be chained with a separate RCE flaw for unauthenticated remote code execution.
A critical security feature bypass vulnerability in Microsoft SharePoint Server caused by weak authentication.
Query your assets running an affected version, and investigate the blast radius.
Every observed campaign linking this CVE to a named adversary.
Malware families riding this exploit, with evidence and IOCs.
YARA, Sigma, Snort, and vendor rules, auto-deployed to your SIEM.
Cross-references every affected SKU, including bundled OEM variants.
Community discussion across Reddit, Mastodon, and other social sources.