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CVE-2026-55040 is an authentication bypass vulnerability in Microsoft SharePoint caused by multiple flaws in the JWT token validation pipeline. A remote unauthenticated attacker can exploit the issue to bypass normal authentication checks on a vulnerable SharePoint server and impersonate an existing SharePoint user, including a site administrator. Reported exploitation requirements indicate the attacker must know the identity of the target account in advance, such as a valid Active Directory Security Identifier or User Principal Name. The vulnerability affects the SharePoint authentication boundary and can be used as an entry point for follow-on attacks against authenticated SharePoint functionality. Research demonstrated that this flaw can be chained with a separate SharePoint remote code execution vulnerability to achieve unauthenticated remote code execution.
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What an attacker gets, and what they’ve been doing with it.
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Patch, then assume compromise.
No public exploits tracked yet. Mallory keeps watching.
No public exploit code observed for this vulnerability.
4 sources tracked across advisories, community write-ups, and news. New activity surfaces here as Mallory finds it.
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.