Unauthenticated user impersonation via unsigned JWT acceptance
Published Jul 14, 2026 · Updated Jul 14, 2026
Weak authentication in Microsoft SharePoint Server allows remote attackers to impersonate any site user via forged JWTs. The JWT validation pipeline accepts unsigned outer tokens, resolves attacker-selected certificates without verifying actor-token signatures, and accepts unregistered issuers. Network access to an on-premises SharePoint site is sufficient, and successful exploitation grants the selected user's permissions without affecting service availability.
Summary
What happened
Weak authentication in Microsoft SharePoint Server allows remote attackers to impersonate any site user via forged JWTs. The JWT validation pipeline accepts unsigned outer tokens, resolves attacker-selected certificates without verifying actor-token signatures, and accepts unregistered issuers. Network access to an on-premises SharePoint site is sufficient, and successful exploitation grants the selected user's permissions without affecting service availability.
The record
- CVE
- CVE-2026-55040
- Published
- Jul 14, 2026
- Updated
- Jul 14, 2026
- Vendor
- Microsoft
- Product
- Microsoft SharePoint Enterprise Server 2016
- Classifications
- CWE-1390, T1606
- Attack vector
- network
- Privileges
- unauthenticated
Timeline
How it unfolded
- Jul 14, 2026CVE publishedPublication date reported by the CVE source.
- Jul 14, 2026Record updatedLatest update available in the CVE record.
- Aug 18, 2026Exploitation reportedCISA Known Exploited Vulnerabilities entry
Exploitability
Present is not the same as exploitable
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Is a vulnerable build present?
Compare these published version ranges with your installed build and any vendor patches.
- Affected versionversion=16.0.0 <16.0.5561.1001
What conditions does exploitation require?
What is affected?
Published CVSS scores
CVSS describes severity. EPSS estimates exploitation probability.
Attacks
What attackers are doing with it
Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.
Public exploit references
- Rapid7 Labs SharePoint JWT authentication-bypass PoCproof of concept · demonstrated
Labels summarize the accepted research assessment. They do not indicate a test against your environment.
Reported exploitation
- CISA Known Exploited Vulnerabilities entryknown exploited catalog
Technologies
Your stack
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Your stack
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