Rapid7 Analysis: Microsoft SharePoint JWT Token Authentication Bypass (CVE-2026-55040)
Admiralty grading (A–F · 1–6)
Source reliability
- A Completely reliable
- B Usually reliable
- C Fairly reliable
- D Not usually reliable
- E Unreliable
- F Cannot be judged
Information credibility
- 1 Confirmed
- 2 Probably true
- 3 Possibly true
- 4 Doubtful
- 5 Improbable
- 6 Cannot be judged
NATO Admiralty (AJP-2.1) grades confidence, independent of the risk score. Cross-source corroboration isn't tracked for non-CVE news, so single-source items are capped at a lower credibility number; a low number does not imply low quality.
Key metrics
- CVSS
- 9.1
- EPSS
- 40%
- KEV due date
- 21 August 2026
Affected versions
Key insight
A critical flaw in SharePoint's JWT token validation logic allows attackers to bypass authentication without valid cryptographic signatures.
Description
CVE-2026-55040 is a critical authentication bypass vulnerability in Microsoft SharePoint rooted in multiple JWT token validation flaws. The vulnerability allows an attacker to craft forged JWT tokens that SharePoint will accept, as critical security checks can be disabled or bypassed. Specifically affected are RequireSignedTokens validation, x5t header resolution without signature verification, faulty issuer validation, and a non-cryptographic signature check. Once authentication is bypassed, an attacker gains access to the authenticated attack surface of the target SharePoint deployment and can perform subsequent operations.
Risk score
- cvss base
- 91.00
- kev bonus
- 20.00
- epss bonus
- 0.00
- poc bonus
- 15.00
- raw before weight
- 126.00
- industry weight
- 1.21
- freshness factor
- 0.50
- exploitability factor
- 1.00
- days old
- 28.00
- vendor mismatch penalty
- 0.00
Path: operational