CVE-2026-56191Network attack vector

Unauthenticated hosted-service tampering

Published Jul 24, 2026 · Updated Jul 24, 2026

Improper authentication in Microsoft Exchange Online allows remote attackers to tamper with the hosted service through network requests. Microsoft has not published the affected endpoint, authentication check, or request operation that accepts an unverified identity. No credentials or user interaction are required, but the exact changes and affected tenant boundary remain undisclosed.

CVSS severity10.0
Critical
EPSS probability0.76%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Improper authentication in Microsoft Exchange Online allows remote attackers to tamper with the hosted service through network requests. Microsoft has not published the affected endpoint, authentication check, or request operation that accepts an unverified identity. No credentials or user interaction are required, but the exact changes and affected tenant boundary remain undisclosed.

The record

CVE
CVE-2026-56191
Published
Jul 24, 2026
Updated
Jul 24, 2026
Vendor
Microsoft
Product
Microsoft Exchange Online
Classifications
CWE-287
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Jul 24, 2026CVE publishedPublication date reported by the CVE source.
  2. Jul 24, 2026Record updatedLatest update available in the CVE record.

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.

  1. Affected versionversion=-

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Microsoft · Microsoft Exchange Onlineversion=-

Published CVSS scores

10.0Microsoft CorporationCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H

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.

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No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-287Improper Authentication

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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