MSRC-CVE-2026-56191 — Microsoft Exchange Online Tampering Vulnerability

Summary

Impact: Tampering. Microsoft rating: Critical. Not known to be exploited. Customer action: Not required.

Details

Why are there no links to an update or instructions with steps that must be taken to protect from this vulnerability?
This vulnerability has already been fully mitigated by Microsoft. There is no action for users of this service to take. The purpose of this CVE is to provide further transparency.
Please see Toward greater transparency: Unveiling Cloud Service CVEs for more information.

Vulnerabilities 1

Critical 10.0

CVE-2026-56191

Microsoft Exchange Online Tampering Vulnerability

Improper authentication in Microsoft Exchange Online allows an unauthorized attacker to perform tampering over a network.

Microsoft Microsoft Exchange Online

Jul 24, 2026

Revision history

2026-07-23 #1
Information published.

References

CVE-2026-56191 Microsoft Exchange Online Tampering Vulnerability - HTML
https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-56191
Microsoft Exploitability Index
https://www.microsoft.com/en-us/msrc/exploitability-index?rtc=1
Microsoft Support Lifecycle
https://support.microsoft.com/lifecycle
Common Vulnerability Scoring System
https://www.first.org/cvss