What is CVE-2026-56191?

CVE-2026-56191 is a critical-severity vulnerability in Microsoft Exchange Online, classified under Improper Authentication. CVSS score: 10.0/10. Published 2026-07-24.

How severe is CVE-2026-56191?

Critical severity. CVSS v3 base score is 10.0 out of 10.

Auth bypass in Microsoft Exchange Online

CVE-2026-56191

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

Vulnerability class: Broken Authentication

CVSS v3 metric

CVSS v3 base score 10.0 (Critical). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H.

Affected products

Microsoft Exchange Online

Weakness classification (CWE)

CWE-287 — Improper Authentication

References

secure@microsoft.com (vendor-advisory, patch)

Frequently asked questions

What is CVE-2026-56191?: CVE-2026-56191 is a critical-severity vulnerability in Microsoft Exchange Online, classified under Improper Authentication. CVSS score: 10.0/10. Published 2026-07-24.
How severe is CVE-2026-56191?: Critical severity. CVSS v3 base score is 10.0 out of 10.