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CVE-2026-56191Critical

Microsoft Exchange Online Tampering Vulnerability

MSRC advisory NVD CVE.org

Assessment

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

Severity
Critical
CVSS base score
10.0CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H/E:U/RL:O/RC:C
Impact
Tampering
EPSS, next 30 days
0.9%Higher than 58.1% of scored CVEs · FIRST model run 26 Sep 2026 · about EPSS
Exploitability
Not assessed
Publicly disclosed
No
Customer action
Not required (service-side fix)
Component
Microsoft Exchange Online
Weakness
CWE-287: Improper Authentication
Issued by
Microsoft
Published
23 Jul 2026 · July 2026
Last revised
23 Jul 2026

Updates that fix it

0 KBs

No KB listed

MSRC lists no downloadable update, which usually means a service-side fix or a release-notes update. Check the MSRC advisory.

Affected products

Server Software
1 affected product
  • Microsoft Exchange Online
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