CVE-2026-56191
A flaw in Microsoft Exchange Online lets an attacker bypass authentication and change data over the network. Because the vulnerability can be exploited from anywhere, it poses a serious risk to all users of the service. The flaw is rated critical due to its potential to compromise confidentiality, integrity, and availability.
No official fix yet. If none appears within 45 days of first tracking, this entry is removed automatically.
Who is affected
Microsoft Exchange Online, the cloud-based email and collaboration service.
Real-world impact
An attacker who exploits this flaw can modify, delete, or add messages and other data in Exchange Online, potentially disrupting business communications and exposing sensitive information.
Why this severity
The CVSS score of 10 reflects that the flaw can be exploited over the network without any authentication, giving an attacker full control over the affected system’s confidentiality, integrity, and availability.
What to do about it
No official fix or mitigation is documented in the sources yet. Monitor the vendor advisory and apply the patch as soon as it is released.
No fix documented in sources
Timeline
- Jul 24, 2026 · 8d agoPublishedDisclosed and added to the National Vulnerability Database.
- Jul 29, 2026 · 3d agoAdvisory updatedThe NVD record was last revised.
How it’s attacked
References & advisories
- msrc.microsoft.com/update-guide/vulnerability/…vendor advisory