Impacts Microsoft Exchange Online, a widely used cloud email platform; auth bypass could disrupt email integrity and tenant security across organizations.
CVE-2026-56191
Authentication bypass in Microsoft Exchange Online allows unauthorized tampering.
Is CVE-2026-56191 being exploited?
Not confirmed. CVE-2026-56191 does not appear in CISA's Known Exploited Vulnerabilities catalog, which records only exploitation that has been observed and reported publicly. That is evidence of absence of a report, not evidence the flaw is unattacked. EPSS currently estimates a 0.76% probability of exploitation in the next 30 days.
How severe is CVE-2026-56191?
CVE-2026-56191 is rated Critical with a CVSS score of 10.0. Severity describes how bad exploitation would be, not how likely it is: pair it with exploitation evidence before deciding what to patch first.
Is there a patch for CVE-2026-56191?
Yes. A fix has been recorded for CVE-2026-56191. The vendor advisory is the authority on the exact fixed version — apply it from there rather than from a summary.
What does CVE-2026-56191 affect?
CVE-2026-56191 affects Microsoft Exchange Online, Microsoft 365 Exchange Online. Confirm the exact affected versions against the vendor advisory before deciding you are exposed.
What should I do about CVE-2026-56191?
Apply vendor mitigations/patch immediately; enforce MFA, restrict admin access, and monitor logs.
Exploitation status reflects CISA's KEV catalog as we last synced it. Check the catalog directly.
Authentication bypass permits remote attackers to tamper with mailboxes, message flow, and tenant configuration; data manipulation and account takeover possible.
Immediate action required