CVE-2026-56191: Improper authentication in Microsoft Exchange Online allows an unauthorized attacker to perform tampering over

This week

CRITICAL 10.0REMOTEWATCHLISTMITIGATE

CVE-2026-56191 is a critical-severity flaw (CVSS 10.0) affecting exchange online. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

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

Why this priority

Critical severity
CISA SSVC: automatable with total technical impact
Remotely exploitable, no login, no user action
On your watchlist

A fixed release or distro patch exists.

Risk facts

CVSS score	10.0 (Critical)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	0.9% (top 41%)
CISA exploited list	No
Weakness	CWE-287
Published	2026-07-24
Last modified	2026-10-24

Publisher (CNA): Microsoft Corporation · Vendor: microsoft

Last updated 2026-10-24 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-07-24 01:17 UTCPublished CVE record published
2026-10-24 13:10 UTCUpdated Record last modified

microsoft exchange online MITIGATE

Affected: all versions

Fixed in: no fixed release known

Safe to downgrade to: no unaffected older release

1. Check installed version

apt-cache policy exchange-online || rpm -q exchange-online || exchange-online --version

No fixed or unaffected release known

Apply the vendor mitigation from the advisory, disable the affected feature, or restrict network access.

Exact fixes by distribution

A distro patch exists: prefer upgrading over downgrading.

Fedora 46 · gnome-shell-extension-tiling-assistant · FIXED · 56-1.fc46 · FEDORA-2026-ebf66a5321 · source

sudo dnf upgrade --advisory=FEDORA-2026-ebf66a5321

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2026-56191?

exchange online: mitigate or apply the listed mitigation. Exact commands for each OS are on this page.

Is CVE-2026-56191 being exploited?

CVE-2026-56191 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2026-56191 affect?

CVE-2026-56191 affects exchange online. Severity is critical (CVSS 10.0). Improper authentication in Microsoft Exchange Online allows an unauthorized attacker to perform tampering over a network.

How urgent is CVE-2026-56191?

Priority 63/100. Critical severity; CISA SSVC: automatable with total technical impact; Remotely exploitable, no login, no user action; On your watchlist

Software pages

Microsoft Exchange vulnerabilities

References

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