How severe is CVE-2026-56191?

CVE-2026-56191 has a CVSS 3.x base score of 10.0, rated critical severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is high, integrity high, and availability high.

Is CVE-2026-56191 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 1% (49th percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2026-56191?

CVE-2026-56191 affects Microsoft Exchange Online. See the affected-products list for the exact vulnerable versions.

How do I fix CVE-2026-56191?

Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its critical severity, prioritise patching exposed systems.

When was CVE-2026-56191 published?

CVE-2026-56191 was published on 2026-07-24 and last updated on 2026-07-29.

CVE-2026-56191

CVE-2026-56191 is a critical-severity vulnerability in Microsoft Exchange Online with a CVSS 3.x base score of 10.0. It is not currently listed as actively exploited by CISA, and its EPSS exploit-prediction score is low. The underlying weakness is classified as CWE-287.

Key facts

Severity: Critical (CVSS 3.x base score 10.0)
EPSS exploit prediction: 1% (49th percentile)
Actively exploited: Not listed in CISA KEV
Weakness: CWE-287
Affected product: Microsoft Exchange Online
Published: 2026-07-24
Last modified: 2026-07-29

Description

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

Frequently asked questions

What is CVE-2026-56191?: Improper authentication in Microsoft Exchange Online allows an unauthorized attacker to perform tampering over a network.
How severe is CVE-2026-56191?: CVE-2026-56191 has a CVSS 3.x base score of 10.0, rated critical severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is high, integrity high, and availability high.
Is CVE-2026-56191 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 1% (49th percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2026-56191?: CVE-2026-56191 affects Microsoft Exchange Online. See the affected-products list for the exact vulnerable versions.
How do I fix CVE-2026-56191?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its critical severity, prioritise patching exposed systems.
When was CVE-2026-56191 published?: CVE-2026-56191 was published on 2026-07-24 and last updated on 2026-07-29.

References

https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-56191

Affected products (1)

cpe:2.3:a:microsoft:exchange_online:-:*:*:*:*:*:*:*

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