What’s Vulnerable › Vulnerabilities

CVE-2026-56191: Microsoft Exchange Online Tampering Vulnerability

• Severity: critical (CVSS 10.0 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.9% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-07-24 · updated 2026-10-28
• Weakness: CWE-287

Description

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

Affected products

• Microsoft Exchange Online: all versions
• Microsoft Exchange Online: all versions

References

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

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