CVE-2026-56191
Received Received - Intake

Improper Authentication in Microsoft Exchange Online

Publication date: 2026-07-24

Last updated on: 2026-07-24

Assigner: Microsoft Corporation

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

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Meta Information

Published
2026-07-24
Last Modified
2026-07-24
Generated
2026-07-24
AI Q&A
2026-07-24
EPSS Evaluated
N/A
NVD
CVE-2026-56191
EUVD
EUVD-2026-48458

Affected Vendors & Products

Showing 1 associated CPE
Vendor Product Version / Range
microsoft exchange_online *

Helpful Resources

Exploitability

CWE
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KEV
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CWE ID Description
CWE-287 When an actor claims to have a given identity, the product does not prove or insufficiently proves that the claim is correct.

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Executive Summary

This vulnerability is an improper authentication flaw in Microsoft Exchange Online. It allows an unauthorized attacker to perform tampering over a network without needing valid credentials.

Detection Guidance

Detection requires monitoring Microsoft Exchange Online for unauthorized tampering attempts. Check Exchange Online logs for unusual authentication failures or suspicious activity. Use Microsoft Defender for Office 365 to scan for tampering indicators. Enable audit logging in the Microsoft 365 compliance center to track changes.

Impact Analysis

An attacker could modify data or communications in your Exchange Online environment. This could lead to unauthorized changes, data corruption, or misinformation being sent from your systems.

Compliance Impact

This vulnerability could violate compliance requirements by allowing unauthorized tampering with sensitive data. GDPR and HIPAA mandate strict access controls and data integrity, which this flaw undermines.

Mitigation Strategies

Apply the latest security updates from Microsoft for Exchange Online immediately. Monitor Microsoft's official advisory for patches and follow their recommended mitigation steps.

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