EUVD-2026-56417
GHSA-255v-3pqf-6g77
Severity by source
AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:H/A:N
Low-privilege network credential is sufficient; missing authorization enables high-integrity writes with no confidentiality or availability impact and no scope change.
Primary rating from Vendor (microsoft).
CVSS VectorVendor: microsoft
Lifecycle Timeline
3DescriptionCVE.org
Missing authorization in Microsoft Exchange Server allows an authorized attacker to bypass a security feature over a network.
AnalysisAI
Missing authorization in Microsoft Exchange Server allows authenticated low-privilege network users to bypass a security feature, achieving high-integrity impact without requiring elevated credentials or user interaction. Rooted in CWE-862, Exchange fails to enforce a required authorization check on at least one network-accessible endpoint, enabling unauthorized high-integrity modifications - such as changes to mailbox permissions, transport rules, or security policy - by any valid account holder. No active exploitation is confirmed by CISA KEV, no public exploit code is identified, and Microsoft has released patches across all affected cumulative update tracks.
Technical ContextAI
CWE-862 (Missing Authorization) describes a condition where an application executes a sensitive action or exposes a protected resource without verifying that the requesting principal holds the necessary authorization. In Microsoft Exchange Server, this class of flaw typically appears when an internal API endpoint or administrative function omits an authorization gate, allowing callers with only basic mailbox-level credentials to invoke privileged operations. The CVSS vector (AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:H/A:N) confirms the flaw is exploitable remotely over the network with low complexity and a low-privilege account. Affected CPE data from EUVD covers Exchange 2016 CU23 (branch 15.01.x), Exchange 2019 CU14 and CU15 (branch 15.02.x), and Exchange Subscription Edition RTM (also branch 15.02.x), spanning both on-premises long-term servicing and current subscription deployments. No third-party library or protocol other than the Exchange application layer itself is implicated by available data.
RemediationAI
Apply Microsoft's released patches immediately by upgrading to the following fixed builds: Exchange 2019 CU15 to build 15.02.1748.049 or later, Exchange 2019 CU14 to build 15.02.1544.044 or later, Exchange 2016 CU23 to build 15.01.2507.072 or later, and Exchange Subscription Edition RTM to build 15.02.2562.046 or later. Full patch guidance is available at https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-62915. As a compensating control prior to patching, restrict Exchange network access to trusted IP ranges or VPN segments to eliminate exposure to untrusted low-privilege users - note this does not protect against insider threats or already-compromised accounts. Enforcing multi-factor authentication on all Exchange-connected accounts raises the credential barrier for exploitation but does not remediate the underlying authorization flaw. Neither compensating control should substitute for patching.
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Same weakness CWE-862 – Missing Authorization
View allSame technique Authentication Bypass
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External POC / Exploit Code
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