Email contains some of a company’s most valuable information. Customer conversations, invoices, contracts, password resets, employee records, and confidential attachments all pass through business inboxes.
That is why an important Microsoft Exchange Server update released on October 2, 2026 deserves prompt attention.
Microsoft issued a second version of its September Exchange Server security update to address an additional vulnerability known as CVE-2026-96940. Microsoft describes it as a weakness that could allow an authenticated attacker to gain privileges over a network.
In plain English, someone who already has access to a lower-level Exchange account could potentially use the flaw to reach mailboxes they should not be able to read.
Microsoft rated the vulnerability as “exploitation more likely” and released the fix ahead of its originally intended schedule. That does not mean every Exchange server is under attack, but it does mean businesses should treat the update as a priority.
This particular warning is primarily for organizations running Microsoft Exchange Server on their own equipment, including some hybrid environments that connect an on-premises Exchange server to Microsoft 365.
A business that uses only Microsoft 365 and Exchange Online may not operate an affected Exchange server.
Unfortunately, many business owners do not know which type of email environment they have. The easiest first step is to ask:
Your IT provider should be able to answer these questions without requiring you to decipher server names or technical reports.
An employee’s email account does not need executive-level access to contain valuable information. Even an ordinary mailbox can reveal customer details, payment requests, internal schedules, vendor relationships, or conversations that help an attacker impersonate the business.
If an attacker could move from one compromised account to other mailboxes, the potential impact could include:
Multifactor authentication remains important because it makes stolen passwords harder to use. However, MFA is not a replacement for installing a security update that corrects a weakness inside the server.
Ask your internal IT contact or service provider whether your email is entirely cloud-based or whether an on-premises Exchange server remains involved.
Do not assume that having Microsoft 365 means there is no local server. Some companies maintain hybrid systems for administration, applications, scanners, compliance needs, or older business workflows.
The October 2 release is a second version of the September security update. A server that received the original September update may still need the newer package.
Exchange Server Subscription Edition has a current update available. Exchange Server 2016 and 2019 organizations may need enrollment in Microsoft’s applicable Extended Security Update program to obtain recent fixes. An unsupported server should trigger a broader migration discussion.
The package must match the server’s exact version. This is a job for an experienced Exchange administrator.
Email-server updates should be handled through a controlled process. Before installation, the administrator should verify backups, confirm available storage, document the current configuration, and plan for a brief interruption if required.
Microsoft lists several known issues with the update, including potential problems involving published calendars and certain hybrid free/busy requests. Businesses that depend on these features should test them after installation.
Microsoft recommends running its Exchange Server Health Checker after the update. The tool can verify server configuration, identify missing security updates, and highlight additional issues that may require attention.
Completing an installation is not the same as proving that every server received it successfully. Verification is an essential part of patching.
The update prevents future use of the corrected vulnerability, but it does not automatically tell a business whether suspicious access occurred earlier.
An administrator should review unusual mailbox activity, unexpected administrative changes, unfamiliar sign-ins, and security alerts. Any credible sign of unauthorized access should be handled through the company’s incident-response process.
The larger lesson is not limited to one vulnerability. A business should know who owns email-server maintenance, how quickly urgent updates are assessed, when installations occur, and how successful deployment is verified.
A short written process can prevent dangerous assumptions between a business, its software vendors, and its IT provider.
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