CVE-2026-77910: 
Microsoft Exchange Server SE vulnerability analysis and mitigation

CVE-2026-77910 is an Incorrect Authorization vulnerability in Microsoft Exchange Server that allows a low-privileged authenticated attacker to elevate privileges over a network. It affects Microsoft Exchange Server 2016 Cumulative Update 23 (versions before 15.01.2507.077), Exchange Server 2019 Cumulative Update 14 (before 15.02.1544.050), Exchange Server 2019 Cumulative Update 15 (before 15.02.1748.055), and Exchange Server Subscription Edition RTM (before 15.02.2562.056). The vulnerability was disclosed and patched on October 13, 2026, as part of Microsoft's monthly security update cycle. It carries a CVSS v3.1 base score of 8.8 (High) (MSRC Advisory).

The vulnerability is classified as CWE-863 (Incorrect Authorization), meaning the server fails to properly verify that an authenticated user has the appropriate level of authorization before granting access to privileged operations or resources. An attacker with low-level network credentials can exploit this flaw remotely without requiring user interaction or elevated initial privileges. The attack vector is network-based with low complexity, making it straightforward to exploit once valid credentials are obtained. No public proof-of-concept or detailed technical write-up has been published as of the disclosure date (MSRC Advisory).

Successful exploitation allows an authenticated low-privileged attacker to gain high-level access to the affected Exchange Server, resulting in high confidentiality, integrity, and availability impact. An attacker could read all mailbox data and sensitive organizational communications, modify server configurations or data, and potentially disrupt mail service availability. Given Exchange Server's central role in enterprise environments, privilege escalation on this platform could facilitate lateral movement, credential harvesting, and broader network compromise (MSRC Advisory).

As of the disclosure date (October 13, 2026), there is no evidence of active in-the-wild exploitation and no public proof-of-concept exploit has been identified. The EPSS score is reported at 0.0, and the NVD SSVC assessment classifies exploitation as "none" and the vulnerability as not automatable. The CVE is not currently listed in the CISA Known Exploited Vulnerabilities catalog. Exploitation requires a valid low-privileged account on the Exchange Server, which limits opportunistic mass exploitation but remains a realistic threat in targeted attacks (MSRC Advisory).

Microsoft released security updates on October 13, 2026, addressing this vulnerability. Administrators should apply the following patched builds immediately:

• Exchange Server 2016 CU23: Update to version 15.01.2507.077 or later
• Exchange Server 2019 CU14: Update to version 15.02.1544.050 or later
• Exchange Server 2019 CU15: Update to version 15.02.1748.055 or later
• Exchange Server Subscription Edition RTM: Update to version 15.02.2562.056 or later

No configuration-based workarounds have been published. Organizations should prioritize patching given the low authentication requirement and high impact of this vulnerability (MSRC Advisory).

The Zero Day Initiative (ZDI) included CVE-2026-77910 in their October 2026 Security Update Review, noting it as part of Microsoft's Patch Tuesday release (ZDI Blog). No significant independent researcher commentary or broader media coverage has been identified beyond standard Patch Tuesday reporting.