Nearly 22,000 Microsoft Exchange servers exposed online remain unpatched against a high-severity authentication bypass vulnerability that allows attackers to hijack all user mailboxes.
Tracked as CVE-2026-62911 and reported by DEVCORE Research Team's Orange Tsai, this security flaw affects Exchange Server 2016, Exchange Server 2019, and Exchange Server Subscription Edition (SE) software. Threat actors with basic privileges on the targeted server can exploit it in low-complexity attacks that require user interaction.
"Authentication bypass by capture-replay in Microsoft Exchange Server allows an authorized attacker to elevate privileges over a network," Microsoft said when it patched the vulnerability during the August 2026 Patch Tuesday. "The attacker would be able to take over the mailboxes of all Exchange users, attackers can send emails, read emails, download attachments."
While Microsoft has yet to update the CVE-2026-62911 advisory to confirm it, the Netherlands National Cyber Security Centre (NCSC-NL) reported last week that exploit code for this vulnerability is already available online.
"Microsoft has made updates available to address the vulnerabilities. Install these updates as soon as possible," NCSC-NL noted. "Exchange Server 2016 and 2019 only receive security updates via the Extended Security Updates Program (ESU). Are you using one of these versions? If so, ensure that the server is accessible only internally and replace it if possible."
