(Pwn2Own) Microsoft Exchange Capture-Replay Authentication Bypass Vulnerability

August 11th, 2026
ZDI-26-534 ZDI-CAN-31262

CVE ID

CVE-2026-62911

CVSS Score

8.1 AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H

Affected Vendors

Microsoft

Affected Products

Exchange

Vulnerability Details

This vulnerability allows remote attackers to bypass authentication on affected installations of Microsoft Exchange. Authentication is not required to exploit this vulnerability.

The specific flaw exists within the handling of authentication requests. The issue results from using an alternative, weak authentication path. An attacker can leverage this vulnerability to bypass authentication on the system.

Additional Details

Microsoft has issued an update to correct this vulnerability. More details can be found at:
https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-62911

Disclosure Timeline

  • 2026-05-21 - Vulnerability reported to vendor
  • 2026-08-11 - Coordinated public release of advisory
  • 2026-08-11 - Advisory Updated

Credit

Orange Tsai (@orange_8361) from DEVCORE Research Team

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