Report: Windows has a new 0-day vulnerability called BlueHammer

The next Windows Patch Day is just a week away and it is unclear whether it will include a fix for a recently disclosed 0-day vulnerability.

The new security vulnerability has been disclosed on GitHub, including proof of concept code to exploit the issue. However, there is no explanation how the issue works.

Well-known security researcher Will Dormann commented on the issue and confirmed that it is working. He admitted that it “may not be 100%” reliable though. It seems that frustration with MSRC, the Microsoft Security Research Center, and how it operates, was the reason for the public disclosure of the vulnerability. Whether that is true or not can’t be verified though.

So, what do we know about the vulnerability so far?

• What it is: “BlueHammer” is an unpatched zero-day Local Privilege Escalation (LPE) vulnerability affecting Microsoft Windows.
• Impact: It allows a local attacker with limited, low-level user access to escalate their permissions to SYSTEM or elevated administrator rights. This effectively grants the attacker full control over the compromised machine.
• Current Status: Microsoft has not yet released an official patch or mitigation, making it a true zero-day.

Security experts (such as Will Dormann) describe it as a flaw that combines a TOCTOU (Time-of-Check to Time-of-Use) vulnerability with path confusion. At a high level, it appears to weaponize Windows Defender-related interfaces (the leaked source code contains files like windefend.idl and windefend_c.c). By bypassing the system’s original validation, a local attacker can gain access to the Security Account Manager (SAM) database, which stores local account password hashes, ultimately allowing them to spawn SYSTEM-level shells.

Good news is that the flaw is a local privilege escalation, which means that attackers can’t exploit it to hack into Windows PCs remotely. However, if they were to gain access to a Windows system, they could use it to expand access or even take over a system completely.