BlueHammer (CVE-2026-33825) is a Windows flaw involving Defender for EoP (Escalation of Privilege; specifically Local Privilege Escalation). According to CISA, ransomware gangs have begun exploiting this vulnerability.
Attackers exploit TOCTOU race conditions. What does that mean?
Time of Check and Time of Use.
To deeper understand this race condition, there’s some fundamental things we need to understand:
1. Multi-threaded: This scares a lot of people, because it can be hard to mentally visualize or keep track of in real time. One of the simplest ways I like to visualize this, is having 2 or 3 workers doing tasks at the same time. Often they have separate tasks that they’re doing (perhaps one is taking orders from customers while another is cleaning the lobby).
Or they could be splitting the same workload (Parallelism). Such as 2 people loading or unloading a truck.
2. TOCTOU happens when a program verifies a condition (does the file exist?) and in that tiny, TINY window of time before the program USES the file, something slips in.
Yes, this occurs extremely fast. The system could check for file “update.exe”, sees that it exists, and before it executes it (has Windows Loader initialize, load, and run the file), the hacker has made a change to the file, or swapped it for another.
But how can they force themselves in? Back to the workers concept…
Let’s say Worker 1 goes and checks the label on a box inside the truck then walks out. – Insert sneaky little racoon character swaps the labels out 🥷 🦝 –
Then Worker 2 comes and grabs the box that Worker 1 already checked and confirmed.
The Solution? Worker 1 checks label and grabs the box.
This can come with some disadvantages, particularly in speed since we are down to one worker (Single Thread). There are other solutions that exist, usually dependent on the intention, requirements, and architecture of the program.
Microsoft patched this vulnerability on April 14th, 2026. So ensure your systems are updated. 🙂
BlueHammer (CVE-2026-33825)
