According to Dark Reading, a researcher known as Nightmare-Eclipse has released another zero-day exploit for Windows Defender, named ShieldCrash. The publication describes the move as part of an ongoing vendetta against Microsoft.
The exploit targets the antivirus component that ships with Windows, which makes the attack notable because Defender is itself a security boundary. The report frames ShieldCrash as the latest in a series of public disclosures by the same researcher rather than an isolated event.
Since there is only one source, there are no conflicting details to compare. The significance here is that a built-in security product is the target, reminding users that even defensive software can become an attack vector.