A 0-Day is a software vulnerability without an available fix, creating risk because defenders have limited time to mitigate exploitation.
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Background for this topic.
0-Day describes a software vulnerability unknown to the software maker or unpatched when first exploited. Attackers can use these flaws immediately, as no official fix or signature exists to block the exploit. Such vulnerabilities often affect widely deployed software or hardware, making them valuable for targeted attacks or widespread campaigns.
Because defenders lack patches or reliable detection signatures initially, they must rely on anomaly detection, network monitoring, and threat intelligence to identify suspicious activity linked to 0-day exploits. Rapid patching once a fix is released is critical to reduce exposure. Tracking emerging 0-day threats helps prioritize defensive measures and informs risk management decisions in environments where unpatched vulnerabilities pose significant security risks.
Fortinet has silently patched a critical zero-day vulnerability in its FortiWeb web application firewall, which is now being widely exploited. [...]
An advanced threat actor exploited the critical vulnerabilities "Citrix Bleed 2" (CVE-2025-5777) in NetScaler ADC and Gateway, and CVE-2025-20337 affecting Cisco Identity Service Engine (ISE) as zero-days to deploy custom malware. [...]
Today is Microsoft's November 2025 Patch Tuesday, which includes security updates for 63 flaws, including one actively exploited zero-day vulnerability. [...]
CISA ordered U.S. federal agencies today to patch a critical Samsung vulnerability that has been exploited in zero-day attacks to deploy LandFall spyware on devices running WhatsApp. [...]