SCIMono - RCE (CVE-2021-21479)

CVE-2021-21479 is a serious vulnerability in SCIMono that enables attackers to inject and execute Java expressions, leading to potential compromise of system availability and integrity.

CrowdSec has been tracking this vulnerability and its exploits since 22nd of May 2025.

Data from the CrowdSec community indicates that exploitation of CVE-2021-21479 is highly selective and intelligence-driven. Threat actors use advanced reconnaissance and carefully choose their targets, often as part of sophisticated campaigns or advanced persistent threat operations. CrowdSec network telemetry also shows that exploitation of CVE-2021-21479 has significantly declined over the past week. Attack volumes are well below the long-term average, suggesting attackers are rapidly losing interest. The vulnerability appears to be falling out of active use across most threat landscapes.

Exploitation attempts commonly target URLs that start with /schemas/ and include suspicious Java expression patterns.