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.

CrowdSec network observations suggest that most exploitation of CVE-2021-21479 involves focused reconnaissance to identify viable targets. Attackers typically tailor their campaigns based on system exposure and configuration. It is unlikely that a given attack is accidental. Telemetry from the CrowdSec network also shows that exploitation activity for CVE-2021-21479 remains steady week-over-week. Attack volumes are consistent with long-term trends, indicating sustained interest from threat actors. CVE-2021-21479 continues to be an active part of the threat landscape and will likely remain this way for the forseeable future.

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