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. Data from the CrowdSec community also indicates a gradual decrease in attacks targeting CVE-2021-21479. While still present in the wild, exploitation levels have dropped noticeably week-over-week. This may signal that the vulnerability is becoming less relevant or that defenses are improving fast enough for attackers to lose interest.

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