A security flaw has been identified in Red Hat Libuser, a component included in many Linux server systems that manages user account information. The flaw is a race condition, a type of bug that occurs when multiple system processes try to access and modify the same file at the same time, leading to unexpected and harmful results.
This specific vulnerability can be exploited by users who already have valid, authenticated access to your server to corrupt the /etc/passwd file, the core system file that stores all user account details and access permissions. If successfully exploited, this could either cause your server to stop working properly (a denial of service) or allow the attacker to gain elevated, unrestricted access to your server (privilege escalation), which could put your website, stored data, and customer information at risk.
Source: CISA Known Exploited Vulnerabilities Catalog — https://nvd.nist.gov/vuln/detail/CVE-2015-3246
