Acmailer - RCE (CVE-2021-20617)

CVE-2021-20617 is a critical improper access control vulnerability in acmailer and acmailer DB that allows remote attackers to execute arbitrary OS commands or escalate privileges to gain administrative access. Exploiting this flaw could enable attackers to obtain sensitive information from the server or fully compromise the affected system.

CrowdSec has been tracking this vulnerability and its exploits since 7th of January 2026.

CrowdSec network observations suggest that most exploitation of CVE-2021-20617 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. CrowdSec network telemetry also shows that exploitation of CVE-2021-20617 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.

Attackers exploit the /init_ctl.cgi endpoint by injecting OS commands into the sendmail_path parameter, enabling remote code execution and potential privilege escalation.