CVE-2021-27104 is a critical vulnerability affecting Accellion File Transfer Appliance (FTA) versions 9_12_370 and earlier. The vulnerability allows an unauthenticated attacker to execute arbitrary operating system commands on the affected appliance by sending a specially crafted POST request to various administrative endpoints. This type of vulnerability is categorized under CWE-78 (Improper Neutralization of Special Elements used in an OS Command), which indicates that the application fails to properly sanitize input before passing it to the OS command interpreter. Exploitation does not require any authentication or user interaction, making it highly accessible to remote attackers. The vulnerability has a CVSS v3.1 base score of 9.8, reflecting its critical severity with high impact on confidentiality, integrity, and availability. Successful exploitation could lead to full system compromise, data theft, or disruption of file transfer services. The vendor addressed this issue in version FTA_9_12_380 and later, so upgrading to the fixed version is essential. Although no public exploits have been reported in the wild at the time of publication, the nature of the vulnerability and its criticality make it a prime target for attackers, especially given the appliance’s role in secure file transfers.

For European organizations, the impact of this vulnerability can be severe. Accellion FTA is used by various enterprises and government agencies for secure file transfers, often involving sensitive or regulated data. Exploitation could lead to unauthorized access to confidential information, including personal data protected under GDPR, intellectual property, or critical business documents. Additionally, attackers could disrupt file transfer operations, impacting business continuity and operational efficiency. The ability to execute arbitrary OS commands could allow attackers to deploy malware, establish persistent backdoors, or pivot to other internal systems, increasing the risk of widespread compromise. Given the criticality of data protection and regulatory compliance in Europe, a breach resulting from this vulnerability could also lead to significant legal and financial penalties, as well as reputational damage.

European organizations should immediately verify if they are running Accellion FTA versions 9_12_370 or earlier. The primary mitigation is to upgrade to version FTA_9_12_380 or later, which contains the patch for this vulnerability. If immediate upgrade is not feasible, organizations should restrict access to the administrative endpoints of the FTA appliance by implementing network segmentation and firewall rules to limit exposure only to trusted management networks. Monitoring and logging of all POST requests to admin endpoints should be enhanced to detect any suspicious activity indicative of exploitation attempts. Additionally, organizations should conduct thorough audits of the appliance and surrounding infrastructure for signs of compromise. Implementing strict input validation and web application firewall (WAF) rules to block malicious payloads targeting command injection patterns can provide temporary protection. Finally, organizations should review and update incident response plans to address potential exploitation scenarios involving this vulnerability.