CVE-2025-65318 is a critical security vulnerability affecting Canary Mail versions 5.1.40 and earlier. The flaw arises from the application's handling of email attachments via its attachment interaction functionality. Specifically, when documents are saved to the file system, Canary Mail fails to apply the Mark-of-the-Web (MOTW) tag, a security feature used by Windows OS and many third-party security solutions to identify files originating from potentially unsafe sources, such as the internet or email attachments. The absence of this tag means that the built-in file protection mechanisms, including Windows SmartScreen and other security controls that rely on MOTW to enforce sandboxing or warnings, are bypassed. This allows attackers to deliver malicious attachments that, once saved, can execute or be opened without triggering typical security warnings or restrictions. The vulnerability has a CVSS v3.1 base score of 9.1, reflecting its critical nature. It requires no privileges, no user interaction, and can be exploited remotely by sending a crafted email with a malicious attachment. The impact includes high confidentiality and integrity compromise, as attackers can execute arbitrary code or manipulate files without detection. Although no known exploits have been reported in the wild yet, the vulnerability poses a significant risk to users of Canary Mail, especially in environments where email is a primary vector for malware delivery. The CWE-693 classification indicates a protection mechanism failure, emphasizing the importance of proper tagging and file origin tracking in secure software design.

For European organizations, this vulnerability presents a severe risk to the confidentiality and integrity of sensitive communications and data. Canary Mail is used by various enterprises and professionals for secure email handling; thus, exploitation could lead to unauthorized access to confidential information, potential lateral movement within networks, and compromise of critical systems. The bypass of Windows and third-party file protections increases the likelihood of successful malware deployment or data exfiltration without triggering alerts. Sectors such as finance, healthcare, government, and critical infrastructure are particularly vulnerable due to the sensitive nature of their communications and regulatory requirements for data protection. The lack of required user interaction or privileges lowers the barrier for attackers, increasing the threat landscape. Additionally, the potential for undetected compromise could lead to prolonged breaches and significant operational disruption.

Immediate mitigation involves restricting the use of Canary Mail versions 5.1.40 and below until a patched version is released. Organizations should enforce strict email attachment handling policies, including disabling automatic saving or opening of attachments and scanning all attachments with updated endpoint protection solutions that do not solely rely on MOTW tagging. Deploy application whitelisting and behavior-based detection tools to identify suspicious file activities. Network-level controls such as email gateway filtering and sandboxing of attachments can reduce exposure. User training to recognize suspicious attachments remains important, despite the lack of required user interaction for exploitation. Monitoring file system changes and unusual process executions related to email attachments can provide early detection. Once a patch is available, prompt deployment is critical. Additionally, organizations should review and harden their Windows security configurations to minimize the impact of MOTW bypasses.