The vulnerability identified as CVE-2025-61084 affects MDaemon Mail Server version 23.5.2. The core issue lies in how the mail server validates SPF, DKIM, and DMARC records by extracting the email address enclosed within angle brackets (<>) in the From: header of SMTP DATA. The server fails to properly handle multiple invisible Unicode thin space characters embedded within the From: header. These invisible characters can be used by an attacker to craft a From: header that visually appears as a legitimate sender to the recipient but is actually a spoofed address that passes SPF, DKIM, and DMARC validation checks. This discrepancy arises because the validation logic processes the underlying email address including the invisible characters, while the displayed header to the user omits or renders them invisibly, enabling the attacker to bypass anti-spoofing protections. This vulnerability undermines the trust model of email authentication mechanisms, allowing attackers to impersonate trusted senders and potentially conduct phishing, business email compromise (BEC), or other social engineering attacks. Although no exploits have been reported in the wild, the vulnerability is significant due to the widespread reliance on SPF, DKIM, and DMARC for email security. The lack of a CVSS score suggests the need for a severity assessment based on impact and exploitability factors. The vulnerability requires vendor patching to correctly parse and normalize Unicode characters in email headers before validation. Until a patch is available, organizations should consider additional email filtering and monitoring controls to detect anomalous sender behavior.

For European organizations, this vulnerability poses a significant risk to email security and trust. Successful exploitation can lead to email spoofing that bypasses SPF, DKIM, and DMARC protections, increasing the likelihood of phishing attacks, credential theft, and business email compromise. This can result in financial losses, data breaches, and reputational damage. Organizations relying on MDaemon Mail Server for critical communications may experience disruption or compromise of sensitive information. The impact extends to sectors with high email dependency such as finance, government, healthcare, and critical infrastructure. Given the subtlety of the spoofing method, end users may be deceived by seemingly legitimate emails, complicating detection and response. The vulnerability undermines the integrity and authenticity of email communications, which are foundational to secure business operations. Without timely mitigation, the threat landscape for European enterprises using this mail server is elevated, especially as attackers may develop exploits once the vulnerability becomes widely known.

1. Apply vendor patches promptly once available that address Unicode handling in From: header parsing and validation logic. 2. Implement additional email gateway filtering rules to detect and block emails with suspicious or multiple invisible Unicode characters in headers. 3. Use advanced email security solutions capable of deep header analysis and anomaly detection beyond standard SPF, DKIM, and DMARC checks. 4. Educate end users to be vigilant for unexpected or unusual email senders, even if authentication checks pass. 5. Monitor email logs for unusual patterns or repeated spoofing attempts targeting the organization. 6. Consider deploying DMARC enforcement policies with quarantine or reject actions to reduce the impact of spoofed emails. 7. Collaborate with incident response teams to prepare for potential phishing campaigns exploiting this vulnerability. 8. Review and harden internal email server configurations to minimize exposure to spoofed emails. 9. Engage with MDaemon support for guidance and interim workarounds if patches are delayed.