CVE-2025-59937 is a vulnerability classified under CWE-88 (Improper Neutralization of Argument Delimiters in a Command, also known as Argument Injection) affecting the go-mail library, a Go language package for sending emails. Versions prior to 0.7.1 improperly handle mail.Address values when constructing SMTP commands MAIL FROM and RCPT TO. Specifically, if the sender or recipient email addresses contain quoted local parts or special characters, the library fails to correctly neutralize delimiters, allowing an attacker to inject additional SMTP parameters or manipulate the command structure. This can result in ESMTP parameter smuggling, causing emails to be routed incorrectly or enabling attackers to influence SMTP session behavior. Exploitation requires that the application accepts arbitrary email addresses from untrusted sources, such as user input via web forms, and forwards them without proper validation or sanitization. Static or hardcoded email addresses without quoted local parts are not vulnerable. The vulnerability does not require authentication or user interaction beyond supplying crafted email addresses. The CVSS 4.0 base score is 8.2 (high), reflecting network attack vector, low attack complexity, no privileges required, no user interaction, and high impact on integrity via mail routing manipulation. No known exploits have been reported in the wild as of now. The vulnerability was published on September 29, 2025, and is fixed in go-mail version 0.7.1. Organizations using go-mail in email sending workflows that accept user-supplied addresses should upgrade promptly and validate inputs to prevent exploitation.

For European organizations, this vulnerability poses a significant risk to the integrity and confidentiality of email communications. Attackers could exploit the flaw to manipulate SMTP commands, causing emails to be misrouted, intercepted, or altered in transit. This could lead to data leakage, phishing, or bypassing of email security controls. Organizations relying on go-mail in customer-facing applications, such as contact forms, newsletters, or automated notifications, are particularly vulnerable if they accept arbitrary email addresses without validation. The impact extends to potential reputational damage, regulatory non-compliance (e.g., GDPR breaches due to data exposure), and disruption of business communications. Since the vulnerability can be exploited remotely without authentication or user interaction, the attack surface is broad. However, the absence of known active exploits reduces immediate risk, though proactive mitigation is critical. The vulnerability could also be leveraged in targeted attacks against European entities with sensitive communications, especially in sectors like finance, healthcare, and government.

1. Upgrade all instances of the go-mail library to version 0.7.1 or later immediately to apply the official fix. 2. Implement strict validation and sanitization of all user-supplied email addresses before passing them to the mail library, ensuring no quoted local parts or special characters that could be used for injection are accepted. 3. Employ allowlists for email address formats and domains where feasible to reduce risk from arbitrary inputs. 4. Monitor email sending logs for unusual SMTP command patterns or routing anomalies that could indicate exploitation attempts. 5. Use application-layer email validation libraries that conform to RFC standards to reject malformed or suspicious addresses. 6. Conduct code reviews and security testing focused on email input handling in all applications utilizing go-mail. 7. Consider deploying outbound email security gateways that can detect and block anomalous SMTP commands or parameter injections. 8. Educate developers and administrators about the risks of argument injection in SMTP commands and the importance of input sanitization. 9. Maintain an incident response plan to quickly address any suspected exploitation. 10. Regularly audit dependencies and update libraries to minimize exposure to known vulnerabilities.