Summary
go-mail has insufficient address encoding when passing mail addresses to the SMTP client
Vulnerability details
Instead of making use of the String() method of mail.Address, which takes care of proper escaping and quotation of mail address, we used the Address value of the mail.Address which is the raw value when passing it to our SMTP client.
This meant, if a mail address like this was set: "[email protected]> [email protected]"@example.com for a sender or recipient, instead of the correctly quoted/escaped address, the SMTP client would get the raw value passed which would translate into something like this being passed to the SMTP server: RCPT TO:<[email protected]> [email protected]@example.com>.
Since ORCTP is a valid command for the SMTP server, the mail would be routed to the wrong address. Additionally, other SMTP commands could potientially be smuggled in using this method causing unexpected behaviour.
Exploitation requirements
For successful exploitation of this vulnerability it is required that the user's code is allowing for arbitrary mail address input (i. e. through a web form or similar). If only static mail addresses are used (i. e. in a config file) and the mail addresses in use do not consist of quoted local parts, this should not affect your code.
Impact
Due to incorrect handling of the mail.Address values when a sender- or recipient address is passed to the corresponding MAIL FROM or RCPT TO commands of the SMTP client, this could lead to a possible wrong address routing or even to ESMTP parameter smuggling.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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The vulnerability has been fixed with PR #496 and the fix has been shipped with the go-mail v0.7.1 release.
Issue #495 holds the full report and discussion.
Frequently Asked Questions
- What is CVE-2025-59937? CVE-2025-59937 is a high-severity security vulnerability in github.com/wneessen/go-mail (go), affecting versions < 0.7.1. It is fixed in 0.7.1.
- Which versions of github.com/wneessen/go-mail are affected by CVE-2025-59937? github.com/wneessen/go-mail (go) versions < 0.7.1 is affected.
- Is there a fix for CVE-2025-59937? Yes. CVE-2025-59937 is fixed in 0.7.1. Upgrade to this version or later.
- Is CVE-2025-59937 exploitable, and should I be worried? Whether CVE-2025-59937 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2025-59937 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2025-59937? Upgrade
github.com/wneessen/go-mailto 0.7.1 or later.