CVE-2025-59937: Pebcak Go-mail SMTP Smuggling Vulnerability

CVE-2025-59937 Overview

CVE-2025-59937 is an Argument Injection vulnerability in the go-mail library, a comprehensive Go library for sending emails. In versions 0.7.0 and below, incorrect handling of mail.Address values when sender or recipient addresses are passed to SMTP MAIL FROM or RCPT TO commands can lead to wrong address routing or ESMTP parameter smuggling.

Critical Impact

Attackers can exploit improper mail address parsing to perform SMTP command injection, potentially routing emails to unintended recipients or smuggling ESMTP parameters when user input is not properly validated.

Affected Products

• pebcak go-mail versions 0.7.0 and below
• Applications using go-mail with user-supplied email addresses
• Go applications accepting arbitrary mail address input through web forms or APIs

Discovery Timeline

• 2025-09-29 - CVE-2025-59937 published to NVD
• 2025-10-16 - Last updated in NVD database

Technical Details for CVE-2025-59937

Vulnerability Analysis

The vulnerability stems from improper handling of mail.Address values in the go-mail SMTP client. When email addresses containing quoted local parts or specially crafted characters are processed, the library fails to properly sanitize them before passing to SMTP MAIL FROM and RCPT TO commands. This creates an opportunity for attackers to inject additional SMTP parameters or manipulate email routing.

The issue is classified under CWE-88 (Improper Neutralization of Argument Delimiters in a Command), as the vulnerability allows attackers to inject arguments into SMTP commands through malformed email addresses. The attack requires network access and can be exploited without authentication or user interaction, though some preparation is necessary for successful exploitation.

Root Cause

The root cause lies in the inadequate validation and sanitization of email address values before they are incorporated into SMTP protocol commands. The mail.Address struct values were not being properly escaped or validated, allowing specially crafted addresses to break out of the expected format and inject additional SMTP parameters.

Attack Vector

Exploitation requires that the target application allows arbitrary mail address input, typically through user-facing interfaces such as web forms, contact pages, or APIs. An attacker can craft a malicious email address containing SMTP command delimiters or ESMTP extension parameters. When this address is processed by the vulnerable go-mail library, the injected content is passed directly to the SMTP server, potentially causing:

1. Email routing to unintended recipients
2. ESMTP parameter injection to modify mail handling behavior
3. Bypass of intended email security controls

Applications using only static mail addresses from configuration files with standard (non-quoted) local parts are not affected by this vulnerability.

The security patch addressed multiple areas of the codebase to improve error handling and address parsing:

// Security patch in b64linebreaker.go
// Source: https://github.com/wneessen/go-mail/commit/42e92cfe027be04aff72921adb0f72f11d517479

 func (l *base64LineBreaker) Write(data []byte) (numBytes int, err error) {
 	if l.out == nil {
 		err = errors.New("no io.Writer set for base64LineBreaker")
-		return
+		return numBytes, err
 	}
 	if l.used+len(data) < MaxBodyLength {
 		copy(l.line[l.used:], data)

// Security patch in client.go
// Source: https://github.com/wneessen/go-mail/commit/42e92cfe027be04aff72921adb0f72f11d517479

 		Reason: ErrConnCheck, errlist: []error{err}, isTemp: isTempError(err),
 		errcode: errorCode(err), enhancedStatusCode: enhancedStatusCode(err, escSupport),
 	}
-		return
+		return returnErr
 	}

 	var errs []error

Detection Methods for CVE-2025-59937

Indicators of Compromise

• Unusual SMTP command sequences in mail server logs containing unexpected parameters
• Email addresses with quoted local parts or special characters in application logs
• Unexpected email routing or delivery to unintended recipients
• SMTP transaction logs showing malformed MAIL FROM or RCPT TO commands

Detection Strategies

• Monitor application logs for email addresses containing quotes, spaces, or SMTP control characters
• Implement input validation monitoring on all email address fields in web applications
• Review SMTP server logs for anomalous command patterns or unexpected ESMTP extensions
• Deploy application-level monitoring to detect injection attempts in email-related functions

Monitoring Recommendations

• Enable detailed SMTP transaction logging on mail servers to capture full command sequences
• Implement alerting for email addresses matching injection patterns (e.g., addresses containing CRLF, quotes, or semicolons)
• Monitor go-mail library usage across your Go applications and track version deployments
• Set up dependency scanning to identify applications using vulnerable go-mail versions

How to Mitigate CVE-2025-59937

Immediate Actions Required

• Upgrade go-mail to version 0.7.1 or later immediately
• Audit all applications using go-mail to identify those accepting user-supplied email addresses
• Implement strict email address validation at the application layer as defense-in-depth
• Review SMTP logs for any evidence of prior exploitation attempts

Patch Information

The vulnerability has been fixed in go-mail version 0.7.1. The fix is available through the following resources:

• GitHub Security Advisory GHSA-wpwj-69cm-q9c5 - Official security advisory with full details
• GitHub Pull Request #496 - The pull request containing the fix
• GitHub Commit 42e92cf - The specific commit addressing the vulnerability

Update your Go application's dependencies by running go get github.com/wneessen/go-mail@v0.7.1 and rebuild affected applications.

Workarounds

• If immediate upgrade is not possible, restrict email address input to only allow standard RFC 5321 compliant addresses without quoted local parts
• Implement server-side validation to reject email addresses containing quotes, CRLF sequences, or SMTP control characters
• Use static mail addresses from configuration files instead of accepting user input where possible
• Deploy a mail proxy or gateway that validates SMTP commands before forwarding to backend servers

# Configuration example
# Update go-mail dependency to patched version
go get github.com/wneessen/go-mail@v0.7.1

# Verify the updated version
go list -m github.com/wneessen/go-mail

# Rebuild your application with the patched library
go build -o myapp ./...