Executive brief
Spring Boot is a widely-used framework for building Java applications, including those that send emails. The framework's default mail configuration fails to verify that the SMTP mail server's SSL certificate matches its hostname, leaving email communications vulnerable to man-in-the-middle attacks. An attacker on the same network could intercept and read email sent by affected applications without detection. Manual configuration can work around the issue, but the default behavior is insecure.
Technical details
This vulnerability is a certificate validation bypass (CWE-295) in Spring Boot's mail auto-configuration. The JavaMail library supports hostname verification via the mail.smtp.ssl.checkserveridentity property, but Spring Boot does not enable this by default. An attacker with network adjacency (e.g., on the same network segment or via compromised network infrastructure) can perform a man-in-the-middle attack by presenting a valid certificate for any domain and intercepting SMTP connections. This requires high attack complexity due to network positioning but results in potential disclosure of email content and integrity compromise. Patches are available: 4.0.7, 3.5.15, and 3.4.17+.
Affected products
- Spring Spring Boot 4.0.0 through 4.0.6
- Spring Spring Boot 3.5.0 through 3.5.14
- Spring Spring Boot 3.4.0 through 3.4.16
Timeline
- 2026-06-11: disclosed: Published to GitHub Advisory Database
- 2026-06-11: patched: Patches released: 4.0.7, 3.5.15