CVE-2026-42246

A flaw was found in the Ruby net-imap library. When upgrading a cleartext IMAP connection to TLS using the Net::IMAP#starttls method, the library improperly handles certain responses received during STARTTLS negotiation. A man-in-the-middle (MITM) attacker can inject a predicted tagged OK response before the client completes the STARTTLS command, causing the operation to appear successful without establishing a TLS session. As a result, the connection may continue to transmit sensitive information in cleartext and enable modification of data exchanged over the affected connection, while the application incorrectly believes that encryption has been enabled.

This vulnerability affects the STARTTLS functionality in the Ruby net-imap library. Red Hat Product Security has assessed this issue as an Important severity vulnerability.

Attack Complexity is considered High (AC:H), because successful exploitation requires an attacker capable of intercepting and modifying network traffic and successfully winning a timing race during the STARTTLS negotiation process.

This may allow exposure of authentication credentials, email contents, and other sensitive information, as well as unauthorized modification of data transmitted over the affected connection.

Red Hat's ruby packages distribute net-imap as a default bundled gem, the ruby package itself is listed affected. Applications relying on the system-provided Ruby installation to handle IMAP connections may be exposed to this flaw.

Red Hat 3scale API Management uses net-imap which is a transitive dependency of mail, which is a dependency of actionmailer and actionmailbox. The images doesn’t load them or use them in any way, hence, they are not affected.

The CVSS score(s) listed for this vulnerability do not reflect the associated product's status, and are included for informational purposes to better understand the severity of this vulnerability.

As a temporary workaround, Users are strongly encouraged to switch from explicit TLS upgrading mechanisms (STARTTLS on port 143) to Implicit TLS connections (such as IMAPS on port 993).

By enforcing implicit TLS via port 993 from the initial socket creation step, the connection is mathematically protected against packet injection and connection degradation tactics entirely, bypassing the vulnerable implementation path.