Net::IMAP STARTTLS Stripping Vulnerability

A critical vulnerability exists within the Net::IMAP library, affecting versions 0.6.0 through 0.6.3, 0.5.0 through 0.5.13, 0.4.0 through 0.4.23, and 0 through 0.3.9. This flaw allows a man-in-the-middle (MitM) attacker to perform a STARTTLS stripping attack. By injecting a specially crafted, tagged “OK” response with a predictable tag before the client completes sending the STARTTLS command, the client prematurely believes TLS negotiation has succeeded. Consequently, the TLS connection is never established, leaving subsequent communication unencrypted. This vulnerability, identified as CVE-2026-42246, enables attackers to intercept and potentially steal sensitive data transmitted in cleartext. Defenders should prioritize patching or implementing mitigations to prevent exploitation.

Attack Chain

1. The client initiates a plaintext IMAP connection to the server.
2. The client issues a STARTTLS command to initiate TLS negotiation.
3. The MitM attacker intercepts the STARTTLS command.
4. The attacker injects a spoofed OK response with a predictable tag before the IMAP server responds.
5. The Net::IMAP#starttls method returns “successfully” due to the premature OK response.
6. The TLS connection is never established, and the socket remains unencrypted.
7. The client continues communication, sending sensitive data (usernames, passwords, emails) in cleartext.
8. The attacker intercepts the cleartext data, compromising the client’s account and potentially gaining access to sensitive information.

Impact

Successful exploitation of this vulnerability results in a complete bypass of TLS encryption for IMAP communication. This allows a man-in-the-middle attacker to eavesdrop on sensitive information transmitted between the client and the server, including usernames, passwords, email content, and other confidential data. The vulnerability poses a significant risk to any application using the affected versions of the net-imap gem, potentially impacting a large number of users and organizations.

Recommendation

• Upgrade to a patched version of the net-imap gem that raises an exception when #starttls fails to establish TLS, as described in the advisory.
• If upgrading is not immediately feasible, explicitly verify Net::IMAP#tls_verified? returns true after calling #starttls before transmitting any sensitive data.
• Consider using implicit TLS connections (connecting directly to a TLS port) instead of relying on STARTTLS, following the recommendations in RFC 8314.