Man-in-the-Middle TLS Bypass in Ruby Net::IMAP

Mitigation Strategies

The primary mitigation is to upgrade the ruby/net-imap library to a patched version where this vulnerability is fixed. The patched versions are 0.3.10, 0.4.24, 0.5.14, and 0.6.4.

Alternatively, instead of using STARTTLS to upgrade a plaintext connection, connect directly to an implicit TLS port to ensure the connection is encrypted from the start.

Another recommended step is to explicitly verify the tls_verified? flag after calling the #starttls method to ensure that TLS negotiation was successful.

The patches also ensure that the client disconnects immediately if an invalid or unexpected response is detected during the STARTTLS handshake, preventing the connection from remaining unencrypted.

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Compliance Impact

CVE-2026-42246 allows a man-in-the-middle attacker to bypass the STARTTLS encryption in the Ruby net-imap library, causing the Net::IMAP#starttls method to falsely report a successful TLS connection without actually establishing it.

This vulnerability can lead to sensitive data being transmitted in cleartext over the network, exposing it to interception and unauthorized access.

Such exposure of sensitive data can negatively impact compliance with data protection standards and regulations like GDPR and HIPAA, which require encryption of data in transit to protect confidentiality and integrity.

Failure to properly secure communications could result in violations of these regulations, potentially leading to legal and financial consequences.

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Executive Summary

CVE-2026-42246 is a vulnerability in the Ruby net-imap library's implementation of the STARTTLS command, which is used to upgrade a plaintext IMAP connection to a secure TLS connection.

A man-in-the-middle attacker can exploit this vulnerability by sending a fake 'OK' response prematurely before the client finishes the STARTTLS handshake. This causes the Net::IMAP#starttls method to falsely report a successful TLS connection without actually establishing encryption.

As a result, the connection remains unencrypted, allowing the attacker to intercept or manipulate sensitive data transmitted over the connection.

The vulnerability arises because the client does not properly verify that the STARTTLS command was fully processed before accepting the 'OK' response, allowing the attacker to bypass the TLS negotiation.

This issue has been fixed by adding checks to ensure the STARTTLS command is properly handled and by disconnecting if suspicious or invalid responses are detected.

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Impact Analysis

This vulnerability can have serious security impacts by allowing a man-in-the-middle attacker to intercept or manipulate data that is supposed to be protected by TLS encryption.

• Sensitive information such as login credentials, emails, and other private data can be exposed in plaintext.
• The attacker can eavesdrop on or alter communications without detection.
• It undermines the confidentiality and integrity of the IMAP connection.

Because the vulnerability is relatively easy to exploit over a network and requires no privileges or user interaction, it poses a high risk to users relying on the net-imap library for secure email communication.

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Detection Guidance

This vulnerability involves a man-in-the-middle attacker causing the Net::IMAP#starttls method to falsely report a successful TLS connection without actually establishing it. Detection involves monitoring for unexpected or premature 'OK' responses from the IMAP server during the STARTTLS handshake.

Specifically, the patched versions add a method to detect tagged OK responses sent before the client finishes the STARTTLS command, which could indicate a STARTTLS stripping attack.

While no explicit network or system commands are provided in the resources, detection could involve capturing and analyzing IMAP traffic to check if the STARTTLS command is followed by a proper TLS handshake or if an 'OK' response is sent prematurely.

For example, using packet capture tools like tcpdump or Wireshark to monitor IMAP sessions and verify that the STARTTLS command is properly negotiated and that TLS handshake packets follow.

Additionally, inspecting application logs for errors related to InvalidResponseError or unexpected disconnections during STARTTLS attempts could help detect exploitation attempts.

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