{"description":"Trending threats, MITRE ATT\u0026CK coverage, and detection metadata — refreshed continuously.","feed_url":"https://feed.craftedsignal.io/products/net-imap--0--0.3.9/","home_page_url":"https://feed.craftedsignal.io/","items":[{"_cs_actors":[],"_cs_cves":[],"_cs_exploited":false,"_cs_products":["net-imap (\u003e= 0.6.0, \u003c= 0.6.3)","net-imap (\u003e= 0.5.0, \u003c= 0.5.13)","net-imap (\u003e= 0.4.0, \u003c= 0.4.23)","net-imap (\u003e= 0, \u003c= 0.3.9)"],"_cs_severities":["high"],"_cs_tags":["tls-stripping","man-in-the-middle","net-imap","cve-2026-42246"],"_cs_type":"advisory","_cs_vendors":["rubygems"],"content_html":"\u003cp\u003eA 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 \u0026ldquo;OK\u0026rdquo; 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.\u003c/p\u003e\n\u003ch2 id=\"attack-chain\"\u003eAttack Chain\u003c/h2\u003e\n\u003col\u003e\n\u003cli\u003eThe client initiates a plaintext IMAP connection to the server.\u003c/li\u003e\n\u003cli\u003eThe client issues a \u003ccode\u003eSTARTTLS\u003c/code\u003e command to initiate TLS negotiation.\u003c/li\u003e\n\u003cli\u003eThe MitM attacker intercepts the \u003ccode\u003eSTARTTLS\u003c/code\u003e command.\u003c/li\u003e\n\u003cli\u003eThe attacker injects a spoofed \u003ccode\u003eOK\u003c/code\u003e response with a predictable tag before the IMAP server responds.\u003c/li\u003e\n\u003cli\u003eThe \u003ccode\u003eNet::IMAP#starttls\u003c/code\u003e method returns \u0026ldquo;successfully\u0026rdquo; due to the premature \u003ccode\u003eOK\u003c/code\u003e response.\u003c/li\u003e\n\u003cli\u003eThe TLS connection is never established, and the socket remains unencrypted.\u003c/li\u003e\n\u003cli\u003eThe client continues communication, sending sensitive data (usernames, passwords, emails) in cleartext.\u003c/li\u003e\n\u003cli\u003eThe attacker intercepts the cleartext data, compromising the client\u0026rsquo;s account and potentially gaining access to sensitive information.\u003c/li\u003e\n\u003c/ol\u003e\n\u003ch2 id=\"impact\"\u003eImpact\u003c/h2\u003e\n\u003cp\u003eSuccessful 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 \u003ccode\u003enet-imap\u003c/code\u003e gem, potentially impacting a large number of users and organizations.\u003c/p\u003e\n\u003ch2 id=\"recommendation\"\u003eRecommendation\u003c/h2\u003e\n\u003cul\u003e\n\u003cli\u003eUpgrade to a patched version of the \u003ccode\u003enet-imap\u003c/code\u003e gem that raises an exception when \u003ccode\u003e#starttls\u003c/code\u003e fails to establish TLS, as described in the advisory.\u003c/li\u003e\n\u003cli\u003eIf upgrading is not immediately feasible, explicitly verify \u003ccode\u003eNet::IMAP#tls_verified?\u003c/code\u003e returns \u003ccode\u003etrue\u003c/code\u003e after calling \u003ccode\u003e#starttls\u003c/code\u003e before transmitting any sensitive data.\u003c/li\u003e\n\u003cli\u003eConsider using implicit TLS connections (connecting directly to a TLS port) instead of relying on \u003ccode\u003eSTARTTLS\u003c/code\u003e, following the recommendations in RFC 8314.\u003c/li\u003e\n\u003c/ul\u003e\n","date_modified":"2026-05-05T14:00:00Z","date_published":"2026-05-05T14:00:00Z","id":"/briefs/2026-05-net-imap-starttls-stripping/","summary":"A man-in-the-middle attacker can exploit a vulnerability in Net::IMAP's STARTTLS implementation to bypass TLS encryption, leading to cleartext transmission of sensitive information by injecting a spoofed 'OK' response during the TLS negotiation.","title":"Net::IMAP STARTTLS Stripping Vulnerability","url":"https://feed.craftedsignal.io/briefs/2026-05-net-imap-starttls-stripping/"}],"language":"en","title":"CraftedSignal Threat Feed — Net-Imap (\u003e= 0, \u003c= 0.3.9)","version":"https://jsonfeed.org/version/1.1"}