CVE-2026-47241 Overview
CVE-2026-47241 affects the Net::IMAP Ruby library, which implements Internet Message Access Protocol (IMAP) client functionality. Versions prior to 0.6.5 and 0.5.15 accept raw string arguments in several commands that are only validated for Carriage Return Line Feed (CRLF) injection before being sent verbatim to the IMAP server. When applications derive these strings from user-controlled input, an attacker can force the next IMAP command to be absorbed as a continuation of the first. The first command eventually fails, but blocks until another command is issued from a separate thread, and that subsequent command will not return until the connection closes. This results in a denial of service condition affecting Ruby applications that rely on Net::IMAP for mail processing.
Critical Impact
Attacker-controlled IMAP command arguments cause client commands to hang indefinitely, blocking threads until the underlying connection is terminated.
Affected Products
- Ruby Net::IMAP gem versions prior to 0.6.5
- Ruby Net::IMAP gem versions prior to 0.5.15
- Ruby applications using Net::IMAP commands with user-controlled input
Discovery Timeline
- 2026-06-22 - CVE-2026-47241 published to NVD
- 2026-06-23 - Last updated in NVD database
Technical Details for CVE-2026-47241
Vulnerability Analysis
The vulnerability is classified under [CWE-162: Improper Neutralization of Multiple Newlines]. Net::IMAP exposes several command methods that accept string arguments destined for transmission to a remote IMAP server. The library validates these arguments only for CRLF sequences to block standard CRLF injection attacks. It does not validate other characters that have syntactic meaning in the IMAP protocol, such as literal string markers or quoting characters.
When an attacker supplies crafted input, the IMAP server interprets part of the next command as continuation data belonging to the malicious first command. The first command then fails on the server side. However, the client cannot complete its receive loop until a subsequent command is dispatched from another thread. That second command also blocks, and neither command returns until the TCP connection is closed.
The practical impact is thread starvation and resource exhaustion in long-running Ruby services that process IMAP traffic with user-influenced parameters such as mailbox names or search criteria.
Root Cause
The root cause is incomplete input neutralization in argument-passing code paths within Net::IMAP. The library restricts only CRLF characters, leaving IMAP literal and quoted-string delimiters unhandled. This allows attacker input to alter the boundary between commands on the protocol stream.
Attack Vector
Exploitation requires an application that passes untrusted input into a vulnerable Net::IMAP command argument. The attacker supplies a string containing IMAP syntax that causes command-stream confusion. Successful exploitation hangs IMAP client operations and consumes thread resources. The attack requires low privileges and some user or application interaction to reach the vulnerable code path.
For protocol-level details, see the Net::IMAP GitHub Security Advisory GHSA-c4fp-cxrr-mj66.
Detection Methods for CVE-2026-47241
Indicators of Compromise
- IMAP client threads in a long-lived waiting state with no protocol response from the server
- Unexpected disconnects on IMAP sessions following malformed argument transmission
- Application logs showing Net::IMAP commands that never return until connection teardown
Detection Strategies
- Inventory all Ruby applications using the net-imap gem and identify versions below 0.6.5 and 0.5.15
- Audit source code for Net::IMAP command calls (such as search, examine, select, status) that accept user-controlled arguments
- Review IMAP server-side logs for clients issuing commands with unusual literal or quoted-string delimiters
Monitoring Recommendations
- Track thread counts and blocked thread duration in Ruby processes performing IMAP operations
- Alert on abnormal IMAP connection lifetimes or repeated forced disconnections
- Monitor outbound IMAP traffic for malformed command sequences originating from application servers
How to Mitigate CVE-2026-47241
Immediate Actions Required
- Upgrade the net-imap gem to version 0.6.5 or 0.5.15 or later
- Identify and patch all Ruby applications that pass user input to Net::IMAP command arguments
- Restart affected Ruby services after upgrading to clear any blocked threads
Patch Information
The maintainers fixed this vulnerability in Net::IMAP 0.6.5 and 0.5.15. Patch details are available in the Net::IMAP GitHub Security Advisory. Apply the upgrade through Bundler or RubyGems by updating the gem version constraint and running bundle update net-imap.
Workarounds
- Sanitize and strictly validate any user-supplied input before passing it to Net::IMAP commands
- Reject input containing IMAP literal markers, quote characters, or non-ASCII control sequences
- Implement command timeouts on IMAP client operations to limit thread blocking duration
- Isolate IMAP client operations in dedicated worker pools to contain resource exhaustion
# Configuration example
bundle update net-imap
gem list net-imap
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
