CVE-2026-47240 Overview
CVE-2026-47240 is a command injection vulnerability in the Ruby Net::IMAP library, which implements Internet Message Access Protocol (IMAP) client functionality. The flaw affects versions prior to 0.6.5 and 0.5.15. Several Net::IMAP commands accept a raw data argument that is sent verbatim after validation intended to prevent command injection. When the target IMAP server does not support non-synchronizing literals, attackers can inject arbitrary IMAP commands through carriage return line feed (CRLF) sequences inside literal payloads. The vulnerability is classified under CWE-77: Improper Neutralization of Special Elements used in a Command.
Critical Impact
Attackers controlling search criteria, sort keys, or fetch attributes can inject arbitrary pipelined IMAP commands against servers that lack non-synchronizing literal support, compromising mailbox integrity.
Affected Products
- Ruby net-imap gem versions prior to 0.6.5
- Ruby net-imap gem versions prior to 0.5.15
- Applications using Net::IMAP with IMAP servers that do not support non-synchronizing literals (RFC 7888)
Discovery Timeline
- 2026-06-22 - CVE-2026-47240 published to NVD
- 2026-06-23 - Last updated in NVD database
Technical Details for CVE-2026-47240
Vulnerability Analysis
The Net::IMAP client validates raw data arguments to prevent direct CRLF injection. However, the validation assumes the server supports non-synchronizing literals as defined in RFC 7888. When the server does not support this extension, the literal continuation marker +}\r\n is interpreted differently by the server than by the client.
A non-compliant server treats the +}\r\n sequence as the termination of a malformed command line and replies with a tagged BAD response. The contents intended as literal data are then parsed by the server as one or more new pipelined IMAP commands. This breaks the trust boundary between literal content and command syntax.
Affected APIs include the criteria parameter for #search and #uid_search, the search_keys parameter for #sort, #thread, #uid_sort, and #uid_thread, and the attr parameter for #fetch and #uid_fetch.
Root Cause
The root cause is a protocol-level assumption mismatch. The client library relies on non-synchronizing literal semantics to safely transmit user-controlled data containing CRLF bytes. When that assumption fails, the validation logic becomes insufficient, allowing arbitrary commands to be smuggled into the IMAP session.
Attack Vector
Exploitation requires an attacker to control input passed to one of the affected Net::IMAP methods and the target server to lack non-synchronizing literal support. The attacker supplies search criteria or fetch attributes containing crafted literal sequences. The server then processes the injected commands using the authenticated session's privileges.
No verified public exploit code is available. Refer to the GitHub Security Advisory GHSA-8p34-64r3-mwg8 for additional protocol-level details.
Detection Methods for CVE-2026-47240
Indicators of Compromise
- Unexpected tagged BAD responses in IMAP server logs followed by additional command tags within the same session.
- IMAP sessions containing literal markers such as {n+}\r\n where the server has not advertised the LITERAL+ or LITERAL- capability.
- Application logs showing user-supplied search or fetch arguments containing embedded CRLF byte sequences.
Detection Strategies
- Inspect outbound IMAP traffic for CRLF byte patterns embedded within #search, #sort, #thread, or #fetch arguments.
- Audit Ruby application dependencies for net-imap gem versions earlier than 0.6.5 or 0.5.15.
- Correlate IMAP server BAD responses with subsequent unexpected command execution in the same session.
Monitoring Recommendations
- Enable verbose IMAP protocol logging on mail servers to capture full command sequences for forensic review.
- Monitor process telemetry on hosts running Ruby IMAP clients for anomalous outbound connections to mail servers.
- Alert on dependency manifests (Gemfile.lock) referencing vulnerable net-imap versions during CI/CD scans.
How to Mitigate CVE-2026-47240
Immediate Actions Required
- Upgrade the net-imap gem to version 0.6.5 or 0.5.15 across all Ruby applications.
- Inventory all internal and third-party Ruby code that calls Net::IMAP methods accepting user input.
- Validate that production IMAP servers advertise the LITERAL+ or LITERAL- capability where feasible.
Patch Information
The vulnerability is fixed in net-imap versions 0.6.5 and 0.5.15. The patched versions strengthen validation of raw data arguments to reject CRLF sequences regardless of server literal support. Update via bundle update net-imap or pin the fixed version directly in Gemfile.
Workarounds
- Sanitize all user-supplied input passed to #search, #uid_search, #sort, #thread, #uid_sort, #uid_thread, #fetch, and #uid_fetch to reject CRLF bytes.
- Restrict IMAP client connections to servers known to support non-synchronizing literals (RFC 7888).
- Apply network-level egress filtering to limit which IMAP endpoints Ruby applications can reach.
# Update net-imap to patched version
bundle update net-imap
# Verify installed version meets minimum patched release
gem list net-imap
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
