CVE-2026-42257 Overview
CVE-2026-42257 is a command injection vulnerability in the Ruby Net::IMAP library, which implements Internet Message Access Protocol (IMAP) client functionality. Several Net::IMAP commands accept raw string arguments that are sent to the server without validation or escaping. When these strings derive from user-controlled input, an attacker can embed Carriage Return Line Feed (CRLF) sequences to inject arbitrary IMAP commands into the protocol stream. The maintainers patched the issue in versions 0.4.24, 0.5.14, and 0.6.4. The vulnerability is tracked under [CWE-77: Improper Neutralization of Special Elements used in a Command].
Critical Impact
Attackers can inject arbitrary IMAP commands through unsanitized user input, manipulating mailbox state and integrity on behalf of authenticated sessions.
Affected Products
- Ruby net-imap gem versions prior to 0.4.24
- Ruby net-imap gem versions prior to 0.5.14
- Ruby net-imap gem versions prior to 0.6.4
Discovery Timeline
- 2026-05-09 - CVE-2026-42257 published to NVD
- 2026-05-13 - Last updated in NVD database
Technical Details for CVE-2026-42257
Vulnerability Analysis
The Net::IMAP Ruby library transmits client commands as plain-text lines terminated by CRLF (\r\n), as defined by RFC 3501. Several command-issuing methods accept raw string arguments and pass them directly to the underlying socket without validating or escaping protocol delimiters. An attacker who controls any portion of these arguments can embed \r\n sequences followed by additional IMAP verbs. The IMAP server parses these injected lines as separate commands executed within the authenticated session context. This results in unauthorized actions such as selecting alternate mailboxes, deleting messages, modifying flags, or issuing LOGOUT directives.
Root Cause
The defect is a classic command injection pattern: the client library trusts argument strings without enforcing protocol-level neutralization. Methods that should have rejected or quoted control characters instead concatenated user-provided text directly into the wire-format command buffer. Because IMAP relies on CRLF as its statement separator, unescaped \r\n bytes terminate the intended command and begin a new one parsed by the server.
Attack Vector
Exploitation requires that an application built on Net::IMAP pass attacker-influenced input as an argument to a vulnerable command, for example as a mailbox name, search criterion, or identifier. The attacker supplies a value containing CRLF followed by an arbitrary IMAP command. The library forwards the unsanitized string, and the server executes the injected command under the active authenticated session. The CVSS 4.0 vector indicates local attack vector with passive user interaction, reflecting that the malicious input typically flows through an application-layer channel rather than direct network reach to the IMAP server.
No verified public proof-of-concept code is available. Refer to the GitHub Security Advisory GHSA-hm49-wcqc-g2xg for additional technical context.
Detection Methods for CVE-2026-42257
Indicators of Compromise
- IMAP server logs showing multiple commands originating from a single client write within an unusually short interval.
- Unexpected mailbox state changes such as message deletions, flag modifications, or SELECT operations on mailboxes the application does not normally access.
- Application logs containing user input fields with embedded \r\n, %0d%0a, or literal CRLF byte sequences destined for IMAP method calls.
Detection Strategies
- Audit Ruby applications for calls to Net::IMAP methods such as select, examine, create, delete, rename, subscribe, search, and fetch where arguments derive from HTTP parameters, database fields, or message headers.
- Deploy static analysis rules that flag Net::IMAP argument flows originating from untrusted sources without sanitization.
- Inspect outbound IMAP traffic at the network boundary for command sequences that do not match the application's expected command vocabulary.
Monitoring Recommendations
- Enable verbose IMAP server logging and alert on tag mismatches or unexpected command frequencies tied to service accounts used by Ruby applications.
- Track installed gem versions across the fleet and flag hosts running net-imap below 0.4.24, 0.5.14, or 0.6.4.
- Correlate web application input logs with downstream IMAP command volume to surface injection attempts.
How to Mitigate CVE-2026-42257
Immediate Actions Required
- Upgrade the net-imap gem to 0.4.24, 0.5.14, or 0.6.4 depending on the maintenance branch in use.
- Inventory all Ruby services and background workers that link Net::IMAP, including indirect dependencies pulled in by mail-processing gems.
- Reject or strip CRLF characters from any user-controlled input before it reaches IMAP command arguments.
Patch Information
The Ruby net-imap maintainers released fixed versions on three supported branches: v0.4.24, v0.5.14, and v0.6.4. The patches add validation that rejects argument strings containing CRLF and other illegal protocol characters. Update the Gemfile constraint and run bundle update net-imap to apply the fix.
Workarounds
- Wrap Net::IMAP calls in application-level helpers that validate arguments against an allowlist of permitted characters before invocation.
- Reject input containing \r, \n, or their encoded forms at the earliest trust boundary in the application.
- Restrict the privileges of IMAP service accounts used by the application so that injected commands have limited blast radius.
# Configuration example: pin the patched net-imap version in Gemfile
bundle update net-imap
gem list net-imap
# Verify output shows 0.4.24, 0.5.14, or 0.6.4 or higher
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
