CVE-2026-42258 Overview
CVE-2026-42258 is a Carriage Return Line Feed (CRLF) injection vulnerability in Net::IMAP, the Ruby standard library that implements Internet Message Access Protocol (IMAP) client functionality. The flaw allows attackers to inject arbitrary IMAP commands when applications pass Symbol arguments to IMAP commands without sanitization. Maintainers have 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
An attacker who controls Symbol values passed to Net::IMAP commands can inject CRLF sequences to smuggle additional IMAP protocol commands, potentially manipulating mailbox state or exfiltrating mail data.
Affected Products
- Ruby net-imap gem versions prior to 0.4.24
- Ruby net-imap gem versions 0.5.x prior to 0.5.14
- Ruby net-imap gem versions 0.6.x prior to 0.6.4
Discovery Timeline
- 2026-05-09 - CVE-2026-42258 published to NVD
- 2026-05-13 - Last updated in NVD database
Technical Details for CVE-2026-42258
Vulnerability Analysis
The Net::IMAP library constructs IMAP protocol messages by serializing Ruby arguments into wire-format strings. String arguments are properly validated and quoted to neutralize CRLF sequences, which serve as command terminators in the IMAP protocol. Symbol arguments, however, bypass these checks. The library converts symbols to strings without applying the same sanitization, allowing embedded \r\n sequences to pass through unmodified. An attacker who controls a symbol value reaching commands such as select, examine, status, or fetch can append arbitrary IMAP commands to the request stream.
Root Cause
The root cause is inconsistent input validation across argument types. The protocol serialization code treats Symbol as a trusted type and skips the CRLF neutralization applied to String inputs. This breaks the implicit contract that all user-influenced data is sanitized before being written to the network socket. The defect maps to [CWE-77] because raw protocol delimiters are passed into a command stream without escaping.
Attack Vector
Exploitation requires a Ruby application that converts external input into a Symbol and forwards it to a Net::IMAP command. The attacker supplies a value containing \r\n followed by an injected IMAP command tag and verb. When the IMAP client serializes the request, the server interprets the injected sequence as a second command. Successful exploitation can modify mailbox selection, delete messages, or trigger unintended FETCH operations against arbitrary mailboxes the authenticated session can access. The attack vector is local in CVSS terms because it requires the attacker to influence values supplied to the in-process IMAP client.
No verified public exploit code is available. See the GitHub Security Advisory GHSA-75xq-5h9v-w6px for full technical details.
Detection Methods for CVE-2026-42258
Indicators of Compromise
- Unexpected IMAP commands appearing in mail server logs from sessions originating at Ruby application hosts.
- Multiple IMAP command tags transmitted within a single client request frame.
- Mailbox state changes (SELECT, DELETE, STORE) that do not correspond to documented application workflows.
Detection Strategies
- Inventory Ruby applications and check Gemfile.lock for net-imap versions below 0.4.24, 0.5.14, or 0.6.4.
- Perform static code review for call sites that pass Symbol values derived from user input into Net::IMAP methods such as select, examine, status, list, and fetch.
- Inspect IMAP server logs for command tags containing embedded CRLF artifacts or anomalous command sequencing on a single TCP connection.
Monitoring Recommendations
- Forward IMAP server access and command logs to a centralized analytics platform and alert on unexpected DELETE, EXPUNGE, or cross-mailbox FETCH activity.
- Monitor outbound IMAP traffic from application servers for protocol anomalies using network detection rules.
- Track dependency manifests in CI pipelines and alert on builds that pin vulnerable net-imap versions.
How to Mitigate CVE-2026-42258
Immediate Actions Required
- Upgrade net-imap to 0.4.24, 0.5.14, or 0.6.4 depending on the branch currently deployed.
- Audit application code for IMAP command invocations that accept external input and ensure values are passed as validated String objects.
- Rebuild and redeploy any container images, gems, or bundled artifacts that include the vulnerable library.
Patch Information
The Ruby net-imap maintainers released fixes in v0.4.24, v0.5.14, and v0.6.4. The patches extend CRLF neutralization to Symbol arguments so they are validated using the same logic as String inputs. Application owners should update Gemfile constraints and run bundle update net-imap.
Workarounds
- Convert all Symbol arguments to String and reject values containing \r or \n before invoking any Net::IMAP command.
- Restrict IMAP service accounts used by Ruby applications to least-privilege mailboxes to limit the blast radius of injected commands.
- Where upgrades are blocked, wrap Net::IMAP calls in an internal helper that enforces an allowlist of permitted mailbox identifiers.
# Update the vulnerable gem to a patched release
bundle update net-imap
# Verify the installed version is patched
bundle exec gem list net-imap
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
