CVE-2026-47241

What it actually does

This is an input-validation flaw in the Ruby Net::IMAP client library. Several IMAP command methods accept raw string arguments that are only checked for CRLF injection before being written directly to the IMAP connection. A crafted argument can alter IMAP command framing so that a subsequent command is treated as a continuation of the previous command rather than as a separate request. The original command may eventually fail, but it can block until another command is sent, and that later command can remain blocked until the IMAP connection is closed. The practical technical impact is low-grade availability degradation, connection/thread blocking, request hangs, and potential resource exhaustion in applications that pass user-controlled input into affected Net::IMAP raw string arguments.

Why it matters to the business

Applications that use affected Ruby Net::IMAP versions may experience stuck mail operations, delayed background jobs, exhausted worker threads, or degraded email-related functionality if attacker-controlled values reach vulnerable IMAP command arguments. This is not known to enable data theft or code execution, has low EPSS likelihood, is not in CISA KEV, and has no reported exploit maturity, but it can still disrupt business workflows that depend on reliable mailbox access, ticket ingestion, notification processing, or email synchronization.

How an attack unfolds

An attacker first needs a path to influence input that the application passes into a vulnerable Net::IMAP command as a raw string argument, such as a mailbox name, search expression, or other IMAP command component. The attacker supplies a specially crafted value that does not use blocked CRLF characters but still changes the way the IMAP server interprets command framing, for example by causing the command to expect continuation data. The application sends the malformed command over an established IMAP session. The vulnerable command does not return normally and waits until another command is sent on the same connection, often from another thread or shared client object. That subsequent command can then be absorbed into the first command’s continuation flow and also remain blocked until the connection is closed. Repeated attempts can tie up IMAP connections, application threads, or background workers, producing denial-of-service symptoms in mail-processing features.

How to fix it

Upgrade Net::IMAP to a fixed release: use 0.6.5 or later for the 0.6.x line, or 0.5.15 or later for the 0.5.x line. If Net::IMAP is pulled transitively by another Ruby package or framework, update the dependency lockfile and rebuild/redeploy the application so the fixed gem is actually loaded at runtime. Where immediate upgrade is not possible, avoid passing user-controlled values to raw string IMAP command arguments; use structured APIs, validated mailbox names, allowlists, and safe quoting/encoding routines instead of verbatim strings. Also consider connection-level safeguards such as per-command timeouts, connection recycling after protocol errors, bounded worker pools, and isolation of IMAP sessions so one blocked command cannot stall unrelated work.

How to confirm the fix worked

Confirm the deployed runtime is loading Net::IMAP 0.6.5 or later, or 0.5.15 or later. In Ruby environments, validate with dependency tooling such as `bundle info net-imap`, `bundle list | grep net-imap`, `gem list net-imap`, and inspection of `Gemfile.lock`. Confirm container images, serverless packages, background-worker images, and cron/job hosts all contain the updated gem, not only developer workstations. Exercise application IMAP workflows that previously accepted user-controlled mailbox names, search terms, or command arguments and verify malformed inputs are rejected, safely encoded, or time out cleanly without leaving blocked threads or open IMAP sessions. Monitor application and mail-client metrics after deployment for reduced IMAP command timeouts, stuck jobs, and connection resets.

Typical attack path

1. Identify an application feature that interacts with IMAP using Ruby Net::IMAP, such as mailbox browsing, message search, synchronization, or inbound email processing.
2. Find an input field or upstream data source whose value is passed into a Net::IMAP command as a raw string argument.
3. Submit a crafted string that avoids CRLF characters but manipulates IMAP command continuation/framing behavior.
4. Trigger the application to send the affected IMAP command to the mail server.
5. Cause the command to hang or fail only after additional command activity on the same connection.
6. Repeat or automate the trigger to consume IMAP connections, application threads, job workers, or request capacity.

How to detect it

There are no reliable public exploit-specific network or file indicators for this issue. Detection should focus on behavioral evidence in application, worker, and IMAP client logs. Review logs for IMAP commands or mail-processing jobs that hang until timeout, repeated connection closures after malformed mailbox/search operations, spikes in blocked Ruby threads, and increases in IMAP connection pool exhaustion. Correlate these events with user-controlled inputs to email features, especially mailbox names, search parameters, folder selectors, or synchronization requests submitted shortly before the hang. On mail servers, look for sessions where a client begins a command and then sends unexpected continuation data or where commands remain incomplete until disconnect. In application telemetry, investigate repeated timeouts from the same account, IP address, API token, session, or request path that exercises IMAP operations. For forensics, preserve application request logs, background job logs, IMAP debug/protocol logs if enabled, dependency manifests, and runtime gem inventories to determine whether vulnerable Net::IMAP versions were in use at the time of suspicious hangs.

• Gemfile.lock: net-imap (0.6.4)
• Gemfile.lock: net-imap (0.6.3)
• Gemfile.lock: net-imap (0.6.2)
• Gemfile.lock: net-imap (0.6.1)
• Gemfile.lock: net-imap (0.6.0)
• Gemfile.lock: net-imap (0.5.14)
• bundle list | grep net-imap
• gem list net-imap
• Net::IMAP
• IMAP command timed out
• Net::IMAP::NoResponseError
• Net::IMAP::ByeResponseError
• connection is closed
• execution expired

What your security team should do

• Inventory Ruby applications and services that use Net::IMAP directly or transitively, prioritizing internet-facing workflows and systems that process untrusted email-related input.
• Upgrade affected deployments to Net::IMAP 0.6.5 or later, or 0.5.15 or later, and ensure dependency lockfiles and production artifacts are refreshed.
• Review code paths that call Net::IMAP commands with raw string arguments and remove direct use of user-controlled input where possible.
• Add defensive timeouts, connection recycling, and worker limits around IMAP operations to reduce denial-of-service impact from malformed protocol state.
• Search recent logs for unexplained IMAP command hangs, worker timeouts, mailbox-operation errors, and connection resets around user-triggered mail actions.
• Because this issue is not KEV-listed, not associated with ransomware use, and has low EPSS, handle it through normal vulnerability SLAs unless the affected application is business-critical or exposed to untrusted users.