CVE-2021-46853

What it actually does

The flaw permits an unauthenticated remote attacker to trigger an application crash in Alpine by issuing LIST or LSUB IMAP commands prior to STARTTLS negotiation, resulting in denial of service with no impact on confidentiality or integrity.

Why it matters to the business

Organizations relying on Alpine for email access may experience unexpected service interruptions and application restarts, leading to temporary loss of email functionality until the process recovers.

How an attack unfolds

An attacker connects to an Alpine-based IMAP service and sends LIST or LSUB commands before the STARTTLS handshake completes; the server fails to handle the pre-encryption state correctly and crashes.

How to fix it

Upgrade Alpine to version 2.25 or later; if immediate upgrade is not feasible, restrict network access to the IMAP service or enforce STARTTLS at the network layer.

How to confirm the fix worked

Confirm the installed Alpine version is 2.25 or higher; test by attempting LIST or LSUB before STARTTLS and verify the process no longer crashes.

Typical attack path

1. Establish TCP connection to the Alpine IMAP listener
2. Send LIST or LSUB command without first issuing STARTTLS
3. Observe application crash and service unavailability
4. Repeat as needed to maintain denial of service

How to detect it

Review application and system logs for unexpected Alpine process terminations occurring shortly after inbound IMAP connections; correlate with network captures showing LIST or LSUB commands issued prior to TLS negotiation.

What your security team should do

• Prioritize upgrade to Alpine 2.25+ across all deployments
• Monitor for application crashes correlated with IMAP traffic
• Apply network-level controls to enforce encryption before command processing if patching is delayed