CVE-2021-46853 is a medium-severity vulnerability affecting Alpine versions prior to 2.25. The vulnerability allows remote attackers to cause a denial of service (DoS) by crashing the application when the IMAP commands LIST or LSUB are sent before the STARTTLS command is issued. This indicates a flaw in the handling of IMAP protocol commands related to secure session initiation. Specifically, the application does not properly handle these commands if they are sent before the secure TLS session is established, leading to an application crash. The vulnerability is categorized under CWE-367, which relates to time-of-check/time-of-use (TOCTOU) race conditions or improper synchronization issues, suggesting a logic flaw in the sequence enforcement of commands. The CVSS v3.1 base score is 5.9, reflecting a medium severity with network attack vector, high attack complexity, no privileges required, no user interaction, unchanged scope, and impact limited to availability (application crash). There are no known exploits in the wild, and no vendor or product information is explicitly provided, but the reference to Alpine and the nature of the commands strongly suggest this affects Alpine's IMAP server or client implementations prior to version 2.25. The lack of patch links indicates that users should verify if updates or mitigations are available from Alpine or related software providers.

For European organizations, this vulnerability could disrupt email services that rely on Alpine's IMAP client or server implementations, particularly if they use versions prior to 2.25. The denial of service caused by remote attackers sending specific IMAP commands before STARTTLS could lead to temporary loss of email access or service interruptions. This can impact business communications, delay critical information exchange, and reduce operational efficiency. While the vulnerability does not compromise confidentiality or integrity, the availability impact could be significant for organizations with high email traffic or those relying on Alpine in automated or embedded systems. Additionally, organizations in regulated sectors such as finance, healthcare, or government may face compliance risks if email service disruptions affect data handling or reporting obligations. The lack of known exploits reduces immediate risk, but the medium severity and network accessibility mean that attackers could potentially develop exploits, especially in environments where Alpine is widely deployed.

European organizations should first identify if Alpine versions prior to 2.25 are in use within their infrastructure, particularly in email clients or servers handling IMAP connections. If Alpine is deployed, upgrading to version 2.25 or later is the primary mitigation step to address this vulnerability. In the absence of an available patch, organizations should consider implementing network-level controls such as filtering or blocking IMAP commands LIST and LSUB before STARTTLS is negotiated, using mail gateway appliances or firewalls with protocol inspection capabilities. Monitoring IMAP traffic for anomalous sequences of commands can help detect potential exploitation attempts. Additionally, enforcing strict STARTTLS usage policies and ensuring clients and servers do not accept commands before secure session establishment can reduce exposure. Regularly reviewing and updating email infrastructure configurations to comply with best practices for secure IMAP usage is recommended. Finally, organizations should maintain incident response readiness to quickly address any service disruptions caused by exploitation attempts.