CVE-2025-43857 is a medium-severity vulnerability affecting the Ruby net-imap library, which implements the Internet Message Access Protocol (IMAP) client functionality. The vulnerability arises from uncontrolled resource consumption, specifically memory exhaustion, when the net-imap client reads server responses containing "literal" byte counts. In affected versions prior to 0.5.7, 0.4.20, 0.3.9, and 0.2.5, the client automatically allocates memory based on the byte count specified by the server response without sufficient validation or limits. A malicious or compromised IMAP server can exploit this behavior by sending an excessively large literal byte count, causing the client to allocate large amounts of memory, potentially leading to denial of service (DoS) through memory exhaustion. This vulnerability is particularly relevant when connecting to untrusted or insecure IMAP servers, such as those specified by user-supplied hostnames or servers vulnerable to compromise. Secure connections to trusted servers are less likely to be affected. The issue has been patched in the specified versions, and no known exploits are currently reported in the wild. The CVSS 4.0 score is 6.0 (medium), reflecting network attack vector, low attack complexity, no privileges required, partial user interaction, and high impact on availability. The vulnerability is categorized under CWE-400 (Uncontrolled Resource Consumption), CWE-770 (Allocation of Resources Without Limits or Throttling), CWE-789 (Uncontrolled Memory Allocation), and CWE-405 (Missing Initialization).

For European organizations, this vulnerability could lead to denial of service conditions in applications or services relying on the Ruby net-imap client library to access IMAP servers. This may disrupt email retrieval processes, impacting business communications and automated workflows dependent on email data. Organizations using custom or third-party software built on vulnerable net-imap versions, especially those connecting to external or user-supplied IMAP servers, are at higher risk. The DoS attack could degrade system performance or cause crashes due to memory exhaustion, potentially affecting availability of critical email-based services. While confidentiality and integrity impacts are minimal, availability degradation can have operational consequences. This is particularly relevant for sectors with high reliance on email infrastructure such as finance, government, healthcare, and critical infrastructure in Europe. The risk is amplified in environments where IMAP clients connect to untrusted or compromised servers, including scenarios involving remote or mobile users connecting to external mail servers. Given the medium severity and absence of known exploits, the immediate threat level is moderate but warrants proactive mitigation to prevent service disruption.

Upgrade all Ruby net-imap client libraries to patched versions 0.5.7, 0.4.20, 0.3.9, or 0.2.5 or later to ensure the vulnerability is fixed. Implement strict validation and limits on literal byte counts when processing IMAP server responses in custom implementations or wrappers around net-imap. Restrict connections to trusted IMAP servers only, avoiding user-supplied or unverified hostnames to reduce exposure to malicious servers. Use secure connection methods such as TLS to protect against man-in-the-middle attacks that could inject malicious responses. Monitor memory usage of applications using net-imap clients and set resource limits or alerts to detect abnormal consumption patterns. In environments where upgrading is not immediately feasible, consider isolating vulnerable applications in containers or sandboxes with memory limits to contain potential DoS impact. Educate developers and system administrators about the risks of connecting to untrusted IMAP servers and encourage secure configuration practices. Review and audit third-party software dependencies to identify usage of vulnerable net-imap versions and coordinate patching efforts.