CVE-2026-27852 in Dovecot Pro
Summary
by MITRE • 08/28/2026
An attacker that can send mail to a user can craft a message whose headers contain a very large number of email addresses or MIME parameters, which causes excessive memory usage when the message is later parsed. The message is still delivered, but reading it over IMAP can exhaust the memory limit of the process and terminate it, causing denial of service for the affected user. Update to non-vulnerable version. No publicly available exploits are known.
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Analysis
by VulDB Data Team • 08/28/2026
The vulnerability described constitutes a resource exhaustion flaw within an email processing system, specifically affecting the parsing logic when handling messages with excessively large header fields or MIME parameters. This type of issue is categorized under CWE-789: Memory Excessiveness and often aligns with CWE-400: Uncontrolled Resource Consumption from the Common Weakness Enumeration database. The core technical flaw lies in the application's failure to enforce strict limits on the size or complexity of email headers during the parsing phase. When an attacker sends a message containing a disproportionately large number of email addresses or MIME parameters, the server allocates memory dynamically to process these fields without adequate bounds checking. This behavior allows for the creation of input that triggers disproportionate resource allocation relative to the actual utility of the data being processed.
The operational impact of this vulnerability is primarily focused on availability rather than confidentiality or integrity. Although the message is successfully delivered and stored, the subsequent action of reading the email via IMAP forces the server process to parse these malformed headers in real-time. This parsing operation consumes an excessive amount of memory resources allocated to that specific user session or service thread. As the memory usage exceeds predefined limits, the affected process terminates abruptly due to out-of-memory conditions. This termination results in a denial of service for the targeted user, preventing them from accessing their mailbox and disrupting normal email operations until the service is restarted or the resource constraints are adjusted.
This attack vector leverages the trust placed in incoming mail headers by the server's parsing engine. It does not require authentication to execute if the attacker can send mail to a valid user account, making it accessible to any external actor with SMTP access. The lack of publicly available exploits suggests that while the vulnerability is theoretically exploitable for denial-of-service purposes, practical exploitation may be limited by factors such as rate limiting, network bandwidth constraints, or specific server configurations that mitigate extreme memory spikes in production environments. However, the potential remains significant for targeted attacks against high-value targets where service disruption causes substantial business impact.
Mitigation strategies should prioritize updating to a non-vulnerable version of the email software, which presumably includes patches designed to enforce stricter limits on header sizes and MIME parameter counts. In addition to patching, administrators can implement defensive measures such as configuring mail transfer agents or webmail proxies to reject messages with headers exceeding specific byte thresholds before they reach the vulnerable parsing engine. Implementing rate limiting for incoming connections and monitoring memory usage patterns can also help detect and mitigate attempts to exploit this flaw. Furthermore, adhering to industry standards like RFC 5321 regarding SMTP extensions and best practices for input validation ensures that systems are resilient against malformed or oversized inputs designed to trigger resource exhaustion vulnerabilities.