CVE-2026-107577 in hMailServer
Summary
by MITRE • 10/08/2026
Inefficient algorithmic complexity and a non-terminating loop in the MIME processing of received messages in Progressive Robot hMailServer 6.0.0 through 6.3.5 allow a remote unauthenticated attacker to make the mail services unavailable by sending a message. Removing a MIME header parameter whose value is empty and directly followed by a semicolon (for example a Content-Disposition with 'filename=a.bat; filename=;') entered a loop that never terminates, holding a worker thread at full load until the server is restarted; this is reached when the attachment blocker renames a blocked attachment or a filename is set over the REST API. Separately, decoding a header field that holds many RFC 2047 encoded words of an encoding other than base64 or quoted-printable, removing a parameter with many RFC 2231 continuations, and deleting many header fields of one name each took time growing with the square of the message, on the small thread pools that serve IMAP, SMTP and POP3 connections, delivery and the REST API.
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Analysis
by VulDB Data Team • 10/08/2026
The vulnerability in hMailServer versions 6.0.0 through 6.3.5 represents a critical service availability risk stemming from two distinct technical flaws within its MIME processing engine: an infinite loop condition causing thread exhaustion and inefficient algorithmic complexity leading to resource starvation. As a cyber security specialist, it is imperative to recognize that these issues allow unauthenticated remote attackers to execute Denial of Service attacks against the mail infrastructure without requiring valid credentials or prior interaction with the system beyond sending a single crafted email message. The primary vector for immediate service disruption involves manipulating MIME header parameters in specific ways that trigger non-terminating loops within the application logic. Specifically, when processing attachments via mechanisms such as the attachment blocker renaming functionality or filename configuration through the REST API, certain malformed inputs cause the server to enter an infinite loop state.
The technical root of this infinite loop lies in the parsing of MIME header parameters where a parameter value is empty and immediately followed by a semicolon. A concrete example of this malicious input involves setting a Content-Disposition header with a structure such as filename=a.bat; filename=;. In standard compliant parsers, an empty parameter value following a delimiter should be handled gracefully or ignored depending on the specific RFC interpretation. However, in hMailServer versions 6.0.0 through 6.3.5, this specific pattern causes the parsing routine to fail to advance its internal pointer correctly, resulting in it re-evaluating the same position repeatedly without termination condition satisfaction. This behavior holds a dedicated worker thread at one hundred percent CPU utilization indefinitely. Since hMailServer relies on a finite pool of threads to handle incoming SMTP connections, IMAP sessions, POP3 access, and REST API requests, the occupation of even a single thread by this infinite loop can rapidly exhaust available resources if multiple such messages are processed concurrently or if the server is under moderate load. The operational impact is severe: mail services become unresponsive, legitimate users cannot send or receive emails, and administrative functions via the web interface may fail due to lack of available threads for request handling.
Separately from the infinite loop issue, a significant performance degradation vulnerability exists characterized by inefficient algorithmic complexity during MIME header decoding. This flaw manifests when processing messages containing numerous RFC 2047 encoded words using encodings other than base64 or quoted-printable, as well as headers with extensive RFC 2231 continuations or multiple duplicate header fields of the same name. The parsing logic exhibits quadratic time complexity relative to the size and number of these elements in the message header. On systems configured with small thread pools typical for IMAP, SMTP, POP3, delivery agents, and REST API services, this inefficiency translates into substantial latency spikes and temporary unresponsiveness rather than immediate total failure like the infinite loop case. An attacker can exploit this by sending messages with heavily encoded or fragmented headers to consume excessive CPU cycles during parsing, effectively throttling the server's ability to process other legitimate traffic efficiently. This aligns with CWE-400 (Uncontrolled Resource Consumption) and specifically reflects patterns associated with inefficient regular expression usage or poor string manipulation algorithms often categorized under CWE-1341 in broader contexts of input validation failures leading to performance issues.
From a threat intelligence perspective, these vulnerabilities map directly to MITRE ATT&CK techniques related to Service Stop via resource exhaustion. The infinite loop scenario corresponds to T1499 (Endpoint Denial of Service) where the attacker leverages application logic flaws to consume computational resources indefinitely. The quadratic complexity issue relates to T1496 (Resource Hijacking) or general DoS vectors that degrade service quality rather than causing immediate crash, though in constrained environments with limited thread pools, it effectively achieves a similar denial-of-service outcome by blocking new connections from being accepted or processed promptly. These flaws highlight the dangers of insufficient input validation and lack of complexity bounds checking in core mail processing libraries.
Mitigation strategies must prioritize immediate patching to the latest stable version of hMailServer where these specific MIME parsing bugs have been addressed by the development team. Until patches are applied, network-level mitigations should be implemented at the perimeter firewall or load balancer level. This includes rate limiting incoming SMTP connections from untrusted sources and inspecting inbound email headers for known malicious patterns such as empty parameter values followed immediately by semicolons in Content-Disposition fields. Additionally, administrators should consider increasing thread pool sizes cautiously to absorb some of the quadratic complexity impact, although this is a temporary workaround rather than a fix. It is also advisable to deploy intrusion detection systems capable of identifying anomalous header structures that deviate from standard RFC 2047 and RFC 2231 compliance norms, thereby filtering out potentially exploitative messages before they reach the vulnerable hMailServer instance for processing. Regular auditing of MIME parsing logic in any mail server software is essential to prevent similar algorithmic vulnerabilities from impacting service availability.