CVE-2026-107583 in hMailServer
Summary
by MITRE • 10/08/2026
Inefficient algorithmic complexity in the webmail's message view of the REST API in Progressive Robot hMailServer 6.3.2 through 6.3.5 allows a remote unauthenticated attacker to make the webmail, the administration console and the REST API unavailable by sending a message. The route that renders a received message's HTML replaced each reference to an embedded image in place, with work that grew with the square of the number of references, and wrote the image out for every reference while counting it against its size limit only once. A message whose HTML refers to one small embedded image a very large number of times, opened in the webmail by its recipient, therefore keeps one of the listener's four worker threads busy for minutes and makes it build a document of gigabytes, so that a few such messages leave the HTTP listener unable to answer anybody.
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Analysis
by VulDB Data Team • 10/08/2026
The vulnerability identified as an inefficient algorithmic complexity flaw in Progressive Robot hMailServer versions 6.3.2 through 6.3.5 represents a significant denial-of-service risk within its webmail interface and associated REST API infrastructure. This issue stems from the server's handling of HTML content containing embedded images, specifically during the rendering process where references to these images are replaced in place. The core technical flaw lies in the computational complexity of this operation; rather than optimizing for repeated references or caching resources efficiently, the system performs a replacement and write-out action that scales with the square of the number of image references within the HTML document. This quadratic time complexity means that as the count of embedded image tags increases, the processing effort required grows exponentially, leading to severe performance degradation under specific conditions.
From an operational perspective, this flaw allows for a remote unauthenticated denial-of-service attack. An attacker can craft and send a malicious email containing HTML with a single small embedded image referenced an extremely large number of times. When a recipient or administrator views this message through the webmail interface, the server attempts to process the document by replacing each reference individually. Because the algorithm does not optimize for duplicate references, it performs redundant work for every instance of the same image tag. This results in one of the HTTP listener's worker threads being occupied for several minutes while processing a single request. During this time, the thread is unable to handle other incoming connections or requests, effectively blocking access for legitimate users.
The impact extends beyond simple latency; the vulnerability also triggers excessive memory allocation and disk I/O operations. The system writes out the image data for every reference in the HTML document rather than referencing a single cached instance. Consequently, even with a small base image size, the resulting generated document can grow to gigabytes in size if the number of references is sufficiently high. This massive output consumes significant server resources and contributes to the exhaustion of available worker threads on the HTTP listener. With only four worker threads typically available for handling such requests, just a few carefully crafted messages are sufficient to saturate these threads, rendering the webmail interface, administration console, and REST API completely unavailable to all users until the backlog is cleared or the service restarts.
This vulnerability aligns with CWE-400, which describes Uncontrolled Resource Consumption, as the application fails to limit the amount of resources consumed by a single request due to inefficient algorithmic design. Furthermore, it maps to MITRE ATT&CK technique T1499, Endpoint Denial of Service, specifically under the sub-category of resource exhaustion via overwhelming requests or processing loops. The attack vector is remote and unauthenticated, meaning no valid credentials are required to exploit this flaw, which significantly lowers the barrier for entry for malicious actors aiming to disrupt mail services.
Mitigation strategies should prioritize updating hMailServer to a version where this algorithmic inefficiency has been addressed by the vendor. In the interim, administrators can implement network-level controls such as rate limiting on HTTP requests or restricting access to the webmail and REST API endpoints to trusted IP addresses only. Additionally, deploying an intrusion prevention system capable of inspecting email payloads for excessively large HTML documents with high ratios of embedded object references may help filter out malicious messages before they reach the mail server's processing engine. Regular patch management is critical here, as this type of vulnerability relies on specific software versions and can be resolved through official updates that optimize resource handling during HTML rendering.