CVE-2026-107579 in hMailServer
Summary
by MITRE • 10/08/2026
Inefficient algorithmic complexity in the bounce and complaint processing of Progressive Robot hMailServer 6.3.4 and 6.3.5 allows a remote unauthenticated attacker to stop mail delivery by sending messages, when bounce processing or complaint processing is enabled or a mailing list is managed by the server (none is by default). The readers of incoming delivery status notifications (RFC 3464) and abuse feedback reports (RFC 5965) removed the blank lines at the start of the returned headers part two bytes at a time, copying the rest of the part each time, so their work grew with the square of the number of blank lines. A message shaped like such a report, whose headers part begins with a very large number of blank lines within the reader's 2 MB limit, keeps a delivery thread busy for over a minute while it is delivered, and a few such messages a minute keep every delivery thread busy.
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Analysis
by VulDB Data Team • 10/08/2026
The vulnerability identified in hMailServer versions 6.3.4 and 6.3.5 represents a critical denial of service condition stemming from inefficient algorithmic complexity within the bounce and complaint processing modules. This flaw allows remote unauthenticated attackers to disrupt mail delivery services by exploiting how the server handles incoming delivery status notifications and abuse feedback reports. The issue is specifically triggered when bounce processing or complaint processing features are enabled, or if the server manages a mailing list, although these features are disabled by default in standard configurations. An attacker can exploit this weakness by sending specially crafted messages that mimic legitimate electronic return receipts or abuse complaints, thereby forcing the server into an excessive computational state that halts normal operations for all users relying on the mail service.
The technical root cause lies in the implementation of the readers responsible for parsing incoming delivery status notifications as defined in RFC 3464 and abuse feedback reports as specified in RFC 5965. When processing these messages, the software attempts to clean up the headers by removing blank lines located at the start of the returned headers part. The flawed algorithm removes these blank lines two bytes at a time while simultaneously copying the remainder of the header data for each iteration. This approach results in quadratic time complexity relative to the number of blank lines present in the message. Consequently, as the volume of leading blank lines increases, the processing time grows exponentially rather than linearly, creating a severe performance bottleneck that consumes significant CPU resources and blocks delivery threads for extended periods.
The operational impact of this vulnerability is substantial, particularly under sustained attack conditions. A single crafted message containing a large number of blank lines within the server's two megabyte limit can keep a delivery thread occupied for over one minute during processing. If an attacker sends just a few such messages per minute, they can effectively saturate all available delivery threads on the mail server. This results in a complete denial of service where legitimate email traffic is queued indefinitely or dropped due to resource exhaustion. The attack does not require authentication, making it easily executable by any external actor with network access to the SMTP port, and it directly undermines the availability aspect of the CIA triad for organizations dependent on this mail server software.
From a classification perspective, this vulnerability aligns with CWE-400, which denotes uncontrolled resource consumption, specifically manifesting as inefficient algorithmic complexity that leads to denial of service conditions. In terms of offensive security frameworks, this exploit maps to MITRE ATT&CK technique T1496, representing Resource Hijacking where an attacker consumes resources to degrade or deny service to legitimate users. The attack vector is classified under Network Access with no additional privileges required, highlighting the severity of exposing such processing logic without adequate input validation or rate limiting mechanisms in place for incoming mail traffic.
Mitigation strategies must focus on both immediate remediation and long-term architectural improvements. Users running hMailServer versions 6.3.4 and 6.3.5 should immediately upgrade to a patched version that addresses this algorithmic flaw, as the vendor has released updates correcting the inefficient processing logic for bounce and complaint handling. In environments where upgrading is not immediately feasible, administrators can mitigate the risk by disabling bounce processing and complaint processing features if they are not strictly required for their operational workflow. Additionally implementing strict rate limiting on incoming SMTP connections and configuring intrusion detection systems to flag messages with abnormally large header sections or excessive blank lines can help reduce the impact of such attacks until a permanent fix is applied. Regular auditing of mail server configurations against industry best practices will further enhance resilience against similar algorithmic complexity vulnerabilities in future software versions.