CVE-2026-107575 in hMailServer
Summary
by MITRE • 10/08/2026
Inefficient algorithmic complexity in the SPF macro expansion of Progressive Robot hMailServer 6.3.4 and 6.3.5 allows a remote unauthenticated attacker to consume worker-thread time by publishing a crafted SPF record. RFC 7208 section 7.1 requires a name too long to look up to lose whole labels from the left; the server did this by removing one label at a time and copying the rest of the name each time, so the work grew with the square of the expansion. An attacker who publishes an SPF record for a domain they control, with a mechanism whose domain-spec expands through macros to a name far longer than 253 characters, makes the SPF check of a message from that domain take several seconds. The expansion is bounded by SPF's own per-term and per-macro limits, so the loss of availability is partial.
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Analysis
by VulDB Data Team • 10/08/2026
The vulnerability identified in Progressive Robot hMailServer versions 6.3.4 and 6.3.5 represents a significant efficiency flaw within the implementation of Sender Policy Framework macro expansion logic. This issue stems from an inefficient algorithmic complexity that results in quadratic time consumption relative to the length of the domain name being processed. The core technical defect lies in how the server handles label removal during SPF record evaluation when encountering names that exceed standard lookup limits. According to RFC 7208 section 7.1, if a DNS query for an SPF mechanism's domain-spec results in a name too long to look up, the protocol mandates that whole labels be removed from the left side of the name until it is short enough or no more labels remain. The flawed implementation adheres to this requirement but does so with poor computational efficiency by removing only one label at a time and then copying the remainder of the domain string for each subsequent iteration. This approach causes the processing work to grow quadratically, meaning that as the length of the crafted SPF record increases, the CPU cycles required to process it increase exponentially rather than linearly.
An unauthenticated remote attacker can exploit this vulnerability by publishing a malicious SPF record under a domain they control. By constructing an SPF mechanism whose domain-spec expands through macros into a name significantly longer than the standard maximum limit of two hundred fifty-three characters, the attacker forces hMailServer to perform excessive string manipulation operations during message processing. When a legitimate email is received from this controlled domain and subjected to SPF verification by the vulnerable mail server, the inefficient macro expansion triggers the quadratic complexity flaw. This results in the consumption of substantial worker-thread time, causing the SPF check for that specific message to take several seconds to complete. The impact is primarily a denial-of-service condition affecting availability, as the excessive CPU usage can degrade performance for other concurrent email processing tasks on the server.
The scope and severity of this vulnerability are bounded by certain constraints inherent in the SPF specification itself. Specifically, the expansion process is limited by per-term and per-macro length restrictions defined within the protocol standards. These limits prevent the attacker from creating an infinitely long string that would cause total system paralysis or complete denial of service for all mail flow. Consequently, the loss of availability is partial rather than absolute. However, even a partial degradation in performance can be significant in high-throughput email environments where timely message delivery and processing are critical operational requirements. The vulnerability highlights how adherence to protocol logic without consideration for algorithmic efficiency can lead to resource exhaustion issues that attackers can leverage effectively.
From a classification perspective, this issue aligns with CWE-400, which describes Uncontrolled Resource Consumption, specifically regarding the inefficient use of CPU resources due to poor algorithm design. It also relates to CWE-756, Missing or Incorrect Calculation of Minimum Required Resources, as the implementation fails to account for the computational cost associated with string copying operations in a loop. In terms of attack patterns, this vulnerability facilitates an impact consistent with ATT&CK technique T1499, Endpoint Denial of Service, where the attacker uses resource exhaustion via inefficient processing logic rather than direct exploitation of memory corruption or privilege escalation flaws. The lack of authentication required to trigger this condition further exacerbates its risk profile, as any external entity can contribute to server load degradation simply by controlling a DNS record.
Mitigation strategies for this vulnerability primarily involve updating hMailServer to a patched version that addresses the macro expansion inefficiency. Administrators should ensure their mail servers are running software versions released after the identification of this flaw, where the algorithm has likely been optimized to avoid quadratic complexity during label removal and string copying operations. In addition to patching, organizations can implement rate limiting or connection throttling at the network perimeter to mitigate the impact of such resource-intensive requests if immediate patching is not feasible. Monitoring server CPU usage for anomalies correlated with SPF checks from specific domains may also aid in early detection of exploitation attempts. Ultimately, resolving this issue requires a code-level fix that optimizes the string manipulation logic during SPF macro expansion to ensure linear or near-linear time complexity regardless of domain name length.