CVE-2026-107581 in hMailServer
Summary
by MITRE • 10/08/2026
Progressive Robot hMailServer 6.0.0 through 6.3.5 processes several IMAP commands from a signed-in account in time quadratic in the command's length or in the number of elements it names, and can be made to hold memory out of proportion to a command. The FETCH data-item parser normalised a BODY[] section with a case-insensitive string replacement that copied the whole item per occurrence and split the item list by repeatedly copying the remainder of the command; the server's case-insensitive search compared a needle afresh at every position, so a long SEARCH TEXT key over a message cost the product of the two lengths; SEARCH message sets and the saved-result marker ($), SORT criteria, HEADER.FIELDS name lists and UID ranges were each read once per message rather than once per command; and a FETCH naming a message's sections very many times read and held every section in memory before sending any. An IMAP command may continue past one line through non-synchronizing literals, so one command can reach about eleven megabytes. The IMAP worker threads are a small pool shared with SMTP and POP3, so a signed-in user sending such commands can make the IMAP, SMTP and POP3 services stop responding (CWE-407) and can consume excessive memory (CWE-400).
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Analysis
by VulDB Data Team • 10/08/2026
The vulnerability identified in hMailServer versions 6.0.0 through 6.3.5 represents a critical class of performance-related flaws rooted in inefficient algorithmic complexity within the IMAP protocol implementation. This issue manifests as quadratic time complexity relative to command length or element count, alongside excessive memory consumption that is disproportionate to the input size. The core technical flaw lies in how several specific IMAP commands are parsed and processed by the server's backend logic. Specifically, the FETCH data-item parser employs a case-insensitive string replacement mechanism for BODY[] sections that inadvertently copies the entire item for every occurrence found. Furthermore, it splits the command list by repeatedly copying the remainder of the command text during processing. This approach results in significant computational overhead and memory allocation as the size of the input increases, rather than scaling linearly or with a more efficient complexity class.
In addition to the FETCH parsing inefficiencies, the server's case-insensitive search functionality exhibits similar performance degradation. When executing a SEARCH TEXT query against a message, the system compares the search needle fresh at every position within the text body. Consequently, the computational cost becomes the product of the length of the search key and the length of the message content. This quadratic behavior is not isolated to text searches; it extends to other IMAP operations such as processing SEARCH message sets, handling the saved-result marker ($), parsing SORT criteria, evaluating HEADER.FIELDS name lists, and managing UID ranges. In these instances, data structures are read once per individual message rather than being processed efficiently on a command-wide basis, leading to substantial resource exhaustion when dealing with large mailboxes or complex queries.
The impact of this vulnerability is exacerbated by the IMAP protocol's support for non-synchronizing literals, which allow an IMAP command to span multiple lines and reach sizes of approximately eleven megabytes in a single request. When such oversized commands are processed using the flawed algorithms described above, the server experiences severe performance degradation. The memory consumption becomes excessive as the system attempts to hold entire sections or repeatedly copy large data structures into RAM before any response can be sent back to the client. This behavior is particularly dangerous because it allows an authenticated user to trigger these conditions deliberately by crafting specific IMAP commands that exploit the quadratic complexity and inefficient string handling mechanisms inherent in the software's design.
From a security operations perspective, this vulnerability poses a significant risk to service availability due to the architecture of hMailServer's thread management. The IMAP worker threads are part of a small pool shared with SMTP and POP3 services. When an authenticated user sends commands that trigger these inefficient processing paths, they can exhaust the available thread resources or consume excessive memory on the server host. This leads to a denial-of-service condition where not only the IMAP service becomes unresponsive but also the co-located SMTP and POP3 services stop responding entirely. The vulnerability is classified under CWE-407 for understanding mechanisms that lead to resource exhaustion, specifically highlighting the improper handling of input size relative to processing time. Additionally, it aligns with CWE-400 regarding unchecked resource consumption, as the application fails to limit or monitor the memory and CPU resources allocated per command execution effectively.
Mitigation strategies must focus on immediate patching and architectural review. The primary remediation is to upgrade hMailServer to a version that has addressed these algorithmic inefficiencies through optimized parsing logic and bounded processing limits. Until an update is applied, administrators should consider implementing network-level rate limiting or input size restrictions for IMAP connections if the infrastructure supports it. Furthermore, monitoring server resource usage patterns can help detect potential exploitation attempts characterized by unusually high memory consumption or thread pool saturation originating from specific user accounts. Long-term mitigation involves ensuring that all protocol implementations adhere to strict complexity bounds and avoid copying large data structures unnecessarily during parsing operations.