CVE-2026-107572 Details
Description
Inefficient complexity in the Sieve filter evaluation of Progressive Robot hMailServer 6.2.24 through 6.3.5 allows an authenticated account holder to make the mail services unavailable with their own filter. A ':matches' pattern was matched by a backtracking descent whose time grew with the matched value's length raised to the number of wildcards, so a pattern such as '*a*a*a*b' over 800 characters took 44 seconds; and 'deleteheader' erased the fields it removed one at a time, so removing many fields of one name over a message's header cost O(N^2) (80,000 fields took 18.8 seconds). Sieve filters run on the small, shared delivery thread pool, so an account holder whose active script does this, fed a few messages they can send themselves, empties the pool and stops delivery for the whole server. The script is the account holder's own and cannot be set for another user.
A denial-of-service vulnerability has been identified in Progressive Robot hMailServer versions 6.2.24 through 6.3.5. This issue arises from an inefficient complexity in the evaluation of Sieve filters, allowing an authenticated user to disrupt mail services by exploiting their own filter. The vulnerability is rooted in the ':matches' pattern, which was processed by a backtracking algorithm. The time taken for evaluation increased exponentially with the length of the matched value and the number of wildcards. For instance, a pattern like '*a*a*a*b' over 800 characters required 44 seconds to process. Additionally, the 'deleteheader' command removed specified fields one at a time, leading to quadratic time complexity. An account holder could exploit this by sending themselves messages that triggered the resource-intensive filter, effectively halting mail delivery for the entire server.
Users are advised to upgrade to hMailServer version 6.3.6, which addresses this vulnerability by improving the efficiency of the Sieve ':matches' pattern evaluation and modifying the 'deleteheader' command to reduce processing time. Instructions for upgrading are available in the hMailServer release notes for version 6.3.6.
Metrics
CVSS 4.0 Severity and Vector Strings:
VolerionCVSS 3.x Severity and Vector Strings:
No data available for CVSS Version 2.0 on this CVE.
Volerion
Assessed Oct 8, 2026CISA-ADP
Assessed Oct 8, 2026References to Advisories, Solutions, and Tools
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| URL | Source(s) | Tag(s) |
|---|---|---|
| https://gitlab.com/hmailserver/hmailserver/-/releases/v6.3.6 | [email protected] | Release NotesVendor |
| https://gitlab.com/hmailserver/hmailserver/-/work_items/75 | [email protected] | Issue TrackingTechnical DescriptionVendor |
Weakness Enumeration
| CWE-ID | CWE Name | Source |
|---|---|---|
| CWE-1333 | Inefficient Regular Expression Complexity | [email protected] |
Affected Products
| Product | Versions |
|---|---|
| Progressive Robot hMailServer | >= 6.2.22, <= 6.3.5 (semver) |
CPE
Remediation
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Change History
2 change records found show changes
| Date | Action | Recorded By |
|---|---|---|
| Oct 8, 2026 | CVE Modified | CISA-ADP |
| Oct 8, 2026 | New CVE Received | [email protected] |