Inefficient Algorithmic Complexity in hMailServer

MITRE NVD CVE.ORG JSON API

Summary

CVECVE-2026-107580
StatePUBLISHED
AssignerGitLab
Source PriorityCVE Program / NVD first with legacy fallback
Published2026-10-08 12:17:16 UTC
Updated2026-10-08 12:17:16 UTC
DescriptionInefficient algorithmic complexity in the decoding of message header fields in Progressive Robot hMailServer 6.0.0 through 6.3.5 allows a remote unauthenticated attacker to make the IMAP, SMTP and POP3 services, or the webmail, unavailable by sending a message. The server unfolded a decoded header value by finding each line break from the end of the value and removing it, moving the rest of the value each time, so its work grew with the square of the number of line breaks, and an RFC 2047 encoded word may decode to any number of them. A received message whose Subject or other header field holds such a value keeps a worker thread busy for minutes or longer each time the value is read: when the recipient's IMAP client searches, sorts or threads the folder by a header, when the webmail lists the folder, and, where configured, when a rule tests a header, a spam tag is added to the Subject or an abuse report is read during delivery. The IMAP worker threads are shared with SMTP and POP3, so a few such messages stop those services responding. The server's reading of a message header from its file also searched everything read so far after each 4,000 bytes, costing seconds for a header of tens of megabytes.

Risk And Classification

Primary CVSS: v3.1 6.5 MEDIUM from [email protected]

CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H

Problem Types: CWE-407 | CWE-407 CWE-407: Inefficient Algorithmic Complexity

VersionSourceTypeScoreSeverityVector
3.1[email protected]Secondary6.5MEDIUMCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H
3.1CNACVSS6.5MEDIUMCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H

CVSS v3.1 Breakdown

Attack Vector
Network
Attack Complexity
Low
Privileges Required
None
User Interaction
Required
Scope
Unchanged
Confidentiality
None
Integrity
None
Availability
High

CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H

Vendor Declared Affected Products

SourceVendorProductVersionPlatforms
CNA Progressive Robot Ltd HMailServer affected 6.0.0 6.3.6 semver Not specified

References

ReferenceSourceLinkTags
gitlab.com/hmailserver/hmailserver/-/releases/v6.3.6 [email protected] gitlab.com
gitlab.com/hmailserver/hmailserver/-/work_items/69 [email protected] gitlab.com
CVE Program record CVE.ORG www.cve.org canonical
NVD vulnerability detail NVD nvd.nist.gov canonical, analysis

Vendor Comments And Credit

Discovery Credit

CNA: Found in the hMailServer project's own security review (Progressive Robot Ltd) (en)

Additional Advisory Data

Solutions

CNA: Upgrade to hMailServer 6.3.6, which unfolds a header value in one pass and searches a header being read only in what each read adds. Until then: lower the maximum message size, which bounds the cost (it grows with the square of the value's size); remove such a message over POP3; or keep the REST listener off (RestApiPort 0, the default).

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