CVE-2026-126623 - Vulnerability Details

- Inefficient Algorithmic Complexity in hMailServer

Description

Inefficient algorithmic complexity in the JSON reader of Progressive Robot hMailServer 6.3.6 and 6.3.7 allows a remote attacker to hold the server's mail delivery threads or its REST API's worker threads, making mail delivery, the REST API and the webmail unavailable. 6.3.6 fixed CVE-2026-107574, in which reading a JSON object searched the member names already read for each new name, by keeping the names in a hash set; the set uses the standard library's hash, which takes no secret, so member names chosen to fall in one bucket of it still cost time growing with the square of their number. Against the released reader, one such object of 16 MB took about 5 seconds to read in the Linux build, and one of under 6 MB about 23 seconds in the Windows build, where objects of the same sizes with other names took under a tenth of a second. A remote unauthenticated attacker reaches the reader by mailing such an object as a TLS-RPT report, up to 16 MB once decompressed, to a hosted domain's report mailbox, where an administrator has set one; it is read on a mail delivery thread (ten by default), and those threads deliver all mail, local and outbound, so a stream of such reports stops all delivery for as long as it continues. It is also reached without an account through the reply to a password-protected message, with the message's link alone, where the REST API is on (it is off by default) and a public base URL for such links is set; and by any account through the webmail's routes granted 16 MB, and by an application's API key through the send API. Each such request holds one of the REST API's four worker threads, in the process that also serves SMTP, IMAP and POP3.

Published: 2026-11-11

Score: 7.5 High

EPSS: n/a

KEV: No

Impact:

Action:

Analysis

Analysis and contextual insights are available on OpenCVE Cloud.

Default status is the baseline for the product, each version can override it (e.g. patched versions marked unaffected).

Vendor Product Default status Versions

Progressive Robot Ltd

hMailServer

unaffected

Version	Status	Constraints
`6.3.6`	affected	≤ 6.3.7

No data.

No data available yet.

Remediation

Vendor Solution

Upgrade to hMailServer 6.3.8, whose JSON reader keeps an object's member names in an ordered set, whose cost does not depend on which names are chosen, and holds a REST request body and a mailed TLS-RPT report to 10,000 members in an object; the reply to a password-protected message is held to 2 MB. Until then, clear each domain's report mailbox, or move it off the domains the server hosts, so that no report is read by the server; and switch OpenPGP in the webmail off (portal_enabled under /api/v1/settings/openpgp), and off for any domain or account that holds openpgp_portal_enabled on, which refuses the reply to a password-protected message before its body is parsed; clearing the public base URL is not enough, since links already made stay usable. For the routes that take an account or an API key, only limiting who can reach the REST API avoids it (RestApiPort 0, the default, turns it off), or a reverse proxy in front of it that holds request bodies well below 16 MB. Restarting the service ends a busy thread.

Tracking

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Advisories

No advisories yet.

No CVSS v4.0

Attack Vector Network

Attack Complexity Low

Privileges Required None

Scope Unchanged

Confidentiality Impact None

Integrity Impact None

Availability Impact High

User Interaction None

No CVSS v3.0

No CVSS v2

This CVE is not in the KEV list.

No EPSS score available.

Key SSVC decision points have not yet been added.

References

Link	Providers
https://gitlab.com/hmailserver/hmailserver/-/releases/v6.3.8
https://gitlab.com/hmailserver/hmailserver/-/work_items/111
https://www.cve.org/CVERecord?id=CVE-2026-107574

History

Wed, 11 Nov 2026 08:45:00 +0000

Type	Values Removed	Values Added
Description		Inefficient algorithmic complexity in the JSON reader of Progressive Robot hMailServer 6.3.6 and 6.3.7 allows a remote attacker to hold the server's mail delivery threads or its REST API's worker threads, making mail delivery, the REST API and the webmail unavailable. 6.3.6 fixed CVE-2026-107574, in which reading a JSON object searched the member names already read for each new name, by keeping the names in a hash set; the set uses the standard library's hash, which takes no secret, so member names chosen to fall in one bucket of it still cost time growing with the square of their number. Against the released reader, one such object of 16 MB took about 5 seconds to read in the Linux build, and one of under 6 MB about 23 seconds in the Windows build, where objects of the same sizes with other names took under a tenth of a second. A remote unauthenticated attacker reaches the reader by mailing such an object as a TLS-RPT report, up to 16 MB once decompressed, to a hosted domain's report mailbox, where an administrator has set one; it is read on a mail delivery thread (ten by default), and those threads deliver all mail, local and outbound, so a stream of such reports stops all delivery for as long as it continues. It is also reached without an account through the reply to a password-protected message, with the message's link alone, where the REST API is on (it is off by default) and a public base URL for such links is set; and by any account through the webmail's routes granted 16 MB, and by an application's API key through the send API. Each such request holds one of the REST API's four worker threads, in the process that also serves SMTP, IMAP and POP3.
Title		Inefficient Algorithmic Complexity in hMailServer
Weaknesses		CWE-407
References		https://gitlab.com/hmailserver/hmailserver/-/releases/v6.3.8 https://gitlab.com/hmailserver/hmailserver/-/work_items/111 https://www.cve.org/CVERecord?id=CVE-2026-107574
Metrics		cvssV3_1 `{'score': 7.5, 'vector': 'CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H'}`