VulnerabilityPublished 11 Nov 2026

CVE-2026-126623: Progressive Robot hMailServer · Inefficient Algorithmic Complexity

Affects Progressive Robot hMailServerSource CVE Database V5

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Mentioned vulnerabilities

CVE-2026-107574

Not known to be exploited · CVSS 7.5 High.

Data as of 11 Nov 2026, 08:49 UTC

CVSS 3.1 base score

7.5High

How severe the flaw is on the 0–10 CVSS scale.

Thin evidence

One scorer or none

CVSS scores by source
CNA (GitLab)	CVSS 3.1	7.5

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

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

Bold values are the worst case: easiest to exploit or largest impact.

Attack Vector: Network: The vulnerable component is bound to the network stack and the set of possible attackers extends beyond the other options listed, up to and including the entire Internet. Such a vulnerability is often termed 'remotely exploitable' and can be thought of as an attack being exploitable at the protocol level one or more network hops away (e.g., across one or more routers).
Attack Complexity: Low: Specialized access conditions or extenuating circumstances do not exist. An attacker can expect repeatable success against the vulnerable component.
Privileges Required: None: The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files to carry out an attack.
User Interaction: None: The vulnerable system can be exploited without any interaction from any user.
Scope: Unchanged: An exploited vulnerability can only affect resources managed by the same security authority. In this case, the vulnerable component and the impacted component are either the same, or both are managed by the same security authority.
Confidentiality: None: There is no loss of confidentiality within the impacted component.
Integrity: None: There is no loss of integrity within the impacted component.
Availability: High: There is total loss of availability, resulting in the attacker being able to fully deny access to resources in the impacted component; this loss is either sustained (while the attacker continues to deliver the attack) or persistent (the condition persists even after the attack has completed). Alternatively, the attacker has the ability to deny some availability, but the loss of availability presents a direct, serious consequence to the impacted component (e.g., the attacker cannot disrupt existing connections, but can prevent new connections; the attacker can repeatedly exploit a vulnerability that, in each instance of a successful attack, leaks a only small amount of memory, but after repeated exploitation causes a service to become completely unavailable).

Definitions: FIRST CVSS v3.1 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 11 Nov 2026
Detectable: no evidence on record.
No public nuclei template indexed
Weaponized: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: no evidence on record.
Not in CISA KEV and no source reports exploitation
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 10 Nov 2026.

Disclosed: The CVE record is public on cve.org. Its publication is t = 0.
Detectable: A public nuclei template can find exposed instances, for defenders and attackers alike.
Weaponized: A Metasploit exploit module or Exploit-DB entry exists (Metasploit scanner modules do not count), or CISA SSVC or the CVSS exploit maturity reports a public proof of concept.
Exploited in the wild: CISA KEV lists it, CISA SSVC rates exploitation active, or a source reports attacks.
Observed by OffSeq Mirage: OffSeq Mirage honeypot sensors recorded attack traffic for this CVE, or found exposed hosts without attack traffic.

Moments are spaced evenly, not to scale; a dashed line marks a gap of six months or more. Deltas count from t = 0. Sources that give only a date (CISA KEV, Exploit-DB, or a time of exactly 00:00Z) read “same day” rather than hours. Metasploit modules carry the flaw’s disclosure date, not their release date, and nuclei templates carry no date, so both appear above but not on the timeline.

What to do

Affected software

Product	Affected	Fixed in
Progressive Robot LtdhMailServer	Affected >=6.3.6 <=6.3.7	Fixed in–

AI-suggested

No explicit patch or fix is stated in the provided data. Since the affected versions are 6.3.6 and 6.3.7, users should upgrade to versions later than 6.3.7 once available. Until then, administrators should consider disabling the REST API if not required, limit or monitor incoming TLS-RPT reports, and restrict access to webmail and API endpoints to trusted users to reduce exposure.

Description

CVE-2026-126623 is a high-severity vulnerability in Progressive Robot Ltd's hMailServer versions 6.3.6 and 6.3.7. It involves inefficient algorithmic complexity in the JSON reader, which can be exploited remotely by unauthenticated attackers to hold mail delivery threads or REST API worker threads. This results in denial of service by making mail delivery, the REST API, and webmail unavailable. The issue arises from the JSON reader's hash set implementation, which can be forced to degrade performance quadratically with specially crafted JSON objects up to 16 MB in size.

Impact

A remote unauthenticated attacker can cause denial of service by exhausting mail delivery threads and REST API worker threads in hMailServer 6.3.6 and 6.3.7. This results in mail delivery, REST API, and webmail becoming unavailable. The attack leverages inefficient JSON parsing that degrades performance significantly when processing specially crafted large JSON objects, effectively blocking server resources.

Summary

The vulnerability CVE-2026-126623 affects hMailServer 6.3.6 and 6.3.7 due to inefficient algorithmic complexity in the JSON reader component. The JSON reader uses a hash set based on the standard library's hash function without a secret, allowing crafted JSON member names to collide in one bucket. This causes the time to parse the JSON object to grow quadratically with the number of members. An attacker can send a large JSON object (up to 16 MB decompressed) as a TLS-RPT report to a hosted domain's report mailbox or exploit the REST API and webmail interfaces to consume mail delivery threads or REST API worker threads. This leads to denial of service by blocking mail delivery and API responsiveness. The vulnerability was introduced in version 6.3.6, which fixed a prior related issue (CVE-2026-107574) but did not mitigate this complexity attack. The CVSS v3.1 score is 7.5 (high severity) with network attack vector, low attack complexity, no privileges or user interaction required, and impact limited to availability.

CWE-407Inefficient Algorithmic Complexity (opens in a new tab)
Class weaknessLikelihood of exploit: Low
An algorithm in a product has an inefficient worst-case computational complexity that may be detrimental to system performance and can be triggered by an attacker, typically using crafted manipulations that ensure that the worst case is being reached.
OWASP Cheat Sheets
File Upload (opens in a new tab)
OWASP ASVS 4.0.3
V12.1.1 (opens in a new tab)
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).

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Exploitation evidence

What to do

Affected software

Description

AI analysisMachine-generated, updated 11 Nov 2026, 08:49 UTC

WeaknessesCWE-407

Technical details

References

Community reviews

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