CVE-2026-126623Progressive Robot Ltd hMailServer inefficient algorithmic complexity
CVE-2026-126623 is a inefficient algorithmic complexity in Progressive Robot Ltd hMailServer, rated high (7.5 on CVSS 3.1).
Why this priority: High severity, with no exploitation signal yet.
Published · Updated · NVD status: Received
What it is
Crafted input triggers a worst-case algorithm that ties up the service.
How it is reached
An attacker can exploit it over the network, without an account, and nobody has to click anything.
What an attacker gains
A successful attack can take it offline. In practice, this kind of flaw lets an attacker exhaust CPU, memory or connections until the service stops answering.
Is it being exploited?
There is no public report of exploitation yet.
What to do
- No fixed version is listed yet. Find every place Progressive Robot Ltd hMailServer runs, watch the references for a fix, and plan a replacement if it is no longer supported.
- While there is no fix, keep it off the internet or behind a VPN, allowlist or authenticating proxy.
Description from NVD
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.
What this means for your architecture.
A CVE names a product. Whether it matters depends on where that product sits in your systems: what reaches it, which trust boundary it guards and what it can reach once compromised. Veriom maps your components, their relationships and your trust boundaries from your code, cloud and delivery evidence, so you can see where a component like this sits. The radar itself does not look at your systems.
Components it touches
- Public entry points and shared services that must stay up. The flaw sits between untrusted traffic and shared capacity.
Trust boundaries in play
- Internet-facing entry points. No account is needed. Any instance reachable from outside your perimeter, through a load balancer, gateway, VPN or exposed admin port, is in scope.
- Between untrusted traffic and shared capacity. An attacker who crosses it can exhaust CPU, memory or connections until the service stops answering.
Questions to answer about your own systems
- Where does Progressive Robot Ltd hMailServer run in your systems, and at which version?
- Which of those instances can be reached from the internet or partner networks?
- What can it reach once compromised: which data stores, secrets and services?
Sources
- CVE records and CVSS scores from the NVD CVE API. This product uses the NVD API but is not endorsed or certified by the NVD.
- Exploitation status from CISA’s Known Exploited Vulnerabilities catalog.
- Exploit likelihood from FIRST’s Exploit Prediction Scoring System (EPSS), refreshed daily.
- Explanations are written by Veriom from fixed templates over each record’s data, not by a person reviewing each CVE. Descriptions are quoted from the records as published.