CVE-2026-126621Progressive Robot Ltd hMailServer improper certificate validation
CVE-2026-126621 is a improper certificate validation 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
TLS certificates are not checked properly, so an attacker on the path can pose as the server.
How it is reached
An attacker can exploit it over the network, with an ordinary user account, and nobody has to click anything. Success also depends on conditions the attacker does not fully control.
What an attacker gains
A successful attack can read the data it protects, change data or behaviour and take it offline. In practice, this kind of flaw lets an attacker read or tamper with traffic, or pass a forged message as genuine.
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, review who holds accounts that can reach it.
Description from NVD
Progressive Robot hMailServer 6.0.0-B1 through 6.3.7 on Windows hands OpenSSL the names of its own TLS files - the listeners' certificates, chains, private keys and Diffie-Hellman parameters, a port's client-certificate CA file (from 6.2.19), the object store's CA file (from 6.3.4) and the ACME client's keys and certificates - as Latin-1 bytes, which on code page 1252 are the code page's own for U+00A0 to U+00FF, and OpenSSL's file opening on Windows reads a name as UTF-8 first and opens the file at that reading when it is there, or creates it there when its folder is. When every byte of a configured name outside ASCII also reads as UTF-8 - for example U+00C3 U+00A9, the bytes C3 A9, which are U+00E9 in UTF-8 - the server used the file at that other name whenever one was there, which allows a local attacker who can create that file or its folder but cannot change the configured one, as any user can by default beside a file an administrator keeps under ProgramData, to have a port that logs on by client certificate (from 6.2.25) accept certificates from a certificate authority of the attacker's own and so sign in over the network as any mailbox without its password; to have the server trust such an authority for its object store and, intercepting that connection, read and modify the messages it stores; to receive the private keys the ACME client writes, the key of a certificate issued for the server's names among them; or to keep TLS ports from starting. The configuration is not the default: by default these files have names in ASCII, client certificates are off on every port, and the object store and ACME are off. The Linux builds are not affected.
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
- TLS termination and service-to-service channels. The flaw sits on the network path between clients and services.
- Signature checks. The flaw sits on the network path between clients and services.
Trust boundaries in play
- Authenticated network access. An account is needed, so exposure follows who can sign in: customers, partners or staff.
- On the network path between clients and services. An attacker who crosses it can read or tamper with traffic, or pass a forged message as genuine.
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.