CVE-2026-103649
HIGHDescription
Missing network timeouts in the Linux builds of Progressive Robot hMailServer 6.3.0 through 6.3.5 allow a remote attacker to hold server threads indefinitely and so stop outbound mail delivery (denial of service). The server set its socket timeouts in the form Windows takes, which Linux refuses, and its HTTPS clients read without a deadline, so a peer that accepts a connection and then sends nothing held the waiting thread for as long as the connection stayed open. The MTA-STS policy fetch, enabled by default, is made during outbound delivery to mta-sts.<recipient domain>, so anyone who can make the server deliver mail to a domain they control - for example as the envelope sender of a message that bounces - can hold delivery threads until outbound delivery stops. The same flaw affects the DANE TLSA query, the OAuth2 token request, the ACME client, and the ManageSieve and metrics listeners, which a silent client stops from serving anyone else. Windows builds are not affected.
How to fix
No published remediation has been found for this vulnerability's affected products yet.
Mitigation guidance may be in the linked vendor advisories in the References section below.
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CVSS v3 Vector
Exploitability
Impact
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Exploit Intelligence
Low risk: more likely to be exploited than 18% of all known CVEs.
References
Related Vulnerabilities
Other CWE-1088 vulnerabilities, ordered by exploit likelihood. View all
| CVE | Severity | CVSS | EPSS | Exploited | Fix |
|---|---|---|---|---|---|
| CVE-2026-89147 | High | 7.5 | 0.5% | - | Fix |
| CVE-2024-8062 | High | 7.5 | 0.5% | - | - |
| CVE-2024-8061 | High | 7.5 | 0.5% | - | - |
| CVE-2024-12777 | Medium | 5.9 | 0.5% | - | - |
| CVE-2020-14483 | Medium | 4.3 | 0.4% | - | - |
| CVE-2026-81520 | High | 7.5 | 0.4% | - | Fix |
Common questions
How do I fix CVE-2026-103649?
No published fix for CVE-2026-103649 has been found yet. Check the linked vendor advisories for guidance as it becomes available.
Is CVE-2026-103649 being actively exploited?
Not that we know of. CVE-2026-103649 is not in the CISA Known Exploited Vulnerabilities catalog. Its EPSS score of 0.27% is the estimated probability that it will be exploited in the next 30 days. That is higher than 18% of all scored CVEs.
How severe is CVE-2026-103649?
CVE-2026-103649 has a CVSS v3 base score of 7.5, rated high. CVSS rates the technical impact if the vulnerability is exploited, not how likely that is, so weigh it alongside the exploit-prediction score when you decide what to patch first.
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This product uses NVD data but is not endorsed or certified by the NVD. EPSS scores courtesy of FIRST.org (https://www.first.org/epss). Source: CISA KEV Catalog. Data as of 2026-10-08.