CVE-2026-103649 — Synchronous Access of Remote Resource without Timeout in hMailServer
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 outboun…
Published: 2026-10-08 · CVSS: 7.5 · CVSS 3.1
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.
Source: CVE Program · Revision 8b16575c39f1e1810c5e863010da073d2d3285e7