CVE-2026-107580
Description
Inefficient algorithmic complexity in the decoding of message header fields in Progressive Robot hMailServer 6.0.0 through 6.3.5 allows a remote unauthenticated attacker to make the IMAP, SMTP and POP3 services, or the webmail, unavailable by sending a message. The server unfolded a decoded header value by finding each line break from the end of the value and removing it, moving the rest of the value each time, so its work grew with the square of the number of line breaks, and an RFC 2047 encoded word may decode to any number of them. A received message whose Subject or other header field holds such a value keeps a worker thread busy for minutes or longer each time the value is read: when the recipient's IMAP client searches, sorts or threads the folder by a header, when the webmail lists the folder, and, where configured, when a rule tests a header, a spam tag is added to the Subject or an abuse report is read during delivery. The IMAP worker threads are shared with SMTP and POP3, so a few such messages stop those services responding. The server's reading of a message header from its file also searched everything read so far after each 4,000 bytes, costing seconds for a header of tens of megabytes.
Predictions
Heuristic predictions, AS-IS, for prioritization only.
Mitigations
No mitigations published for this CVE yet.
The vendor-content worker queues fetches as references arrive (check back in a few minutes). Or — if you've already worked around this in production — publish your fix to the community-verified tier.
Propose a mitigation on Community Mitigations published via the community go through AI scoring + 2 human reviewers + 7-day silent objection window before landing here withsource_tier=community-verified.
References
CWEs
CWE-407
Community-verified mitigations for this CVE will appear above when contributors publish them.
Verify integrity in audit chain (admin only). AS-IS.