CVE-2026-107579: Inefficient Algorithmic Complexity in hMailServer
Inefficient algorithmic complexity in the bounce and complaint processing of Progressive Robot hMailServer 6.3.4 and 6.3.5 allows a remote unauthenticated attacker to stop mail delivery by sending messages, when bounce processing or complaint processing is enabled or a mailing list is managed by the server (none is by default). The readers of incoming delivery status notifications (RFC 3464) and abuse feedback reports (RFC 5965) removed the blank lines at the start of the returned headers part two bytes at a time, copying the rest of the part each time, so their work grew with the square of the number of blank lines. A message shaped like such a report, whose headers part begins with a very large number of blank lines within the reader's 2 MB limit, keeps a delivery thread busy for over a minute while it is delivered, and a few such messages a minute keep every delivery thread busy.
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
Progressive Robot hMailServerto a version that resolves this vulnerability.Fixed in 6.3.6 - Configuration
Until upgrading, turn bounce processing, complaint processing, and list management off.
hMailServer bounce processing, complaint processing, and list management = off
Event History
Frequently Asked Questions
Which deployments are exposed to this denial-of-service condition?
hMailServer 6.3.4 and 6.3.5 are exposed when bounce processing or complaint processing is enabled, or when the server manages a mailing list. None of these conditions is enabled by default.
What does an attacker need to exploit it?
An attacker can be remote and unauthenticated. They need to send specially shaped messages resembling delivery status notifications or abuse feedback reports, with a very large number of leading blank lines in the headers part within the reader's 2 MB limit.
What is the operational impact of an attack?
A crafted message can occupy a delivery thread for more than a minute while it is delivered. A few such messages per minute can keep all delivery threads busy and stop mail delivery.
What can be done if an immediate upgrade is not possible?
Disable bounce processing and complaint processing, and avoid managing mailing lists on the server, where operationally feasible. These are the affected processing paths and are not enabled by default.