CVE-2026-107576 Inefficient Algorithmic Complexity in hMailServer
Remote attackers can cause denial of service in hMailServer by sending crafted emails with complex DKIM/ARC headers, exhausting server resources.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- None listed
What is CVE-2026-107576?
CVE-2026-107576 is a high-severity denial of service vulnerability in Progressive Robot Ltd hMailServer, published on 8 Oct 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
Inefficient algorithmic complexity in the inbound DKIM and ARC signature verification of Progressive Robot hMailServer 6.0.0 through 6.3.5 allows a remote unauthenticated attacker to make the mail services unavailable by sending a message. Building the canonical header and choosing the header fields named in a signature's h= tag took time growing with the square of the message's header: the 'simple' canonicalisation prepended each continuation line of a folded field to the lines already gathered, and both canonicalisations searched the gathered fields from the bottom for each h= name and erased the match from the middle of the list. A message whose header holds very many fields, or a field folded over very many lines, with a DKIM-Signature the attacker signs for a domain they control, keeps a worker thread busy for tens of seconds per signature; up to ten signatures are evaluated per message by each of the DKIM and DMARC tests, on the threads that serve delivery and SMTP.
Weakness: CWE-407
Which versions are affected by CVE-2026-107576?
How do I fix CVE-2026-107576?
No fix has been published for CVE-2026-107576 yet. Limit exposure of the affected software until one is available.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2026-107576 being exploited?
There is no known exploitation of CVE-2026-107576 in the wild and no public exploit code.
Sources
Also known as GHSA-6pj8-9c42-v5rp.