CVE-2026-107572 Inefficient Regular Expression Complexity in hMailServer
An authenticated user can cause a denial of service in hMailServer by creating a Sieve filter with inefficient regex patterns that exhaust the delivery thread pool.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- None listed
What is CVE-2026-107572?
CVE-2026-107572 is a medium-severity denial of service vulnerability in Progressive Robot Ltd hMailServer, published on 8 Oct 2026. Any signed-in user can exploit it over the network, with no action from the victim.
Official description
Inefficient complexity in the Sieve filter evaluation of Progressive Robot hMailServer 6.2.24 through 6.3.5 allows an authenticated account holder to make the mail services unavailable with their own filter. A ':matches' pattern was matched by a backtracking descent whose time grew with the matched value's length raised to the number of wildcards, so a pattern such as '*a*a*a*b' over 800 characters took 44 seconds; and 'deleteheader' erased the fields it removed one at a time, so removing many fields of one name over a message's header cost O(N^2) (80,000 fields took 18.8 seconds). Sieve filters run on the small, shared delivery thread pool, so an account holder whose active script does this, fed a few messages they can send themselves, empties the pool and stops delivery for the whole server. The script is the account holder's own and cannot be set for another user.
Weakness: CWE-1333
Which versions are affected by CVE-2026-107572?
How do I fix CVE-2026-107572?
No fix has been published for CVE-2026-107572 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-2026-107572 being exploited?
There is no known exploitation of CVE-2026-107572 in the wild and no public exploit code.
Sources
Also known as GHSA-cqgg-3g6j-wqjp.