Unchecked Input for Loop Condition in hMailServer
Summary
| CVE | CVE-2026-107569 |
|---|---|
| State | PUBLISHED |
| Assigner | GitLab |
| Source Priority | CVE Program / NVD first with legacy fallback |
| Published | 2026-10-19 16:17:01 UTC |
| Updated | 2026-10-19 21:14:14 UTC |
| Description | Unchecked input for a loop condition in Progressive Robot hMailServer 6.0.0 through 6.3.6 allows a remote unauthenticated attacker to stop the server carrying mail. The server bounds the mail it generates in response to a message - a rule forward, reply or copy, an account forward, a Sieve redirect, vacation or notify, a bounce, an out-of-office reply - by a counter it carries in the message's X-hMailServer-LoopCount header, and makes no more once the counter reaches the configured rule loop limit (5 by default); that is what ends a forwarding or auto-reply cycle in a bounded number of hops. X-hMailServer-LoopCount is an ordinary header that is not stripped from mail arriving from outside, and the server read it with a signed conversion and no validation, so a count below zero passed every test against the limit: a sender who sets X-hMailServer-LoopCount to a negative value, or on the Linux builds to a value above the 32-bit signed range that the conversion wraps to a negative one, lifts the bound, and a message that enters a generated-mail cycle then loops up to about two thousand million times, each hop a queued message and a delivery, filling the queue and disk and stopping mail service. The Sieve notify action (from 6.2.22) consulted no limit at all, and its increment of the count could overflow a signed integer; and an account could reset the counter on its own mail through a Sieve editheader of the header (from 6.2.22) or a rule Set header value action (all versions) ahead of a forward or redirect. |
Risk And Classification
Primary CVSS: v3.1 7.5 HIGH from [email protected]
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Problem Types: CWE-606 | CWE-606 CWE-606: Unchecked Input for Loop Condition
| Version | Source | Type | Score | Severity | Vector |
|---|---|---|---|---|---|
| 3.1 | [email protected] | Secondary | 7.5 | HIGH | CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H |
| 3.1 | CNA | CVSS | 7.5 | HIGH | CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H |
CVSS v3.1 Breakdown
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Vendor Declared Affected Products
| Source | Vendor | Product | Version | Platforms |
|---|---|---|---|---|
| CNA | Progressive Robot Ltd | HMailServer | affected 6.0.0 6.3.6 semver | Not specified |
References
| Reference | Source | Link | Tags |
|---|---|---|---|
| gitlab.com/hmailserver/hmailserver/-/releases/v6.3.7 | [email protected] | gitlab.com | |
| gitlab.com/hmailserver/hmailserver/-/work_items/81 | [email protected] | gitlab.com | |
| CVE Program record | CVE.ORG | www.cve.org | canonical |
| NVD vulnerability detail | NVD | nvd.nist.gov | canonical, analysis |
Vendor Comments And Credit
Discovery Credit
CNA: Found in the hMailServer project's own security review (Progressive Robot Ltd) (en)
Additional Advisory Data
Solutions
CNA: Upgrade to hMailServer 6.3.7, which reads X-hMailServer-LoopCount in one validated place (a count only when it is digits and never above the limit; anything else is read as the limit so no generated mail is made from it), holds a Sieve notify to the same limit, and refuses an account Sieve script or rule that edits the counter, and writes a line break in a header value as spaces unless it folds the line, so that no value a generated message is built from writes a field of its own. Until then there is no setting that stops the header being read or that turns Sieve off: reduce forwarding and auto-reply arrangements that can feed one another, keep the Sieve script surfaces closed to accounts (ManageSieveServerPort=0, the default, and the REST API off where it is not needed, RestApiPort=0, the default), review active Sieve scripts and account rules for a notify of the account's own address and for an edit of X-hMailServer-LoopCount, and watch the queue for a message generated far past the loop limit.