High · CVSS 7.5CVE-2026-107569

Unchecked Input for Loop Condition in hMailServerCVE-2026-107569

Answer in brief

CVE-2026-107569 records a High severity (CVSS 7.5) vulnerability in Unchecked Input for Loop Condition in hMailServer. The current sources do not mark it as known exploited. The current feed maps Progressive Robot Ltd/hMailServer (generic). Check affected ranges and fixed versions before updating.

Analysis pending evidence review

HOL Guard separates source facts from reviewed analysis. See the methodology.

Published Oct 19, 2026Updated Oct 19, 2026Source checked Oct 19, 2026First seen by HOL Oct 19, 2026Material review Oct 19, 2026

Upstream Advisory

Key facts

Risk: High · CVSS 7.5
Exploitation: Not marked as known exploited
Affected software: 1 mapped package or product
Fix availability: Not reported

Why this deserves its current priority

CVSS is 7.5. The current sources do not mark it as known exploited. Treat this as a source-backed prioritization signal, not a statement about your environment.

Analysis status

Analysis pending evidence review

Factual feed record only; HOL analysis is not approved for indexing. Read the methodology.

Affected scope and exposure questions

The current feed maps Progressive Robot Ltd/hMailServer (generic). Check affected ranges and fixed versions before updating.

Mapped affected packages and fixed versions
Package	Affected range	Fixed version
Progressive Robot Ltd/hMailServergeneric	>=6.0.0 <=6.3.6	Not reported

Recommended response

Check inventory. Check lockfiles and deployed manifests for Progressive Robot Ltd/hMailServer.
Review the reported fix. Source-reported solution: 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.

Evidence timeline and material changes

Published upstream
Oct 19, 2026
Evidence: source:cvelist:source_dates:source-dates:record
Source modified
Oct 19, 2026
Evidence: source:cvelist:source_dates:source-dates:record
First seen by HOL
Oct 19, 2026

Sources and claim methodology

Upstream source 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.

Quoted source text, attributed separately from HOL analysis.

Package

Affected range

Fixed version

Progressive Robot Ltd/hMailServergeneric

>=6.0.0 <=6.3.6

Not reported

Recommended response

Check inventory. Check lockfiles and deployed manifests for Progressive Robot Ltd/hMailServer.

Review the reported fix. Source-reported solution: 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.

Sources and claim methodology

Upstream source description

Quoted source text, attributed separately from HOL analysis.