CVE-2026-107569
7.5HighPublished 19 Oct 2026 · NVD status: Received · AI triage by gemini-3.8-flash
- Attack vector
- Network
- Complexity
- Low
- Privileges needed
- None
- User interaction
- None
- Impact
- High impact on availability
- Weakness
- CWE-606
Impact
Remote unauthenticated attackers can cause a denial of service by exhausting disk space and mail queues. They only need to send an email with a forged X-hMailServer-LoopCount header that triggers an auto-reply or forward loop.
Root cause
The server fails to sanitize incoming X-hMailServer-LoopCount headers and parses them as signed integers without validating against negative numbers, disabling the loop limiter.
Affected systems
Progressive Robot hMailServer 6.0.0 through 6.3.6
CVSS v3.1 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Fix & remediation
Upgrade to hMailServer 6.3.7. As an interim workaround, configure an upstream mail gateway or rule to strip or reject inbound emails containing the X-hMailServer-LoopCount header.
AI-suggested. Run it in staging first and check it against the vendor advisory. Why?
NVD 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.