# CVE-2026-103649: Synchronous Access of Remote Resource without Timeout in hMailServer

> Remote attackers can cause denial of service in hMailServer Linux builds by holding server threads indefinitely via missing network timeouts during outbound mail delivery.

- **Severity:** High 7.5 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 8 Oct 2026
- **Updated:** 8 Oct 2026
- **Also known as:** GHSA-69m7-r4p2-rfcq

## What is CVE-2026-103649?

CVE-2026-103649 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

Missing network timeouts in the Linux builds of Progressive Robot hMailServer 6.3.0 through 6.3.5 allow a remote attacker to hold server threads indefinitely and so stop outbound mail delivery (denial of service). The server set its socket timeouts in the form Windows takes, which Linux refuses, and its HTTPS clients read without a deadline, so a peer that accepts a connection and then sends nothing held the waiting thread for as long as the connection stayed open. The MTA-STS policy fetch, enabled by default, is made during outbound delivery to mta-sts.<recipient domain>, so anyone who can make the server deliver mail to a domain they control - for example as the envelope sender of a message that bounces - can hold delivery threads until outbound delivery stops. The same flaw affects the DANE TLSA query, the OAuth2 token request, the ACME client, and the ManageSieve and metrics listeners, which a silent client stops from serving anyone else. Windows builds are not affected.

Weakness: CWE-1088 (Synchronous Access of Remote Resource without Timeout)

## Which versions are affected by CVE-2026-103649?


## How do I fix CVE-2026-103649?

No fix has been published for CVE-2026-103649 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-103649 being exploited?

There is no known exploitation of CVE-2026-103649 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-103649 and fix it if it is affected.

What it is: CVE-2026-103649 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.
Exploitation: There is no known exploitation of CVE-2026-103649 in the wild and no public exploit code.

Affected product: Progressive Robot Ltd hMailServer. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-103649/ (Markdown: https://insights.sbomify.com/CVE-2026-103649/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-103649
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-103649
- OSV: https://osv.dev/vulnerability/CVE-2026-103649

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-103649/
Summaries are generated and may contain mistakes; the official description above is the record.