# CVE-2026-107580: Inefficient Algorithmic Complexity in hMailServer

> Remote unauthenticated attackers can cause denial of service in hMailServer by sending messages with complex headers that exhaust worker threads.

- **Severity:** Medium 6.5 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:R/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:** A user has to click or open something
- **Fix available:** No
- **Published:** 8 Oct 2026
- **Updated:** 8 Oct 2026
- **Also known as:** GHSA-6877-qwxc-rh5c

## What is CVE-2026-107580?

CVE-2026-107580 is a medium-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, and only if a user opens a file or follows a link.

### Official description

Inefficient algorithmic complexity in the decoding of message header fields in Progressive Robot hMailServer 6.0.0 through 6.3.5 allows a remote unauthenticated attacker to make the IMAP, SMTP and POP3 services, or the webmail, unavailable by sending a message. The server unfolded a decoded header value by finding each line break from the end of the value and removing it, moving the rest of the value each time, so its work grew with the square of the number of line breaks, and an RFC 2047 encoded word may decode to any number of them. A received message whose Subject or other header field holds such a value keeps a worker thread busy for minutes or longer each time the value is read: when the recipient's IMAP client searches, sorts or threads the folder by a header, when the webmail lists the folder, and, where configured, when a rule tests a header, a spam tag is added to the Subject or an abuse report is read during delivery. The IMAP worker threads are shared with SMTP and POP3, so a few such messages stop those services responding. The server's reading of a message header from its file also searched everything read so far after each 4,000 bytes, costing seconds for a header of tens of megabytes.

Weakness: CWE-407 (Inefficient Algorithmic Complexity)

## Which versions are affected by CVE-2026-107580?


## How do I fix CVE-2026-107580?

No fix has been published for CVE-2026-107580 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-107580 being exploited?

There is no known exploitation of CVE-2026-107580 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-107580 and fix it if it is affected.

What it is: CVE-2026-107580 is a medium-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, and only if a user opens a file or follows a link.
Exploitation: There is no known exploitation of CVE-2026-107580 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-2026-107580/ (Markdown: https://insights.sbomify.com/CVE-2026-107580/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-107580
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-107580
- OSV: https://osv.dev/vulnerability/CVE-2026-107580

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-107580/
Summaries are generated and may contain mistakes; the official description above is the record.