# CVE-2026-109786: Business logic in fail2ban

> Remote attackers can ban arbitrary IP addresses by manipulating SSH login usernames, causing denial of service for legitimate users.

- **Severity:** Medium 5.3 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 13 Oct 2026
- **Updated:** 15 Oct 2026
- **Also known as:** DEBIAN-CVE-2026-109786, GHSA-xw36-3g63-5wfw, UBUNTU-CVE-2026-109786

## What is CVE-2026-109786?

CVE-2026-109786 is a medium-severity vulnerability in fail2ban, published on 13 Oct 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

In fail2ban 0.10.5 through 1.1.1 before ddd879d, the sshd filter (config/filter.d/sshd.conf) accepts an "on <token>" suffix after the host address. Because OpenSSH logs the offered username verbatim, a remote unauthenticated attacker can use a username such as "x VICTIM-SOURCE-IP-ADDRESS on" to make fail2ban associate failed login attempts with an arbitrary source IP address and ban it (e.g., to block logins from a legitimate user). The default filter mode is affected.

Weakness: CWE-117 (Improper Output Neutralization for Logs)

## Which versions are affected by CVE-2026-109786?


## How do I fix CVE-2026-109786?

No fix has been published for CVE-2026-109786 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-109786 being exploited?

There is no known exploitation of CVE-2026-109786 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-109786 and fix it if it is affected.

What it is: CVE-2026-109786 is a medium-severity vulnerability in fail2ban, published on 13 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-2026-109786 in the wild and no public exploit code.

Affected product: fail2ban. 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-2026-109786/ (Markdown: https://insights.sbomify.com/CVE-2026-109786/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-109786
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-109786
- OSV: https://osv.dev/vulnerability/CVE-2026-109786

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-109786/
Summaries are generated and may contain mistakes; the official description above is the record.