# CVE-2026-59843: Libssh: libssh: denial of service via zero advertised channel packet size

> A remote authenticated peer can cause a denial of service in libssh by advertising a zero maximum packet size, leading to an infinite loop and CPU exhaustion.

- **Severity:** Medium 6.5 (`CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 21 Jul 2026
- **Updated:** 1 Sep 2026
- **Also known as:** AZL-93243, BELL-CVE-2026-59843, DEBIAN-CVE-2026-59843, ECHO-41e0-1521-8b3d, GHSA-4846-7xwg-pw5m, UBUNTU-CVE-2026-59843

## What is CVE-2026-59843?

CVE-2026-59843 is a medium-severity denial of service vulnerability in Redhat Enterprise Linux, published on 21 Jul 2026. Any signed-in user can exploit it over the network, with no action from the victim.

### Official description

A flaw was found in libssh. A remote authenticated peer can advertise a zero maximum packet size in SSH_MSG_CHANNEL_OPEN, causing later channel writes to loop indefinitely and consume CPU, leading to denial of service.

Weakness: CWE-400 (Uncontrolled Resource Consumption), CWE-835 (Loop with Unreachable Exit Condition)

## Which versions are affected by CVE-2026-59843?

It affects 3 products.


Products: Libssh Libssh, Redhat Enterprise Linux, Redhat Hardened Images.

## How do I fix CVE-2026-59843?

Apply the vendor's patch.

## Is CVE-2026-59843 being exploited?

There is no known exploitation of CVE-2026-59843 in the wild and no public exploit code.

## Other vulnerabilities in Redhat Enterprise Linux

- [CVE-2026-59851](https://insights.sbomify.com/CVE-2026-59851/): Libssh: libssh: authentication bypass via missing gssapi principal check (high 8.8)
- [CVE-2025-5987](https://insights.sbomify.com/CVE-2025-5987/): Libssh: invalid return code for chacha20 poly1305 with openssl backend (high 8.1)
- [CVE-2025-14821](https://insights.sbomify.com/CVE-2025-14821/): Libssh: libssh: insecure default configuration leads to local man-in-the-middle attacks on windows (high 7.8)
- [CVE-2012-4560](https://insights.sbomify.com/CVE-2012-4560/): Memory corruption in n/a (high 7.5)
- [CVE-2012-4562](https://insights.sbomify.com/CVE-2012-4562/): Integer overflow in n/a (high 7.5)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-59843 and fix it if it is affected.

What it is: CVE-2026-59843 is a medium-severity denial of service vulnerability in Redhat Enterprise Linux, published on 21 Jul 2026. Any signed-in user can exploit it over the network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-59843 in the wild and no public exploit code.

Affected product: Redhat Enterprise Linux. 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-2026-59843/ (Markdown: https://insights.sbomify.com/CVE-2026-59843/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-59843
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-59843
- OSV: https://osv.dev/vulnerability/CVE-2026-59843

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-59843/
Summaries are generated and may contain mistakes; the official description above is the record.