# CVE-2026-55996: Unauthenticated Denial-of-Service via TLS SAN Stuffing in Rancher and cattle-cluster-agent

> An unauthenticated attacker can cause denial of service in Rancher by stuffing TLS certificates with excessive SANs if CN filtering is not configured.

- **Severity:** Medium 4.3 (`CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 5 Aug 2026
- **Updated:** 5 Aug 2026
- **Also known as:** GHSA-9jxv-832x-45q9

## What is CVE-2026-55996?

CVE-2026-55996 is a medium-severity denial of service vulnerability in SUSE Rancher, published on 5 Aug 2026. Any signed-in user can exploit it over the network, with no action from the victim.

### Official description

A denial-of-service vulnerability was identified in multiple TLS listeners in Rancher. Both the cattle-cluster-agent component running in downstream clusters and the Rancher server itself use the dynamiclistener library to serve TLS traffic. Without an effective CN filter configured, dynamiclistener automatically appended to each serving certificate any hostname presented via Server Name Indication (SNI) in incoming TLS requests.

An unauthenticated attacker with network access within the affected cluster could send a large number of TLS requests with distinct hostnames, causing the serving certificate to accumulate an unbounded number of Subject Alternative Names (SANs). Eventually, the certificate grows large enough that TLS handshakes fail with an excessive message size error, causing a denial of service on the affected listeners.

Weakness: CWE-770 (Allocation of Resources Without Limits or Throttling)

## Which versions are affected by CVE-2026-55996?


## How do I fix CVE-2026-55996?

No fix has been published for CVE-2026-55996 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-55996 being exploited?

There is no known exploitation of CVE-2026-55996 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-55996 and fix it if it is affected.

What it is: CVE-2026-55996 is a medium-severity denial of service vulnerability in SUSE Rancher, published on 5 Aug 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-2026-55996 in the wild and no public exploit code.

Affected product: SUSE Rancher. 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-2026-55996/ (Markdown: https://insights.sbomify.com/CVE-2026-55996/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-55996
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-55996
- OSV: https://osv.dev/vulnerability/CVE-2026-55996

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-55996/
Summaries are generated and may contain mistakes; the official description above is the record.