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.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- None listed
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
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.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2026-55996 being exploited?
There is no known exploitation of CVE-2026-55996 in the wild and no public exploit code.
Sources
Also known as GHSA-9jxv-832x-45q9.