DoS via unbounded SAN growth in Rancher TLS listeners
CVE-2026-55996 Published on August 5, 2026
Unauthenticated Denial-of-Service via TLS SAN Stuffing in Rancher and cattle-cluster-agent
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.
Vulnerability Analysis
CVE-2026-55996 is exploitable with network access, and requires small amount of user privileges. This vulnerability is considered to have a low attack complexity. The potential impact of an exploit of this vulnerability is considered to have no impact on confidentiality and integrity, and a small impact on availability.
Weakness Type
Allocation of Resources Without Limits or Throttling
The software allocates a reusable resource or group of resources on behalf of an actor without imposing any restrictions on the size or number of resources that can be allocated, in violation of the intended security policy for that actor.
Products Associated with CVE-2026-55996
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Affected Versions
SUSE Rancher:- Version 2.11.0 and below 2.11.16 is affected.
- Version 2.12.0 and below 2.12.12 is affected.
- Version 2.13.0 and below 2.13.8 is affected.
- Version 2.14.0 and below 2.14.4 is affected.