CVE-2026-55996 in Rancher
Summary
by MITRE • 08/05/2026
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.
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Analysis
by VulDB Data Team • 08/05/2026
This vulnerability represents a critical denial-of-service condition affecting Rancher's TLS implementation through the dynamiclistener library. The flaw manifests in how the system handles Server Name Indication (SNI) requests during TLS handshakes, creating an exploitable path for authenticated and unauthenticated attackers alike. The root cause lies in the absence of proper certificate name filtering mechanisms within the cattle-cluster-agent component and the Rancher server itself, which together utilize the dynamiclistener library for serving TLS traffic.
The technical implementation of this vulnerability stems from the dynamiclistener library's behavior of automatically appending any hostname presented via SNI to each serving certificate. This design decision creates a memory leak scenario where certificates grow indefinitely with each unique hostname request, effectively turning what should be a bounded data structure into an uncontrolled expansion. The absence of a Certificate Name filter configuration allows malicious actors to exploit this behavior by sending numerous TLS requests containing distinct hostnames, causing the certificate to accumulate Subject Alternative Names without bounds.
The operational impact of this vulnerability extends beyond simple service disruption to encompass complete system unavailability on affected listeners. As certificates grow larger and exceed implementation-defined message size limits, TLS handshakes fail catastrophically, rendering the entire TLS endpoint unusable. This affects both downstream cluster components and the Rancher server itself, creating a cascading failure that can compromise cluster-wide operations and management capabilities. The vulnerability demonstrates how seemingly innocuous TLS protocol features can be weaponized to create denial-of-service conditions.
This vulnerability aligns with CWE-1278, which addresses improper certificate handling in TLS implementations, and maps to attack patterns within the ATT&CK framework under T1499.1 for Network Denial of Service and T1071.001 for Application Layer Protocol. The exploitation requires minimal privileges and network access within the cluster, making it particularly dangerous in environments where internal network segmentation is insufficient. Organizations should implement certificate name filtering configurations to prevent automatic SNI hostname appending, limit the number of SAN entries per certificate, and monitor TLS endpoint behavior for unusual certificate growth patterns.
Mitigation strategies include configuring explicit CN filters within the dynamiclistener library, implementing maximum certificate size limits, and establishing monitoring protocols to detect abnormal certificate expansion. Organizations should also consider implementing rate limiting on SNI requests to prevent rapid certificate growth and ensure proper certificate lifecycle management. The fix requires modifications to the cattle-cluster-agent component and Rancher server configurations to enforce bounded certificate generation while maintaining legitimate TLS functionality. Security teams should conduct thorough vulnerability assessments of all TLS endpoints using similar libraries and implement automated certificate health monitoring to detect potential exploitation attempts before they cause service disruption.