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K3s apiserver port is vulnerable to unauthenticated remote denial-of-service (DoS) attack via TLS SAN stuffing attack

High
cwayne18 published GHSA-m4hf-6vgr-75r2 Sep 8, 2023

Package

gomod k3s-io/k3s (Go)

Affected versions

< v1.24.17+k3s1
< v1.25.13+k3s1
< v1.26.8+k3s1
< v1.27.5+k3s1
< v1.28.1+k3s1

Patched versions

v1.24.17+k3s1
v1.25.13+k3s1
v1.26.8+k3s1
v1.27.5+k3s1
v1.28.1+k3s1

Description

Impact

An issue was found in K3s where an attacker with network access to K3s servers' apiserver/supervisor port (TCP 6443) can force the TLS server to add entries to the certificate's Subject Alternative Name (SAN) list, through a stuffing attack, until the certificate grows so large that it exceeds the maximum size allowed by TLS client implementations. OpenSSL for example will raise an excessive message size error when this occurs. No authentication is necessary to perform this attack, only the ability to perform a TLS handshake against the apiserver/supervisor port (TCP 6443).

Affected servers will continue to operate, but clients (including both external administrative access with kubectl and server or agent nodes) will fail to establish new connections, thus leading to a denial of service (DoS) attack.

Remediation

Upgrade to a fixed release:

  • v1.28.1+k3s1
  • v1.27.5+k3s1
  • v1.26.8+k3s1
  • v1.25.13+k3s1
  • v1.24.17+k3s1

If you are using K3s 1.27 or earlier, you must also add the parameter tls-san-security: true to the K3s configuration to enable enhanced security for the supervisor's TLS SAN list. This option defaults to true starting with K3s 1.28.

Note that this flag changes the behavior of K3s servers. You should ensure that you configure node-external-ip on servers that will be connected to via an external IP, and add tls-san entries for any load-balancers or VIP addresses that will be associated with the supervisor port. External IPs and load-balancer/VIP addresses will no longer be added to the supervisor certificate's SAN list unless explicitly configured.

Mitigation

If you cannot upgrade to a fixed release, the certificate can be "frozen" by running the following command against the cluster:

kubectl annotate secret -n kube-system k3s-serving listener.cattle.io/static=true

⚠️ IMPORTANT CAUTION: Note that this mitigation will prevent the certificate from adding new SAN entries when servers join the cluster, and automatically renewing itself when it is about to expire. If you do this, you should delete the annotation when adding new servers to the cluster, or when the certificate is within 90 days of expiring, so that it can be updated. Once that is done, you can freeze it again.

Affected certificates can be reset by performing the following steps:

  1. Run kubectl --server https://localhost:6444 delete secret -n kube-system k3s-serving
  2. Delete /var/lib/rancher/k3s/server/tls/dynamic-cert.json from all servers, and restart the k3s service.

Background

The K3s apiserver/supervisor listener on port TCP 6443 and uses the rancher/dynamiclistener library to dynamically generate TLS certificates that contain TLS Subject Alternative Names (SAN) for any host name or IP address requested by a client. This is done to allow servers and external load-balancers to be added to the cluster without the administrator having to explicitly know and configure in advance a fixed list of endpoints that the supervisor may be hosted at.

The library allows the embedding application to configure a callback that is used to filter addresses requested by clients; but this was not previously implemented in K3s.

For more information

If you have any questions or comments about this advisory:

Severity

High

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v3 base metrics

Attack vector
Network
Attack complexity
Low
Privileges required
None
User interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
None
Availability
High

CVSS v3 base metrics

Attack vector: More severe the more the remote (logically and physically) an attacker can be in order to exploit the vulnerability.
Attack complexity: More severe for the least complex attacks.
Privileges required: More severe if no privileges are required.
User interaction: More severe when no user interaction is required.
Scope: More severe when a scope change occurs, e.g. one vulnerable component impacts resources in components beyond its security scope.
Confidentiality: More severe when loss of data confidentiality is highest, measuring the level of data access available to an unauthorized user.
Integrity: More severe when loss of data integrity is the highest, measuring the consequence of data modification possible by an unauthorized user.
Availability: More severe when the loss of impacted component availability is highest.
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

CVE ID

CVE-2023-32187

Weaknesses

No CWEs