CVE-2023-32187 — k3s
HIGHCVE-2023-32187 is a high-severity (CVSS 7.5) CWE-770 vulnerability in github.com/k3s-io/k3s. A fix is available for github.com/k3s-io/k3s — see the affected versions and patch details below.
K3s apiserver port is vulnerable to unauthenticated remote denial-of-service (DoS) attack via TLS SAN stuffing attack
Exploitation Status
No confirmed exploitation observed yet
- CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
- CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.
Exploitation and automatability from CISA’s SSVC triage for CVE-2023-32187.
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
Where this sits among everything scored
Real-World Exposure
github.com/k3s-io/k3s🐹github.com/k3s-io/k3s🐹github.com/k3s-io/k3s🐹github.com/k3s-io/k3s🐹github.com/k3s-io/k3sReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects Go packages — download data is not available via public APIs for these ecosystems.
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:
- Run
kubectl --server https://localhost:6444 delete secret -n kube-system k3s-serving - Delete
/var/lib/rancher/k3s/server/tls/dynamic-cert.jsonfrom all servers, and restart thek3sservice.
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:
- Reach out to the K3s Security team for security related inquiries.
- Open an issue in the K3s repository.
- Verify with our support matrix and product support lifecycle.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐹Go | github.com/k3s-io/k3s | all versions | 1.24.17go get github.com/k3s-io/k3s@v1.24.17 |
| 🐹Go | github.com/k3s-io/k3s | ≥ 1.25.0&&< 1.25.13 | 1.25.13go get github.com/k3s-io/k3s@v1.25.13 |
| 🐹Go | github.com/k3s-io/k3s | ≥ 1.26.0&&< 1.26.8 | 1.26.8go get github.com/k3s-io/k3s@v1.26.8 |
| 🐹Go | github.com/k3s-io/k3s | ≥ 1.27.0&&< 1.27.5 | 1.27.5go get github.com/k3s-io/k3s@v1.27.5 |
| 🐹Go | github.com/k3s-io/k3s | ≥ 1.28.0&&< 1.28.1 | 1.28.1go get github.com/k3s-io/k3s@v1.28.1 |
Affected Products
k3sk3sDetection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/k3s-io/k3s, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update github.com/k3s-io/k3s to 1.24.17 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2023-32187 is resolved across your whole dependency graph.
Workarounds
Cap what an attacker can consume: apply request size, rate and timeout limits in front of the affected component, and run it with memory and CPU limits so exhaustion degrades one worker rather than the whole service.
Frequently Asked Questions
Is CVE-2023-32187 in your dependencies?
Find it across Go, including transitive dependencies.