[Security Advisory] CVE-2021-25737: Holes in EndpointSlice Validation Enable Host Network Hijack

Announcements
1

A security issue was discovered in Kubernetes where a user may be able to redirect pod traffic to private networks on a Node. Kubernetes already prevents creation of Endpoint IPs in the localhost or link-local range, but the same validation was not performed on EndpointSlice IPs.

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This issue has been rated Low (CVSS:3.0/AV:N/AC:L/PR:H/UI:N/S:U/C:L/I:N/A:N), and assigned CVE-2021-25737.

### Affected Component###

kube-apiserver

### Affected Versions###

  • v1.21.0

  • v1.20.0 - v1.20.6

  • v1.19.0 - v1.19.10

  • v1.16.0 - v1.18.18 (Note: EndpointSlices were not enabled by default in 1.16-1.18)

### Fixed Versions###

This issue is fixed in the following versions:

  • v1.21.1

  • v1.20.7

  • v1.19.11

  • v1.18.19

### Mitigation###

To mitigate this vulnerability without upgrading kube-apiserver, you can create a validating admission webhook that prevents EndpointSlices with endpoint addresses in the 127.0.0.0/8 and 169.254.0.0/16 ranges. If you have an existing admission policy mechanism (like OPA Gatekeeper) you can create a policy that enforces this restriction.

### Detection###

To detect whether this vulnerability has been exploited, you can list EndpointSlices and check for endpoint addresses in the 127.0.0.0/8 and 169.254.0.0/16 ranges.

If you find evidence that this vulnerability has been exploited, please contact security@kubernetes.io

### Additional Details###

See Kubernetes Issue #102106 for more details.

Acknowledgements###

This vulnerability was reported by John Howard of Google.

Thank You,

CJ Cullen on behalf of the Kubernetes Product Security Committee