Cilium's CIDR deny policies may not take effect when a more narrow CIDR allow is present

Impact

A policy rule denying a prefix that is broader than /32 may be ignored if there is

• A policy rule referencing a more narrow prefix (CIDRSet or toFQDN) and
• This narrower policy rule specifies either enableDefaultDeny: false or - toEntities: all

Note that a rule specifying toEntities: world or toEntities: 0.0.0.0/0 is insufficient, it must be to entity all.

As an example, given the below policies, traffic is allowed to 1.1.1.2, when it should be denied:

apiVersion: cilium.io/v2
kind: CiliumClusterwideNetworkPolicy
metadata:
  name: block-scary-range
spec:
  endpointSelector: {}
  egressDeny:
  - toCIDRSet:
    - cidr: 1.0.0.0/8

---

apiVersion: cilium.io/v2
kind: CiliumNetworkPolicy
metadata:
  name: evade-deny
spec:
  endpointSelector: {}
  egress:
  - toCIDR:
    - 1.1.1.2/32
  - toEntities:
    - all

Patches

This issue affects:

• Cilium v1.14 between v1.14.0 and v1.14.15 inclusive
• Cilium v1.15 between v1.15.0 and v1.15.9 inclusive

This issue has been patched in:

• Cilium v1.14.16
• Cilium v1.15.10

Workarounds

Users with policies using enableDefaultDeny: false can work around this issue by removing this configuration option and explicitly defining any allow rules required.

No workaround is available to users with egress policies that explicitly specify toEntities: all.

Acknowledgements

The Cilium community has worked together with members of Isovalent to prepare these mitigations. Special thanks to @squeed, @christarazi, and @jrajahalme for their work in triaging and resolving this issue.

For more information

If you have any questions or comments about this advisory, please reach out on Slack.

If you think you have found a vulnerability affecting Cilium, we strongly encourage you to report it to our security mailing list at security@cilium.io. This is a private mailing list for the Cilium security team, and your report will be treated with top priority.