[kubernetes] CVE-2021-25748: Ingress-nginx `path` sanitization can be bypassed with newline character

[CJ Cullen <cjcullen () google com>]

Issue Details

A security issue was discovered in ingress-nginx
<https://github.com/kubernetes/ingress-nginx> where a user that can create
or update ingress objects can use a newline character to bypass the
sanitization of the `spec.rules[].http.paths[].path` field of an Ingress
object (in the `networking.k8s.io` or `extensions` API group) to obtain the
credentials of the ingress-nginx controller. In the default configuration,
that credential has access to all secrets in the cluster.

This issue has been rated High (CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:L/A:L
<https://www.first.org/cvss/calculator/3.1#CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:L/A:L>),
and assigned CVE-2021-25748.
Affected Components and Configurations

This bug affects ingress-nginx. If you do not have ingress-nginx installed
on your cluster, you are not affected. You can check this by running
`kubectl get po -n ingress-nginx`.

If you are running the “chrooted” ingress-nginx controller introduced in
v1.2.0 (gcr.io/k8s-staging-ingress-nginx/controller-chroot), you are not
affected.

Multitenant environments where non-admin users have permissions to create
Ingress objects are most affected by this issue.
Affected Versions

   -

   <v1.2.1

Fixed Versions

   -

   v1.2.1

Mitigation

If you are unable to roll out the fix, this vulnerability can be mitigated
by implementing an admission policy that restricts the
`spec.rules[].http.paths[].path` field on the networking.k8s.io/Ingress
resource to known safe characters (see the newly added rules
<https://github.com/kubernetes/ingress-nginx/blame/main/internal/ingress/inspector/rules.go>,
or the suggested value for annotation-value-word-blocklist
<https://kubernetes.github.io/ingress-nginx/user-guide/nginx-configuration/configmap/#annotation-value-word-blocklist>
).
Detection

If you find evidence that this vulnerability has been exploited, please
contact security () kubernetes io
Additional Details

See ingress-nginx Issue #8686
<https://github.com/kubernetes/ingress-nginx/issues/8686> for more details.
Acknowledgements

This vulnerability was reported by Gafnit Amiga.

Thank You,

CJ Cullen on behalf of the Kubernetes Security Response Committee