===================================================================                                CERT-Renater

                      Note d'Information No. 2023/VULN310

_____________________________________________________________________

DATE                : 12/09/2023

HARDWARE PLATFORM(S): /

OPERATING SYSTEM(S): Systems running rancher/rke2 (Go) versions prior
                    to 1.28.1+rke2r1, 1.27.5+rke2r1, 1.26.8+rke2r1,
                             1.25.13+rke2r1, 1.24.17+rke2r1.

====================================================================https://github.com/rancher/rke2/security/advisories/GHSA-p45j-vfv5-wprq
_____________________________________________________________________


RKE2 supervisor port is vulnerable to unauthenticated remote
denial-of-service (DoS) attack via TLS SAN stuffing attack

High
cwayne18 published GHSA-p45j-vfv5-wprq Sep 8, 2023

Package
rancher/rke2 (Go)

Affected versions
< 1.24.17+rke2r1
< v1.25.13+rke2r1
< v1.26.8+rke2r1
< v1.27.5+rke2r1
< v1.28.1+rke2r1

Patched versions
1.24.17+rke2r1
v1.25.13+rke2r1
v1.26.8+rke2r1
v1.27.5+rke2r1
v1.28.1+rke2r1


Description

Impact

An issue was found in RKE2 where an attacker with network access
to RKE2 servers' supervisor port (TCP 9345) can force the TLS
server to add entries to the certificate's Subject Alternative
Name (SAN) list, through a stuffing attack, until the certificate
grows so large that it exceeds the maximum size allowed by TLS
client implementations. OpenSSL for example will raise an excessive
message size error when this occurs. No authentication is necessary
to perform this attack, only the ability to perform a TLS handshake
against the supervisor port (TCP 9345).

Affected servers will continue to operate, but clients (server or
agent nodes) will fail to establish new connections when joining
or rejoining the cluster, thus leading to a denial of service
(DoS) attack.


Remediation

Upgrade to a fixed release:

     v1.28.1+rke2r1
     v1.27.5+rke2r1
     v1.26.8+rke2r1
     v1.25.13+rke2r1
     1.24.17+rke2r1

If you are using RKE2 1.27 or earlier, you must also add the parameter
tls-san-security: true to the RKE2 configuration to enable enhanced
security for the supervisor's TLS SAN list. This option defaults to
true starting with RKE2 1.28.

Note that this flag changes the behavior of RKE2's supervisor listener.
You should ensure that you configure node-external-ip on servers that
will be connected to via an external IP, and add tls-san entries for
any load-balancers or VIP addresses that will be associated with the
supervisor port. External IPs and load-balancer/VIP addresses will no
longer be added to the supervisor certificate's SAN list unless
explicitly configured.


Mitigation

If you cannot upgrade to a fixed release, the certificate can be
"frozen" by running the following command against the cluster:

kubectl annotate secret -n kube-system rke2-serving 
listener.cattle.io/static=true

⚠️ IMPORTANT CAUTION: Note that this mitigation will prevent the
certificate from adding new SAN entries when servers join the cluster,
and automatically renewing itself when it is about to expire. If you
do this, you should delete the annotation when adding new servers to
the cluster, or when the certificate is within 90 days of expiring,
so that it can be updated. Once that is done, you can freeze it again.

Affected certificates can be reset by performing the following steps:

     Run kubectl delete secret -n kube-system rke2-serving
     Delete /var/lib/rancher/rke2/server/tls/dynamic-cert.json from
all servers, and restart the rke2-server service.


Background

The RKE2 supervisor listens on port TCP 9345 and uses the
rancher/dynamiclistener library to dynamically generate TLS
certificates that contain TLS Subject Alternative Names (SAN)
for any host name or IP address requested by a client. This is done
to allow servers and external load-balancers to be added to the
cluster without the administrator having to explicitly know and
configure in advance a fixed list of endpoints that the supervisor
may be hosted at.

The library allows the embedding application to configure a
callback that is used to filter addresses requested by clients;
but this was not previously implemented in RKE2.


For more information

If you have any questions or comments about this advisory:

     Reach out to the SUSE Rancher Security team for security related
inquiries.
     Open an issue in the RKE2 repository.
     Verify with our support matrix and product support lifecycle.


Severity
High

7.5/ 10

CVSS base metrics

Attack vector
Network

Attack complexity
Low

Privileges required
None

User interaction
None

Scope
Unchanged

Confidentiality
None

Integrity
None

Availability
High

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

CVE ID
CVE-2023-32186


Weaknesses
No CWEs


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