Arista EOS'ta beklenmeyen tünel protokolü çözülmesi ve yönlendirme atlatma açığı

On affected platforms running Arista EOS where a tunnel decapsulation configuration—such as VXLAN (Virtual Extensible LAN), decap-groups, or a GRE (Generic Routing Encapsulation) tunnel interface—is present, the switch will incorrectly decapsulate and forward other unexpected tunneled packet with a destination IP matching its configured decapsulation IP. This occurs because the switch does not verify the tunnel protocol type, potentially leading to the unexpected processing of non-configured tunnel traffic.

This issue has been reported as being exploited in the wild.

Üreticinin çözüm notu (İngilizce)

No software upgrade path is planned to address this issue due to the risk of breaking existing configuration on deployments. The recommended resolution of this issue is to follow the appropriate mitigation instructions detailed in the workaround block.

Üreticinin geçici önlemi (İngilizce)

There are two broad approaches to mitigate this issue - (1) applying ACLs on upstream devices or (2) applying ACLs on the devices where the unexpected decapsulation is happening. In both cases, the idea is to either selectively allow only legitimate tunnel traffic or to selectively block malicious tunnel traffic. For example, if a network is configured to forward VXLAN traffic, but GRE traffic is being unexpectedly forwarded, then ACLs can be used to either selectively allow just VXLAN traffic or selectively block GRE traffic. More details about using the ACL feature can be found in the  Arista User Manual https://www.arista.com/en/um-eos/eos-acls-and-route-maps#xx1150869 .

A note of caution, the following ACL-based mitigation recommendations assume that the tunnel IP is dedicated solely to receiving the configured tunnel protocol traffic. When adapting these rules for your environment, it is important to explicitly permit any additional protocol traffic—such as BGP or SSH—if that IP serves multiple functions. To maintain connectivity, ensure these permit statements are sequenced before any deny statements directed at the decapsulation IP.

The following configurations align with the recommendations outlined in the  Arista EOS Hardening Guide https://arista.my.site.com/AristaCommunity/s/article/arista-eos-hardening-guide#Comm_Kna_ka0Uw00000097VJIAY_71 .

Approach 1 - Applying ACL on Upstream Switches

On upstream devices, applying ACLs to allow specific tunneled traffic is straightforward. ACLs can be applied that match on tunnel destination IP, the IP next protocol field, and (optionally) UDP destination port to selectively allow or block specific tunnel protocols.

Example ACLs for Arista EOS follows.

ACL to permit VXLANv4 Only

This IPv4 ACL matches on VXLAN packets as follows:
(a) IP next protocol = UDP (17)
(b) IP DIP = VXLAN VTEP IP
(c) UDP destination port = VXLAN UDP Port (4789)

It allows VXLAN packets and drops all other packets to the VXLAN Decap IP.

ip access-list foo
   counters per-entry
   1 permit udp any host <vxlan-decap-ip> eq 4789
   2 deny ip any host <decap-ip>
   3 permit ip any any

 

ACL to permit GREv4 Only

This IPv4 ACL matches on GRE packets as follows:
(a) IP next protocol = GRE (47)
(b) IP DIP = GRE Tunnel Destination IP

It allows GRE packets and drops all other packets to the GRE Decap IP.

ip access-list foo
   counters per-entry
   1 permit gre any host <gre-decap-ip>
   2 deny ip any host <gre-decap-ip>
   3 permit any any

 

ACL to permit IP-in-IPv4 Only

This IPv4 ACL matches on IP-in-IPv4 packets as follows:
(a) IP next protocol = IPv4 (4) or IPv6 (41)
(b) IP DIP = IP-in-IP Decap IP

It allows IP-in-IPv4 packets and drops all other packets to the IP-in-IPv4 Decap IP.

ip access-list foo
   counters per-entry
   1 permit 4 any host <ipip-decap-ip>
   2 permit 41 any host <ipip-decap-ip>
   3 deny ip any host <ipip-decap-ip>
   4 permit any any

 

ACL to Permit IP-in-IPv6 Only

This IPv6 ACL matches on IP-in-IPv6 packets as follows:
(a) IP next protocol = IPv4 (4) or IPv6 (41)
(b) IP DIP = IP-in-IP Decap IP

It allows IP-in-IPv6 packets and drops all other packets to the IP-in-IPv6 Decap IP.

ipv6 access-list foo
   counters per-entry
   1 permit 4 any host <ipip-decap-ip>
   2 permit 41 any host <ipip-decap-ip>
   3 deny ipv6 any host <ipip-decap-ip>
   4 permit ipv6 any any

 

ACL to permit GUEv4 Only

This IPv4 ACL matches on GUE packets as follows:
(a) IP next protocol = UDP (17)
(b) IP DIP = GUE Decap IP
(c) UDP destination port = UDP port configured per payload
       (IP = Y or MPLS = Z)

It allows GUE packets and drops all other packets to the GUE Decap IP.

ip access-list foo
   counters per-entry
   1 permit udp any host <decap-ip> eq Y
   2 permit udp any host <decap-ip> eq Z
   3 deny ip any host <decap-ip>
   4 permit ip any any

 

ACL to Permit GUEv6 Only

This IPv6 ACL matches on GUE packets as follows:
(a) IP next protocol = UDP (17)
(b) IP DIP = GUE Decap IP
(c) UDP destination port = UDP port configured per payload
       (IP = Y or MPLS = Z)

It allows GUE packets and drops all other packets to the GUE Decap IP.

ipv6 access-list foo
   counters per-entry
   1 permit udp any host <decap-ip> eq Y
   2 permit udp any host <decap-ip> eq Z
   3 deny ipv6 any host <decap-ip>
   4 permit ipv6 any any

Approach 2 - Applying ACL on Decapsulation Switches

Applying ACLs on the decapsulation device is more complicated. It requires the use of MAC ACLs on 7020R Series, 7280R/R2 Series, and 7500R/R2 Series systems and IP ACLs on 7280R3 Series, 7500R3 Series, and 7800R3 Series systems. In both cases, a TCAM profile update is also required. Note that TCAM profile update is a disruptive operation that could impact traffic forwarding. More information can be found in  User-defined TCAM Profiles https://www.arista.com/en/support/toi/eos-4-26-0f/14755-user-defined-tcam-profiles .

7020R Series, 7280R/R2 Series, and 7500R/R2 Series

Mitigation involves using MAC ACLs to allow specific expected protocol packets and block all other traffic to the configured decap IPs. The suggested MAC ACLs use User Defined Fields (UDFs) to match on specific fields in the packet headers. This requires a TCAM profile update to include the following UDF qualifiers:

* For IPv4 tunnels, 2 16b and 1 32b UDF qualifiers need to be included.
* For IPv6 tunnels, 2 16b and 4 32b UDF qualifiers need to be included.

However, in order to make room for the UDF qualifiers, other TCAM features/qualifiers must be removed due to hardware constraints. Following are some suggested TCAM profile changes to accommodate the required UDF qualifiers:

* TCAM profile that includes the UDF qualifiers for IPv4 tunnels, but removes support for MPLS:

hardware tcam
   profile test copy default
      feature acl port mac
         no key size limit 
         key field udf-16b-1 udf-16b-2 udf-32b-1
      no feature mpls
      no feature mpls pop ingress
      no feature pbr mpls

 

 
* TC…

CISA'nın önerdiği eylem (İngilizce)

Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable.