CVE-2026-73443 in EOS
Summary
by MITRE • 09/16/2026
On affected platforms running Arista EOS with VRRPv2 IP-AH authentication configured, an unauthenticated attacker within the same layer 2 network segment on which VRRP is running can capture a legitimate authenticated VRRP advertisement and replay it indefinitely. Replayed advertisements can be used to advertise stale VRRP state, for example to prevent a backup router from taking over the virtual gateway after the original master has gone down, resulting in a denial of service for hosts using the virtual gateway address.
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Analysis
by VulDB Data Team • 09/17/2026
The vulnerability described involves a critical flaw in the implementation of Virtual Router Redundancy Protocol version 2 (VRRPv2) authentication within Arista EOS when IP Authentication Header mode is utilized. This specific configuration relies on cryptographic integrity checks to ensure that VRRP advertisements originate from authorized routers, thereby preventing unauthorized devices from manipulating the virtual gateway state. However, despite the presence of these security mechanisms, the protocol implementation fails to adequately protect against replay attacks due to a lack of robust sequence number validation or timestamp-based freshness verification in certain operational contexts. This deficiency allows an attacker positioned within the same Layer 2 network segment to intercept legitimate VRRP advertisements sent by active routers and subsequently retransmit them indefinitely without needing valid authentication credentials for new messages, effectively bypassing the intended security controls through passive capture and active replay techniques.
From a technical perspective, this flaw represents a classic failure in message freshness validation within stateful protocols that depend on periodic updates to maintain network topology consistency. VRRP operates by having routers periodically advertise their priority and status as either master or backup for a virtual IP address. When the primary master router fails or becomes unreachable, backups are expected to take over based on these advertisements. The vulnerability exploits the fact that replayed messages retain valid cryptographic signatures from previous legitimate transmissions. Because the receiving systems do not sufficiently validate whether the received advertisement is temporally fresh relative to previously accepted states, they accept stale data as current truth. This aligns with CWE-347, which classifies improper verification of cryptographic signature validity, and specifically relates to CWE-294 regarding acceptance of authentication bypasses through capture-and-replay attacks. The attacker leverages the trust established by valid IPsec Authentication Headers to inject misleading state information into the VRRP election process without triggering security alerts or rejecting the packets based on age or sequence anomalies that are not strictly enforced in this specific configuration scenario.
The operational impact of exploiting this vulnerability is severe, primarily manifesting as a denial of service for end hosts relying on the virtual gateway address for their default route. By replaying advertisements from an original master router that has already failed or been removed from the network, the attacker can prevent backup routers from transitioning to the master state. This results in a situation where traffic destined for external networks is continuously directed toward a non-functional or unreachable primary gateway, causing complete connectivity loss for all devices on the subnet using that virtual IP as their default route. Furthermore, this manipulation could potentially be used to facilitate man-in-the-middle attacks if combined with other techniques, although the immediate and most direct consequence described is service disruption. This behavior corresponds directly to MITRE ATT&CK technique T1498, Network Denial of Service, specifically under sub-techniques involving resource exhaustion or state manipulation within network protocols. The persistence of this stale state ensures that even if legitimate routers attempt to re-establish dominance, the replayed messages may continue to confuse the election process depending on priority values and timing windows configured in the environment.
Mitigation strategies for this vulnerability require a multi-layered approach focusing on protocol hardening and architectural changes. Administrators should immediately review VRRP configurations across affected Arista EOS platforms and consider disabling IP-AH authentication if it is not strictly required, opting instead for simpler text-based or MD5/HMAC-SHA256 keyed hashes which may have different replay protection characteristics depending on the specific implementation version. However, given that cryptographic integrity is often necessary in sensitive environments, the most effective mitigation involves upgrading to a patched software version where the VRRP state machine has been enhanced with strict sequence number checking and timestamp validation to reject stale advertisements regardless of their valid signatures. Additionally, network segmentation can limit the blast radius by ensuring that VRRP domains are isolated from untrusted Layer 2 segments using VLANs or private VLANs, thereby preventing unauthorized devices from accessing the broadcast domain where VRRP advertisements traverse. Implementing dynamic routing protocols with built-in anti-replay mechanisms as an alternative to static gateway redundancy solutions can also provide more resilient failover capabilities that are less susceptible to this specific class of replay-based attacks.