CVE-2026-57027: Juniper EX4100/EX4400 Memory Leak DoS in Virtual Chassis sFlow

Technical summary

CVE-2026-57027 is a CWE-401 (Missing Release of Memory after Effective Lifetime) vulnerability affecting Juniper Junos OS on EX4100 and EX4400 Series devices. The defect manifests in the packet forwarding engine (pfe) when sFlow telemetry is active within a Virtual Chassis topology. Multicast packets originating from one VC member and egressing through another trigger a memory allocation that is never deallocated. Over time, the buffer pool referenced by 'show chassis fpc' output grows monotonically until FPC memory exhaustion forces a line card restart. The vulnerability requires adjacent network access but no authentication or special configuration beyond having sFlow and Virtual Chassis already deployed.

Business impact

Organizations running EX4100 or EX4400 Series switches as Virtual Chassis clusters with sFlow enabled face unplanned downtime of their switching fabric. Repeated crashes of forwarding processors disrupt packet flows, terminate active sessions, and force manual intervention to restore service. In critical environments such as data center core, service provider backbone, or high-availability deployments, this denial-of-service vector poses significant operational risk. The attack surface includes any adjacent network segment capable of transmitting multicast traffic—potentially spanning Layer 2 domains within the customer network or connected peering points.

Affected systems

Juniper EX4100 Series (including EX4100-F, EX4100-H, and EX4100-H-12T variants) and EX4400 Series switches running Junos OS versions prior to: 23.2R2-S7, 23.4R2-S7, 24.2R2-S4, or 24.4R2. The vulnerability only manifests when both sFlow and Virtual Chassis are operationally configured. Single-chassis deployments or deployments without sFlow telemetry are not affected.

Exploitability

Exploitability is straightforward for an adjacent attacker with network access. No authentication is required, no complex preconditions beyond sFlow and VC enablement, and the attack simply involves sourcing multicast traffic that traverses VC member boundaries. The CVSS vector (AV:A/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H) reflects adjacent-only access but low complexity. The barrier to attack is primarily network segmentation; if multicast can reach the switch pair, the DoS is achievable.

Remediation

Upgrade Junos OS to a patched release: 23.2R2-S7 or later for the 23.2 branch, 23.4R2-S7 or later for 23.4, 24.2R2-S4 or later for 24.2, or 24.4R2 or later for 24.4. Interim mitigation options include disabling sFlow on affected Virtual Chassis clusters until patching is feasible, or restricting multicast source addresses via access control lists to reduce exposure. Verify patch availability against Juniper's official security advisory before scheduling maintenance.

Patch guidance

Consult Juniper's security advisory for CVE-2026-57027 to confirm exact patch versions and release notes specific to your EX model and current Junos release train. Test patches in a non-production Virtual Chassis environment first to validate multicast forwarding and sFlow functionality post-upgrade. Schedule upgrades during a maintenance window, as they typically require FPC resets. If you are not yet on a patched 23.x, 24.2, or 24.4 branch, plan an interim mitigation such as sFlow disable until a stable upgrade path is available.

Detection guidance

Monitor the output of 'show chassis fpc' on each Virtual Chassis member, tracking buffer utilization metrics over time. A sustained upward trend in buffer consumption, particularly if correlated with multicast traffic flow across VC members, may indicate the leak. Set alerts on FPC restarts or line card failures that lack a clear environmental cause. Correlate sFlow configuration and VC membership with unexplained system instability. Network-based detection is difficult; focus on operational telemetry and system logs noting crash details.

Why prioritize this

This vulnerability merits priority patching because it directly impacts availability in environments already deploying Virtual Chassis for high availability and sFlow for operational visibility—precisely the deployments least tolerant of unplanned downtime. The attack vector is low-friction (adjacent network, no authentication), and the denial-of-service outcome is severe and sustained. Medium CVSS severity may understate business impact in critical infrastructure or carrier-grade networks. Organizations with multiple EX4100/EX4400 clusters should treat this as high-priority in their vulnerability management queue.

Risk score, explained

The CVSS 3.1 score of 6.5 (MEDIUM) reflects a remote attacker unable to read or modify data, combined with high impact on availability and a relatively short attack chain (adjacent access, low complexity, no authentication). However, contextual risk is higher: the specificity of EX4100/EX4400 hardware, the requirement for two particular configuration features (sFlow and VC), and the likelihood that affected organizations depend on these systems for core switching justify aggressive patching even though the CVSS rating is 'MEDIUM'. The vulnerability's reliability and lack of user interaction elevate real-world exploitability.

Frequently asked questions