CVE-2026-102058 in Junos OS
Summary
by MITRE • 10/14/2026
A Reachable Assertion vulnerability in the bfdd daemon of Juniper Networks Junos OS and Junos OS Evolved platforms allows an unauthenticated adjacent attacker to cause a denial of service by restarting the daemon via crafted BFD session traffic.
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Analysis
by VulDB Data Team • 10/14/2026
The identified vulnerability represents a critical security flaw within the Borderless Forwarding Daemon, commonly referred to as bfdd, which is a core component of Juniper Networks Junos OS and Junos OS Evolved platforms. This specific issue is classified as a reachable assertion failure, a type of software defect that occurs when a program attempts to verify an internal invariant or condition that evaluates to false under unexpected circumstances. In the context of network infrastructure daemons, such assertions are typically designed to catch programming errors during development and testing phases by halting execution if assumptions about data integrity or state consistency are violated. However, in production environments, these mechanisms can be exploited maliciously rather than serving their intended debugging purpose. The vulnerability allows an unauthenticated attacker who is adjacent on the network layer to trigger this assertion failure through specially crafted Bidirectional Forwarding Detection session traffic. BFD is a lightweight protocol used for rapid detection of failures in the path between two forwarding engines, and its reliance on simple packet structures makes it susceptible to manipulation if input validation is insufficient.
From a technical perspective, the root cause lies in inadequate sanitization or boundary checking of incoming BPDUs (BFD Protocol Data Units) processed by the bfdd daemon. When an attacker sends malformed packets that violate expected protocol constraints—such as incorrect session states, invalid multiplier values, or unexpected flag configurations—the daemon's internal logic encounters a state it does not anticipate handling gracefully. Instead of returning an error code and continuing operation, the software triggers an assertion failure. In many Unix-like operating systems, including those underlying Junos OS, triggering an assertion results in the immediate termination of the process via a core dump signal. Since bfdd is responsible for maintaining critical adjacency information used by routing protocols like OSPF, IS-IS, and BGP to determine link availability, its sudden restart disrupts these higher-layer dependencies. The daemon must reload its configuration and re-establish sessions with all neighboring routers, leading to temporary loss of connectivity intelligence across the network segment.
The operational impact of this vulnerability is primarily a denial of service against both the affected device and potentially adjacent systems that rely on accurate BFD state information for fast failover mechanisms. When bfdd restarts, there is a window during which it cannot respond to or process new BFD packets from neighbors. During this interval, neighboring routers may interpret the lack of response as a link failure, causing them to withdraw routes associated with the affected interface. This can lead to traffic blackholing, increased latency due to suboptimal path selection, or complete loss of connectivity if redundant paths are also impacted by cascading state changes. Because the attack requires no authentication and only network adjacency, it is particularly dangerous in shared broadcast networks or directly connected point-to-point links where an attacker might have physical or logical access without needing valid credentials. This aligns with common exploitation patterns seen in infrastructure-level attacks where availability is targeted to disrupt business continuity rather than steal data.
This vulnerability maps closely to CWE-617, which describes Reachable Assertion vulnerabilities, indicating that a security-critical assertion can be triggered by external input due to insufficient validation. Furthermore, from the perspective of the MITRE ATT&CK framework, this exploit falls under Tactic TA0004: Privilege Escalation or more accurately in this context, Impact category TI1537: Service Stop, specifically leveraging application layer protocols for denial of service. The attack vector is classified as Network (Network) with Low Complexity and requires no user interaction beyond sending packets to the adjacent interface. To mitigate this risk, administrators should ensure that Juniper Networks OS versions are updated to include patches addressing this specific assertion handling in bfdd. Additionally, implementing strict access control lists on interfaces running BFD can limit exposure by restricting which IP addresses are allowed to send BPDUs to the device. Network segmentation and monitoring for anomalous traffic patterns involving malformed or unusually high volumes of BFD packets can also provide early detection capabilities while patching strategies are deployed across the infrastructure.