Source: Juniper Networks Security Bulletin JSA104294
Vendor: Juniper Networks
Severity: Medium (CVSS v3.1: 6.5 | CVSS v4.0: 7.1)
Published: 14 January 2026
Juniper Networks has disclosed a medium-severity vulnerability in Junos OS that can result in a DoS condition when a device processes a specifically malformed ICMPv4 packet.
This post summarises the issue and highlights operational considerations for network operators from a traffic visibility and monitoring perspective.
Summary of the vulnerability
According to Juniper, an Improper Handling of Exceptional Conditions issue in Junos OS packet processing allows an unauthenticated, network-adjacent attacker to trigger a restart of the Flexible PIC Concentrator (FPC).
The vulnerability is triggered by a specifically malformed ICMPv4 packet containing a crafted IP header value. When received, the FPC processing the packet may crash and restart, leading to temporary traffic disruption.
Key points:
- Juniper SIRT reports no known malicious exploitation at the time of publication
- Impact is limited to availability (DoS)
- No confidentiality or integrity impact
- ICMPv6 is not affected
Attack surface and scope
Juniper notes that due to the malformed nature of the packet, upstream routers would not forward it, limiting exploitation to network-adjacent environments.
This restricts potential attackers to:
- Devices on the same Layer 2 segment
- Adjacent network peers
- Potentially compromised neighbouring infrastructure
While this reduces internet-wide exposure, the issue may still be relevant in IXPs, data centres, private interconnects, and shared network environments.
Operational visibility considerations
Although this issue cannot be mitigated purely at the traffic level, operators may observe symptoms that correlate with exploitation or accidental triggering, including:
- Unexpected FPC restarts
- Short-lived traffic loss or blackholing
- Sudden availability drops without configuration changes
- ICMPv4 traffic anomalies immediately preceding control plane disruption
Network traffic visibility and monitoring tools can help:
- Correlate ICMP activity with device instability
- Support faster root-cause analysis during incidents
- Distinguish between software defects, malformed traffic, and intentional abuse
This can be especially useful in environments where multiple adjacent parties share network segments.
Recommendations for network operators
- Identify Junos OS versions running on edge and core devices
- Plan upgrades to patched releases as part of regular maintenance
- Monitor for unexplained FPC restarts and availability events
- Review ICMP handling policies, particularly in shared or exposed environments
Juniper reports no available workaround; upgrading to a fixed release is the only remediation.
Disclaimer
This document is provided for informational purposes only to the FastNetMon community. It is based on publicly available information published by Juniper Networks.
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