CVE-2025-52948: Juniper Junos Race Condition Vulnerability

CVE-2025-52948 Overview

CVE-2025-52948 is an Improper Handling of Exceptional Conditions vulnerability [CWE-755] in the Berkeley Packet Filter (BPF) subsystem of Juniper Networks Junos OS. An unauthenticated remote attacker sending specific, unknown traffic patterns can trigger a race condition when a BPF instance is cloned. The condition leaks an internal kernel structure and forces the Flexible PIC Concentrator (FPC) and system to crash and restart. The issue is more likely to occur when packet capturing is enabled on the device. Juniper documented this defect in advisory JSA100052 and released fixed builds across all supported Junos OS trains.

Critical Impact
Successful exploitation crashes the FPC and the Junos OS system, causing sustained denial of service across affected routing and switching platforms.

Affected Products

Juniper Junos OS all versions before 21.2R3-S9
Juniper Junos OS 21.4 before 21.4R3-S10, 22.2 before 22.2R3-S6, 22.4 before 22.4R3-S7
Juniper Junos OS 23.2 before 23.2R2-S3, 23.4 before 23.4R2-S3, 24.2 before 24.2R1-S1 and 24.2R2

Discovery Timeline

2025-07-11 - CVE-2025-52948 published to NVD
2026-06-17 - Last updated in NVD database

Technical Details for CVE-2025-52948

Vulnerability Analysis

Berkeley Packet Filter (BPF) provides a raw, protocol-independent interface to data link layers inside the Junos kernel. Packet capture tools such as tcpdump and monitor commands use this interface to inspect traffic. Junos clones BPF instances when new capture consumers attach to an interface. During this clone operation, a timing window exists where the newly created interface can be referenced before its internal structure is fully initialized or released.

When an attacker sends specific traffic patterns that hit this window, the kernel leaks an internal structure and reaches an inconsistent state. The kernel cannot recover from the exceptional condition, and the FPC panics. Because the FPC handles forwarding on Juniper hardware, its restart interrupts all traffic transiting the device.

Root Cause

The root cause is a race condition in the BPF clone path combined with improper handling of the resulting exceptional state [CWE-755]. Junos does not serialize or validate the cloned BPF instance before further packet processing continues. The exact triggering traffic content is indeterminate, but Juniper reproduced the crash multiple times in a lab environment.

Attack Vector

The attack vector is network-based and requires no authentication or user interaction. An attacker sends crafted or otherwise unknown traffic patterns to an interface on a Junos device configured with packet capture. The condition is passive from the attacker's perspective, since exploitation depends on the internal timing of BPF cloning inside the kernel. Devices without packet capturing enabled are substantially less exposed. Refer to the Juniper Support Advisory JSA100052 for the required configuration that increases likelihood.

Detection Methods for CVE-2025-52948

Indicators of Compromise

Unexpected FPC restarts logged as fpc panic or chassisd alarms in system logs.
Kernel crash dumps referencing BPF, bpfclone, or interface cloning code paths.
Repeated traffic interruptions correlated with active packet capture sessions such as monitor traffic or tcpdump.

Detection Strategies

Audit Junos configurations for enabled packet capture features and confirm they are still required.
Correlate FPC restart events with SNMP traps, syslog RPD_, KERNEL_, and CHASSISD_ messages ingested into a centralized log platform.
Review historical crash reports on Juniper support cases for BPF or packet capture references consistent with JSA100052.

Monitoring Recommendations

Forward Junos syslog to a SIEM and alert on FPC restart, kernel panic, and repeated bpf messages.
Baseline normal packet capture activity so unauthorized or unusual capture sessions surface quickly.
Track EPSS (Exploit Prediction Scoring System) and Juniper advisory updates for CVE-2025-52948 to catch changes in exploit availability.

How to Mitigate CVE-2025-52948

Immediate Actions Required

Upgrade Junos OS to a fixed release identified in Juniper Support Advisory JSA100052, such as 21.2R3-S9, 21.4R3-S10, 22.2R3-S6, 22.4R3-S7, 23.2R2-S3, 23.4R2-S3, 24.2R1-S1, or 24.2R2.
Inventory all Juniper devices and match installed versions against the affected list.
Restrict management and transit interfaces so untrusted networks cannot reach devices with packet capture enabled.

Patch Information

Juniper released fixed Junos OS builds for every affected train. Administrators should follow the upgrade procedure in the Juniper Support Advisory JSA100052 and validate the running version with show version after upgrade. Review the Juniper Documentation on Packet Capture Analysis to confirm which configuration stanzas activate BPF cloning.

Workarounds

Disable packet capture on production devices when it is not actively required for troubleshooting.
Limit the duration and scope of monitor traffic and tcpdump sessions during maintenance windows.
Apply infrastructure ACLs and control-plane filters to reduce exposure of BPF-consuming interfaces to untrusted sources.

bash

# Verify running Junos version and disable packet capture stanza
show version
configure
delete forwarding-options packet-capture
commit and-quit