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CVE-2026-57031 details a flaw in Juniper Networks Junos OS on MX Series.
The vulnerability lies in the packet forwarding engine (PFE).
Affected line cards: MPC10/11, LC4800/9600, and MX304.
The issue occurs with subscribers on static interfaces.
Ingress firewall filters are not enforced correctly.
This allows adjacent subscribers to bypass configured filters.
Neither protocol-level filtering nor upstream bandwidth limits apply.
The PFE improperly checks for unusual or exceptional conditions.
Under specific states, the filter lookup is entirely skipped.
Traffic that should be dropped or limited passes through freely.
The attack vector is adjacent, requiring local network access.
No privileges or user interaction are needed for exploitation.
The CVSS base score is 4.7, categorized as MEDIUM.
The vector is CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:C/C:N/I:L/A:N.
Scope is changed, affecting resources beyond the vulnerable component.
Integrity impact is low, but combined with others can be critical.
Affected versions include 23.2R2-S1 through pre-23.2R2-S7.
Also 23.4R2 to before 23.4R2-S7 are vulnerable.
Versions 24.2 before 24.2R2-S3 are affected.
Versions 24.4 before 24.4R2-S2 are affected.
Versions 25.2 before 25.2R2 are affected.
The root cause is a race condition in PFE’s state machine.
When a packet arrives, PFE checks its internal filter table.
An exceptional condition triggers a bypass path in code.
This bypass path omits the filter application routine.
Consequently, ACLs and policers are not evaluated.
Adjacent attackers can send ICMP, UDP, or TCP freely.
Bandwidth policies cannot throttle malicious traffic.
Juniper has released fixed versions to address this.
Administrators must upgrade to the specified releases.
DailyCVE Form:
Platform: Juniper MX Series
Version: Juniper Junos 23.2-25.2
Vulnerability: PFE Firewall Bypass
Severity: Medium CVSS 4.7
date: 09 July 2026
Prediction: Fixed versions released
What Undercode Say:
Analytics:
Bash commands to check system status and filter configuration:
show version | match "junos" show configuration firewall show interfaces <interface-name> | match "filter" show pfe filter statistics <filter-name> show log messages | match "PFE.filter"
To verify if a specific interface has an applied firewall filter:
show configuration interfaces <interface-name> | match "filter"
To check the PFE state for potential bypass conditions (debug only):
show pfe state <fpc-slot> | match "filter.exception"
Exploit: (Educational Purposes!)
An adjacent subscriber can send traffic to a protected destination that is normally blocked by an ingress firewall filter. Since the PFE skips filter evaluation, packets are forwarded as if no policy exists.
Example using hping3 from a subscriber network segment:
hping3 -S -p 443 -c 10 <restricted-server-ip>
Alternatively, a Scapy script to send crafted UDP packets:
from scapy.all import send(IP(dst="<restricted-server-ip>")/UDP(dport=53), count=20)
These packets would be dropped if the filter were enforced, but due to the vulnerability they reach the target, confirming the bypass.
Protection: from this CVE
Upgrade Junos OS to the following fixed releases: 23.2R2-S7, 23.4R2-S7, 24.2R2-S3, 24.4R2-S2, or 25.2R2. If immediate upgrade is not feasible, consider migrating subscribers from static to dynamic interfaces (where the vulnerability does not manifest) as a temporary workaround, though this is not officially confirmed by Juniper.
Impact:
Adjacent subscribers can completely bypass ingress firewall filters, allowing unauthorized traffic to traverse the MX Series router. This leads to violation of security policies, unchecked bandwidth consumption, potential degradation of upstream services, and increased risk of further exploitation when combined with other vulnerabilities.
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Sources:
Reported By: nvd.nist.gov
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