CVE-2026-57019 in Junos OS
Summary
by MITRE • 07/10/2026
An Improper Validation of Specified Quantity in Input vulnerability in the Packet Forwarding Engine (pfe) of Juniper Networks Junos OS on MX Series allows an unauthenticated, adjacent attacker to cause a Denial-of-Service (DoS).
When a specific packet is received from device in the same broadcast domain, an affected system calculates the packet size incorrectly. This causes further packet processing to fail, which triggers an FPC major error, resulting in a FPC reset impacting traffic until the FPC has automatically recovered.
Affected scenarios are: MAP-T, or non-IP traffic encapsulated in IP (e.g. MPLS over GRE).
When this issue happens the following logs can be observed:
fpc<#> CMError: /fpc/0/pfe/0/cm/0/MQSS(0)/0/MQSS_CMERROR_LI_INT_REG_UNROLL_TAIL_LENGTH_OVF (0x2205eb), scope: pfe, category: functional, severity: major, module: MQSS(0), type: LI: Unroll TAIL length overflow, oc_category: default fpc<#> Performing action reset-fru for error /fpc/0/pfe/0/cm/0/MQSS(0)/0/MQSS_CMERROR_LI_INT_REG_UNROLL_TAIL_LENGTH_OVF (0x2205eb) in module: MQSS(0) with scope: pfe category: functional level: major, oc_category: default
This issue affects Junos OS on MX Series:
* all versions before 23.2R2-S6, * 23.4 versions before 23.4R2-S7, * 24.2 versions before 24.2R2-S4, * 24.4 versions before 24.4R2-S4, * 25.2 versions before 25.2R2.
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Analysis
by VulDB Data Team • 07/10/2026
This vulnerability represents a critical input validation flaw within the Packet Forwarding Engine of Juniper Networks Junos OS running on MX Series devices. The issue manifests as an improper validation of specified quantity in input data, specifically affecting how the system processes packets that are received from adjacent network devices within the same broadcast domain. According to CWE-1037, this type of vulnerability falls under improper validation of specified quantity which can lead to unexpected behavior when processing malformed inputs. The flaw is particularly dangerous because it allows unauthenticated attackers to exploit the vulnerability from directly connected networks, making it a significant concern for network infrastructure security.
The technical mechanism behind this vulnerability involves incorrect packet size calculation when specific packets are received through the Packet Forwarding Engine. When such packets are processed, the system fails to properly validate the quantity specifications in the input data, leading to an overflow condition in the MQSS (Multi-Queue Switching Subsystem) component. This overflow causes the FPC (Flexible Packet Core) to trigger a major error condition that results in automatic FPC reset operations. The vulnerability specifically impacts MAP-T (Mapping of Address and Port for TCP) and non-IP traffic encapsulated in IP protocols such as MPLS over GRE, which are common networking technologies that require proper packet handling mechanisms.
The operational impact of this vulnerability is severe as it can cause complete denial-of-service conditions for network traffic passing through the affected FPC modules. When the FPC resets occur, all traffic processing on that module ceases until the system automatically recovers, potentially disrupting network connectivity for extended periods depending on recovery times. The error logs indicate a specific CMError related to LI (Line Interface) unroll tail length overflow within the MQSS subsystem, which is consistent with buffer overflow conditions that can trigger system instability and component resets. This type of attack aligns with ATT&CK technique T1498.001 which describes Denial of Service through resource exhaustion or system instability.
Network administrators should prioritize immediate patching of affected systems to mitigate this vulnerability. The vulnerability affects multiple Junos OS versions including pre-23.2R2-S6, pre-23.4R2-S7, pre-24.2R2-S4, pre-24.4R2-S4, and pre-25.2R2 releases on MX Series devices. Given the nature of the vulnerability and its potential for causing network disruption, organizations should implement temporary network segmentation measures to limit adjacent network access where possible. Additionally, monitoring for the specific error messages mentioned in the logs should be implemented to detect exploitation attempts. The vulnerability demonstrates how input validation flaws in core network processing components can lead to system-wide availability issues, emphasizing the importance of robust validation mechanisms in network infrastructure software as defined by security standards such as those outlined in ISO/IEC 27001 for information security management.