This vulnerability in Juniper Networks Junos OS on MX Series arises from improper validation of packet size in the Packet Forwarding Engine when processing specific packets from devices in the same broadcast domain. The affected system miscalculates packet size, causing packet processing to fail and triggering a major error in the Flexible PIC Concentrator (FPC). This error results in an FPC reset, causing a temporary denial of service impacting traffic until the FPC recovers automatically. The vulnerability affects traffic scenarios including MAP-T and non-IP traffic encapsulated in IP such as MPLS over GRE. Logs indicating the issue include MQSS_CMERROR_LI_INT_REG_UNROLL_TAIL_LENGTH_OVF errors and subsequent FPC resets. Affected versions include all Junos OS MX Series 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, and 25.2 versions before 25.2R2.

An unauthenticated, adjacent attacker can cause a denial-of-service condition by sending crafted packets that trigger incorrect packet size calculation in the Packet Forwarding Engine. This leads to packet processing failure, a major FPC error, and an FPC reset, disrupting traffic until the FPC recovers. The vulnerability does not impact confidentiality or integrity but causes availability degradation.

Patch status is not yet confirmed — check the vendor advisory for current remediation guidance. No official fix or temporary workaround is stated in the provided data. Monitor Juniper Networks advisories for updates and apply patches once available. Until patched, consider limiting exposure to adjacent attackers in the same broadcast domain if feasible.