Juniper Junos OS MX10k TOCTOU in show system firmware (23.4R2-S3)
CVE-2026-21912 Published on January 15, 2026
Junos OS: MX10k Series: 'show system firmware' CLI command may lead to LC480 or LC2101 line card reset
A Time-of-check Time-of-use (TOCTOU) Race Condition vulnerability in the method to collect FPC Ethernet firmware statistics of Juniper Networks Junos OS on MX10k Series allows a local, low-privileged attacker executing the 'show system firmware' CLI command to cause an LC480 or LC2101 line card to reset.
On MX10k Series systems with LC480 or LC2101 line cards, repeated execution of the 'show system firmware' CLI command can cause the line card to crash and restart. Additionally, some time after the line card crashes, chassisd may also crash and restart, generating a core dump.This issue affects Junos OS on MX10k Series:
* all versions before 21.2R3-S10,
* from 21.4 before 21.4R3-S9,
* from 22.2 before 22.2R3-S7,
* from 22.4 before 22.4R3-S6,
* from 23.2 before 23.2R2-S2,
* from 23.4 before 23.4R2-S3,
* from 24.2 before 24.2R2.
Vulnerability Analysis
CVE-2026-21912 is exploitable with local system access, and requires small amount of user privileges. This vulnerability is considered to have a low attack complexity. The potential impact of an exploit of this vulnerability is considered to have no impact on confidentiality and integrity, and a high impact on availability.
Weakness Type
What is a TOCTTOU Vulnerability?
The software checks the state of a resource before using that resource, but the resource's state can change between the check and the use in a way that invalidates the results of the check. This can cause the software to perform invalid actions when the resource is in an unexpected state. This weakness can be security-relevant when an attacker can influence the state of the resource between check and use. This can happen with shared resources such as files, memory, or even variables in multithreaded programs.
CVE-2026-21912 has been classified to as a TOCTTOU vulnerability or weakness.
Products Associated with CVE-2026-21912
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Affected Versions
Juniper Networks Junos OS:- Before 21.2R3-S10 is affected.
- Version 21.4 and below 21.4R3-S9 is affected.
- Version 22.2 and below 22.2R3-S7 is affected.
- Version 22.4 and below 22.4R3-S6 is affected.
- Version 23.2 and below 23.2R2-S2 is affected.
- Version 23.4 and below 23.4R2-S3 is affected.
- Version 24.2 and below 24.2R2 is affected.
Exploit Probability
EPSS (Exploit Prediction Scoring System) scores estimate the probability that a vulnerability will be exploited in the wild within the next 30 days. The percentile shows you how this score compares to all other vulnerabilities.