VulnerabilityPublished 15 Jan 2026

CVE-2026-21912: Juniper Networks Junos OS · Time-of-check Time-of-use (TOCTOU) Race Condition

Affects Juniper Networks Junos OSSource CVE Database V5

Not known to be exploited · CVSS 5.5 Medium · fixed in 24.2R2 : update Juniper Networks Junos OS when convenient.

Data as of 26 Oct 2026, 12:29 UTC

CVSS 3.1 base score

5.5Medium

How severe the flaw is on the 0–10 CVSS scale.

EPSS: 0.1% chance of exploitation in 30 days – higher than 1% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
Patch: Available – fixed in 24.2R2, 23.4R2-S3, 23.2R2-S2 +4

CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H

Attack Vector: Local
Attack Complexity: Low
Privileges Required: Low
User Interaction: None
Scope: Unchanged
Confidentiality: None
Integrity: None
Availability: High

Bold values are the worst case: easiest to exploit or largest impact.

Attack Vector: Local: The vulnerable component is not bound to the network stack and the attacker’s path is via read/write/execute capabilities. Either: the attacker exploits the vulnerability by accessing the target system locally (e.g., keyboard, console), or remotely (e.g., SSH); or the attacker relies on User Interaction by another person to perform actions required to exploit the vulnerability (e.g., tricking a legitimate user into opening a malicious document).
Attack Complexity: Low: Specialized access conditions or extenuating circumstances do not exist. An attacker can expect repeatable success against the vulnerable component.
Privileges Required: Low: The attacker is authorized with (i.e., requires) privileges that provide basic user capabilities that could normally affect only settings and files owned by a user. Alternatively, an attacker with Low privileges may have the ability to cause an impact only to non-sensitive resources.
User Interaction: None: The vulnerable system can be exploited without any interaction from any user.
Scope: Unchanged: An exploited vulnerability can only affect resources managed by the same security authority. In this case, the vulnerable component and the impacted component are either the same, or both are managed by the same security authority.
Confidentiality: None: There is no loss of confidentiality within the impacted component.
Integrity: None: There is no loss of integrity within the impacted component.
Availability: High: There is total loss of availability, resulting in the attacker being able to fully deny access to resources in the impacted component; this loss is either sustained (while the attacker continues to deliver the attack) or persistent (the condition persists even after the attack has completed). Alternatively, the attacker has the ability to deny some availability, but the loss of availability presents a direct, serious consequence to the impacted component (e.g., the attacker cannot disrupt existing connections, but can prevent new connections; the attacker can repeatedly exploit a vulnerability that, in each instance of a successful attack, leaks a only small amount of memory, but after repeated exploitation causes a service to become completely unavailable).

Definitions: FIRST CVSS v3.1 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 15 Jan 2026
Detectable: no evidence on record.
No public nuclei template indexed
Weaponised: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: no evidence on record.
Not in CISA KEV and no source reports exploitation
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 26 Oct 2026.

Disclosed: The CVE record is public on cve.org. Its publication is t = 0.
Detectable: A public nuclei template can find exposed instances, for defenders and attackers alike.
Weaponised: Public exploit code exists in Metasploit or Exploit-DB, or CISA SSVC or the CVSS exploit maturity reports a public proof of concept.
Exploited in the wild: CISA KEV lists it, CISA SSVC rates exploitation active, or a source reports attacks.
Observed by OffSeq Mirage: OffSeq Mirage honeypot sensors recorded attack traffic for this CVE, or found exposed hosts without attack traffic.

Moments are spaced evenly, not to scale; a dashed line marks a gap of six months or more. Deltas count from t = 0. Sources that give only a date (CISA KEV, Exploit-DB, or a time of exactly 00:00Z) read “same day” rather than hours. Metasploit modules carry the flaw’s disclosure date, not their release date, and nuclei templates carry no date, so both appear above but not on the timeline.

What to do

Upgrade to a fixed release listed under Affected software.

Affected software

Product	Affected	Fixed in
Juniper NetworksJunos OS	Affected >=24.2 <24.2R2	Fixed in 24.2R2
Juniper NetworksJunos OS	Affected >=23.4 <23.4R2-S3	Fixed in 23.4R2-S3
Juniper NetworksJunos OS	Affected >=23.2 <23.2R2-S2	Fixed in 23.2R2-S2
Juniper NetworksJunos OS	Affected >=22.4 <22.4R3-S6	Fixed in 22.4R3-S6
Juniper NetworksJunos OS	Affected >=22.2 <22.2R3-S7	Fixed in 22.2R3-S7
Juniper NetworksJunos OS	Affected >=21.4 <21.4R3-S9	Fixed in 21.4R3-S9

AI-suggested

1. 2. Restrict local CLI access to trusted administrators only, implementing strict access controls and monitoring to prevent unauthorized command execution. 3. Employ role-based access control (RBAC) to limit the ability to execute the 'show system firmware' command to necessary personnel. 4. Monitor system logs and chassisd process behavior for signs of repeated crashes or abnormal restarts indicative of exploitation attempts. 5. Implement network segmentation to isolate management interfaces and reduce the risk of lateral movement by attackers with local access. 6. Conduct regular audits of user accounts and privileges to detect and remove unnecessary or dormant accounts. 7. Prepare incident response plans to quickly address potential denial-of-service events affecting network infrastructure. 8. Consider deploying additional redundancy in network paths to mitigate the impact of line card failures.

Description

CVE-2026-21912 is a medium severity TOCTOU race condition vulnerability in Juniper Networks Junos OS affecting MX10k Series with LC480 or LC2101 line cards. A local, low-privileged attacker can repeatedly execute the 'show system firmware' CLI command to cause the affected line cards to crash and restart, potentially leading to chassisd process crashes and core dumps. This vulnerability impacts multiple Junos OS versions prior to specific patch releases. Exploitation does not require user interaction but requires local access with low privileges. The vulnerability results in availability disruption without compromising confidentiality or integrity. No known exploits are currently in the wild. European organizations using MX10k Series routers with these line cards should prioritize patching to prevent potential denial-of-service conditions. Countries with significant Juniper infrastructure deployments and critical network backbones are at higher risk. Mitigation involves applying vendor patches once available and restricting local CLI access to trusted administrators only.

Impact

For European organizations, this vulnerability poses a significant risk to network availability, particularly for those relying on Juniper MX10k Series routers with LC480 or LC2101 line cards in their core or edge network infrastructure. A successful exploitation can cause line card resets and chassisd crashes, leading to temporary loss of routing capabilities, potential network outages, and degraded service quality. This can disrupt business operations, especially for ISPs, data centers, and enterprises with critical network dependencies. While confidentiality and integrity remain unaffected, the availability impact can cascade, affecting dependent services and customers. The requirement for local access limits the attack surface but insider threats or compromised administrative accounts could exploit this vulnerability. Additionally, the generation of core dumps may expose sensitive debugging information if not properly secured. The absence of known exploits reduces immediate risk but does not eliminate the threat, especially as attackers may develop exploits post-disclosure.

Summary

CVE-2026-21912 identifies a Time-of-check Time-of-use (TOCTOU) race condition vulnerability classified under CWE-367 in Juniper Networks Junos OS running on MX10k Series routers equipped with LC480 or LC2101 line cards. The vulnerability arises in the method responsible for collecting Flexible PIC Concentrator (FPC) Ethernet firmware statistics when the 'show system firmware' CLI command is executed. A local attacker with low privileges can exploit this race condition by repeatedly invoking this command, causing the targeted line card to crash and restart. Subsequently, the chassisd daemon, responsible for chassis management, may also crash and generate core dumps, further impacting system stability. The affected Junos OS versions span multiple releases before specific patch versions: 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, and from 24.2 before 24.2R2. The CVSS v3.1 base score is 5.5 (medium), reflecting the local attack vector (AV:L), low attack complexity (AC:L), low privileges required (PR:L), no user interaction (UI:N), unchanged scope (S:U), no impact on confidentiality or integrity (C:N/I:N), but high impact on availability (A:H). There are no known exploits in the wild at this time. The vulnerability does not allow remote exploitation or privilege escalation but can cause denial-of-service conditions on critical network infrastructure components.

CWE-367Time-of-check Time-of-use (TOCTOU) Race Condition (opens in a new tab)
Base weaknessLikelihood of exploit: Medium
The product 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.
Implementation
The most basic advice for TOCTOU vulnerabilities is to not perform a check before the use. This does not resolve the underlying issue of the execution of a function on a resource whose state and identity cannot be assured, but it does help to limit the false sense of security given by the check.
Implementation
When the file being altered is owned by the current user and group, set the effective gid and uid to that of the current user and group when executing this statement.
Architecture and Design
Limit the interleaving of operations on files from multiple processes.
OWASP Cheat Sheets
Abuse Case (opens in a new tab)
OWASP ASVS 4.0.3
V1.11.3 (opens in a new tab)
V11.1.6 (opens in a new tab)
V1.11.2 (opens in a new tab)
Attack patterns
Leveraging Race Conditions via Symbolic Links (opens in a new tab)
Leveraging Time-of-Check and Time-of-Use (TOCTOU) Race Conditions (opens in a new tab)
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).

References

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Exploitation evidence

What to do

Affected software

Description

AI analysisMachine-generated, updated 22 Jan 2026, 21:38 UTC

WeaknessesCWE-367

Technical details

References

Community reviews

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