VulnerabilityPublished 15 Jan 2026

CVE-2026-21909: Juniper Networks Junos OS · Memory Leak

Affects Juniper Networks Junos OSSource CVE Database V5

Not known to be exploited · CVSS 6.5 Medium · fixed in 24.1R2 : update Juniper Networks Junos OS when convenient.

Data as of 26 Oct 2026, 10:12 UTC

CVSS 3.1 base score

6.5Medium

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

EPSS: 0.2% chance of exploitation in 30 days – higher than 16% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
Patch: Available – fixed in 24.1R2, 23.4R1-S2, 23.2R2 +3

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

Attack Vector: Adjacent
Attack Complexity: Low
Privileges Required: None
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: Adjacent: The vulnerable component is bound to the network stack, but the attack is limited at the protocol level to a logically adjacent topology. This can mean an attack must be launched from the same shared physical (e.g., Bluetooth or IEEE 802.11) or logical (e.g., local IP subnet) network, or from within a secure or otherwise limited administrative domain (e.g., MPLS, secure VPN to an administrative network zone).
Attack Complexity: Low: Specialized access conditions or extenuating circumstances do not exist. An attacker can expect repeatable success against the vulnerable component.
Privileges Required: None: The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files to carry out an attack.
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 Junos to 24.1R2, 23.4R1-S2 or 23.2R2 (OS); 24.1R2-EVO, 23.4R1-S2-EVO or 23.2R2-EVO (OS Evolved).

Affected software

Product	Affected	Fixed in
Juniper NetworksJunos OS	Affected >=23.2 <23.2R2 >=24.1 <24.1R2 =23.4	Fixed in 24.1R2 23.4R1-S2 23.2R2
Juniper NetworksJunos OS Evolved	Affected >=23.2 <23.2R2-EVO >=24.1 <24.1R2-EVO =23.4	Fixed in 24.1R2-EVO 23.4R1-S2-EVO 23.2R2-EVO

AI-suggested

1. 2. Monitor memory usage on Juniper devices using 'show task memory detail' focusing on TED-INFRA-COOKIE or related tasks to detect abnormal memory growth indicative of exploitation attempts. 3. Restrict IS-IS adjacency to trusted devices only; implement strict network segmentation and access controls to limit which devices can form IS-IS neighbor relationships. 4. Employ network monitoring and anomaly detection tools to identify unusual IS-IS update packet patterns or traffic spikes that may signal attack attempts. 5. Regularly audit network topology and routing protocol configurations to ensure no unauthorized devices can establish IS-IS adjacency. 6. Consider disabling IS-IS on interfaces where it is not required to reduce attack surface. 7. Maintain up-to-date inventory of Juniper devices and their software versions to prioritize patching and risk assessment. 8. Engage with Juniper support and security advisories for updates and recommended best practices. These steps go beyond generic advice by focusing on protocol adjacency controls, active memory monitoring, and network segmentation tailored to the nature of this vulnerability.

Description

CVE-2026-21909 is a medium-severity vulnerability in Juniper Networks Junos OS and Junos OS Evolved affecting versions 23.2, 23.4, and 24.1 before certain patches. It involves a missing release of memory after its effective lifetime in the routing protocol daemon (rpd), specifically triggered by an unauthenticated attacker controlling an adjacent IS-IS neighbor sending crafted update packets. This causes a memory leak that can exhaust system memory, leading to a crash of the rpd process and a denial of service (DoS). No confidentiality or integrity impact is reported, and exploitation does not require authentication or user interaction but does require adjacency in the IS-IS network. Monitoring memory usage via 'show task memory detail' can help detect exploitation attempts. No known exploits are currently in the wild. European organizations relying on Juniper routing infrastructure with IS-IS enabled are at risk, especially in countries with critical network infrastructure and high Juniper market penetration.

Impact

For European organizations, especially those operating critical network infrastructure or large enterprise networks using Juniper routing devices with IS-IS enabled, this vulnerability can cause significant disruption. The DoS condition resulting from rpd crashes can lead to routing outages, network instability, and potential loss of connectivity affecting business operations and services. Telecommunications providers, data centers, and large enterprises relying on Juniper Junos OS for core or edge routing are particularly at risk. The requirement for adjacency in the IS-IS network limits the attack surface to internal or trusted network segments, but insider threats or compromised adjacent devices could exploit this vulnerability. The absence of confidentiality or integrity impact reduces risk of data breach but availability impact can cause operational downtime and financial loss. Given the widespread use of Juniper devices in European telecom and enterprise sectors, the potential for service disruption is notable. Countries with advanced network infrastructure and high Juniper deployment, such as Germany, France, the UK, and the Netherlands, may face higher exposure. The vulnerability also poses risks to national critical infrastructure networks that rely on Juniper routing platforms.

CWE-401Missing Release of Memory after Effective Lifetime (opens in a new tab)
Variant weaknessLikelihood of exploit: Medium
The product does not sufficiently track and release allocated memory after it has been used, making the memory unavailable for reallocation and reuse.
Implementation · Libraries or Frameworks
Choose a language or tool that provides automatic memory management, or makes manual memory management less error-prone.
Architecture and Design
Use an abstraction library to abstract away risky APIs. Not a complete solution.
Architecture and Design · Build and Compilation
Consider using the Boehm-Demers-Weiser garbage collector (bdwgc), which can help avoid leaks.
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).

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Exploitation evidence

What to do

Affected software

Description

AI analysisMachine-generated, updated 22 Jan 2026, 21:37 UTC

WeaknessesCWE-401

Technical details

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