CVE-2026-21918VulnerabilityPublished 15 Jan 2026

CVE-2026-21918: Juniper Networks Junos OS · Double Free

Affects Juniper Networks Junos OSSource CVE Database V5

Not known to be exploited · CVSS 7.5 High · fixed in 24.2R2 : patch Juniper Networks Junos OS soon.

Data as of 25 Oct 2026, 19:02 UTC

CVSS 3.1 base score

7.5High

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

EPSS: 0.4% chance of exploitation in 30 days – higher than 34% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
Patch: Available – fixed in 24.2R2, 23.4R2-S4, 23.2R2-S3 +1

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

Attack Vector: Network
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: Network: The vulnerable component is bound to the network stack and the set of possible attackers extends beyond the other options listed, up to and including the entire Internet. Such a vulnerability is often termed 'remotely exploitable' and can be thought of as an attack being exploitable at the protocol level one or more network hops away (e.g., across one or more routers).
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 2026-10-25.

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 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 Juniper Networks Junos OS to 24.2R2, 23.4R2-S4, 23.2R2-S3 or 22.4R3-S7.

Affected software

Product	Affected	Fixed in
Juniper NetworksJunos OS	Affected <22.4R3-S7 >=23.2 <23.2R2-S3 >=23.4 <23.4R2-S4 >=24.2 <24.2R2	Fixed in 24.2R2 23.4R2-S4 23.2R2-S3 22.4R3-S7

AI-suggested

Organizations should immediately inventory their Juniper SRX and MX Series devices to identify affected Junos OS versions. Until patches are released and applied, network administrators should implement strict ingress filtering and firewall rules to limit exposure to untrusted networks, especially blocking suspicious TCP session establishment packets that could trigger the vulnerability. Deploying network intrusion detection/prevention systems (IDS/IPS) with signatures tuned to detect anomalous TCP sequences targeting flowd may help mitigate exploitation attempts. Monitoring device logs for flowd crashes and FPC restarts can provide early warning of exploitation attempts. Network segmentation and redundancy can reduce the impact of potential DoS events. Once Juniper releases patches, organizations must prioritize timely updates to affected devices. Additionally, maintaining up-to-date backups and incident response plans tailored to network device outages will improve resilience.

Description

CVE-2026-21918 is a high-severity double free vulnerability in the flow processing daemon (flowd) of Juniper Networks Junos OS running on SRX and MX Series devices. An unauthenticated attacker can trigger this flaw remotely by sending a specific sequence of TCP packets during session establishment, causing flowd to crash and the Flexible PIC Concentrator (FPC) to restart, resulting in a denial-of-service (DoS). This vulnerability affects multiple Junos OS versions prior to 22.4R3-S7, 23.2R2-S3, 23.4R2-S4, and 24.2R2. Exploitation requires no authentication or user interaction and impacts availability without compromising confidentiality or integrity. No known exploits are currently reported in the wild. European organizations using Juniper SRX or MX Series devices should prioritize patching to prevent potential service disruptions, especially in critical network infrastructure.

Impact

For European organizations, this vulnerability poses a significant risk to network availability, particularly for those relying on Juniper SRX and MX Series devices for firewalling, routing, and security gateway functions. A successful exploit could cause flowd crashes and FPC restarts, leading to temporary loss of network traffic processing and potential service outages. This can disrupt business operations, especially in sectors such as telecommunications, finance, government, and critical infrastructure where Juniper devices are commonly deployed. The lack of confidentiality or integrity impact limits data breach concerns, but the denial-of-service effect can degrade network reliability and availability, potentially affecting compliance with regulatory requirements like NIS2. The unauthenticated, remote nature of the exploit increases the threat surface, making perimeter defenses critical. Organizations with large-scale or geographically distributed Juniper deployments may experience cascading effects if multiple devices are targeted simultaneously.

CWE-415Double Free (opens in a new tab)
Variant weaknessLikelihood of exploit: High
The product calls free() twice on the same memory address.
Architecture and Design
Choose a language that provides automatic memory management.
Implementation
Ensure that each allocation is freed only once. After freeing a chunk, set the pointer to NULL to ensure the pointer cannot be freed again. In complicated error conditions, be sure that clean-up routines respect the state of allocation properly.
Implementation
Use a static analysis tool to find double free instances.
OWASP Cheat Sheets
XML Security (opens in a new tab)
Mass Assignment (opens in a new tab)
OWASP ASVS 4.0.3
V13.3.1 (opens in a new tab)
V5.1.4 (opens in a new tab)
V5.1.3 (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, 21:35 UTC

WeaknessesCWE-415

Technical details

References

Community reviews

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