CVE-2026-21920VulnerabilityPublished 15 Jan 2026

CVE-2026-21920: Juniper Networks Junos OS · Unchecked Return Value

Affects Juniper Networks Junos OSSource CVE Database V5

Not known to be exploited · CVSS 7.5 High · fixed in 24.4R2 : patch Juniper Networks Junos OS soon.

Data as of 26 Oct 2026, 06:26 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 41% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
Patch: Available – fixed in 24.4R2, 24.2R2-S1, 23.4R2-S5

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 2026-10-26.

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 24.4R2, 24.2R2-S1 or 23.4R2-S5.

Affected software

Product	Affected	Fixed in
Juniper NetworksJunos OS	Affected >=23.4 <23.4R2-S5 >=24.2 <24.2R2-S1 >=24.4 <24.4R2	Fixed in 24.4R2 24.2R2-S1 23.4R2-S5

AI-suggested

European organizations should prioritize upgrading affected Junos OS versions to the fixed releases: 23.4R2-S5 or later for 23.4, 24.2R2-S1 or later for 24.2, and 24.4R2 or later for 24.4. If you cannot patch yet, organizations should consider implementing network-level mitigations such as filtering or rate-limiting DNS traffic to SRX devices, especially from untrusted sources. Monitoring for unusual DNS traffic patterns and flowd process crashes can provide early detection of exploitation attempts. Deploying intrusion detection/prevention systems (IDS/IPS) with signatures targeting malformed DNS packets may help block exploit attempts. Network segmentation to isolate critical SRX devices and restricting management access can reduce exposure. Regularly reviewing Juniper security advisories and subscribing to vendor notifications will ensure timely awareness of updates. Finally, organizations should test patches in controlled environments before deployment to avoid service disruptions.

Description

CVE-2026-21920 is a high-severity vulnerability in Juniper Networks Junos OS affecting SRX Series devices configured for DNS processing. An unauthenticated, network-based attacker can send a specially crafted DNS request that triggers a crash and restart of the flowd process, causing a denial-of-service (DoS) condition. This vulnerability affects Junos OS versions 23.4 prior to 23.4R2-S5, 24.2 prior to 24.2R2-S1, and 24.4 prior to 24.4R2. No user interaction or authentication is required to exploit this flaw.

Impact

For European organizations, the primary impact of CVE-2026-21920 is the potential for denial-of-service attacks against Juniper SRX Series firewalls, which are widely deployed in enterprise and service provider networks. A successful exploit can disrupt DNS processing and flow management, leading to temporary loss of firewall functionality and network interruptions. This can affect business continuity, especially for organizations relying on these devices for perimeter defense, VPN termination, and DNS security. Critical infrastructure sectors such as finance, telecommunications, energy, and government agencies in Europe could face operational disruptions and increased exposure to secondary attacks during downtime. The lack of authentication and ease of exploitation increase the risk of opportunistic attacks from external threat actors. Although no data confidentiality or integrity is directly compromised, the availability impact can have cascading effects on dependent systems and services. The vulnerability also raises concerns for managed security service providers (MSSPs) and cloud providers using Juniper SRX devices in their infrastructure.

CWE-252Unchecked Return Value (opens in a new tab)
Base weaknessLikelihood of exploit: LowOWASP Top 10 A10:2025: Mishandling of Exceptional Conditions (opens in a new tab)
The product does not check the return value from a method or function, which can prevent it from detecting unexpected states and conditions.
Implementation
Check the results of all functions that return a value and verify that the value is expected.
Implementation
For any pointers that could have been modified or provided from a function that can return NULL, check the pointer for NULL before use. When working with a multithreaded or otherwise asynchronous environment, ensure that proper locking APIs are used to lock before the check, and unlock when it has finished.
Implementation
Ensure that you account for all possible return values from the function.
OWASP Cheat Sheets
Logging (opens in a new tab)
Error Handling (opens in a new tab)
OWASP ASVS 4.0.3
V11.1.7 (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, 21:36 UTC

WeaknessesCWE-252

Technical details

References

Community reviews

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