VulnerabilityPublished 15 Jan 2026

CVE-2026-21917: Juniper Networks Junos OS · Improper Validation of Syntactic Correctness of Input

Affects Juniper Networks Junos OSSource CVE Database V5

Not known to be exploited · CVSS 7.5 High · fixed in 24.4R1-S3 : patch Juniper Networks Junos OS soon.

Data as of 26 Oct 2026, 10:12 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 35% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
Patch: Available – fixed in 24.4R1-S3, 24.2R2-S2, 23.4R2-S5 +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 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 Juniper Networks Junos OS to 24.4R1-S3, 24.2R2-S2, 23.4R2-S5 or 23.2R2-S5.

Affected software

Product	Affected	Fixed in
Juniper NetworksJunos OS	Affected >=23.2R2-S2 <23.2R2-S5 >=23.4R2-S1 <23.4R2-S5 >=24.2 <24.2R2-S2 =24.4	Fixed in 24.4R1-S3 24.2R2-S2 23.4R2-S5 23.2R2-S5

AI-suggested

For versions without available patches, consider disabling UTM Web-Filtering if feasible or limiting exposure by restricting access to SRX management and filtering interfaces via network segmentation and firewall rules. Implement strict ingress filtering to block malformed or suspicious SSL traffic from untrusted sources. Monitor SRX device logs and network traffic for signs of malformed SSL packets or FPC crashes. Employ redundancy and failover mechanisms to maintain network availability in case of device restarts. Engage with Juniper support for guidance on interim mitigations and updates. Regularly review and update incident response plans to address potential DoS scenarios targeting network security appliances. Finally, maintain an inventory of Juniper SRX devices and their firmware versions to ensure timely patch management.

Description

CVE-2026-21917 is a high-severity vulnerability in Juniper Networks Junos OS affecting SRX Series devices with UTM Web-Filtering enabled. An unauthenticated attacker can send a specially crafted malformed SSL packet to cause an FPC crash and restart, resulting in a denial-of-service (DoS) condition. The flaw arises from improper validation of syntactic correctness of input in the Web-Filtering module. Affected versions include multiple recent releases from 23.2 through 24.4, with no fixes available for earlier versions. Exploitation requires no authentication or user interaction and can be performed remotely over the network. Although no known exploits are currently reported in the wild, the vulnerability poses a significant risk to availability of critical network security infrastructure. European organizations relying on Juniper SRX devices for perimeter security and web filtering are particularly at risk. Mitigation involves upgrading to fixed Junos OS versions where available or applying compensating controls to limit exposure.

Impact

For European organizations, the impact of CVE-2026-21917 can be severe, especially for those relying on Juniper SRX Series devices for perimeter defense, Unified Threat Management (UTM), and web filtering. A successful exploit results in denial-of-service by crashing the FPC, which can disrupt network traffic inspection and filtering, potentially exposing the network to other threats or causing service outages. Critical infrastructure sectors such as finance, telecommunications, government, and energy that depend on continuous network availability and security enforcement could face operational disruptions. The lack of authentication and user interaction requirements means attackers can launch attacks remotely and anonymously, increasing the risk of widespread exploitation. Additionally, the inability to patch older versions leaves some organizations exposed indefinitely. This vulnerability could be leveraged as part of a broader attack campaign to degrade network defenses or cause targeted disruption. The availability impact could also affect compliance with regulatory requirements around network security and uptime.

CWE-1286Improper Validation of Syntactic Correctness of Input (opens in a new tab)
Base weakness
The product receives input that is expected to be well-formed - i.e., to comply with a certain syntax - but it does not validate or incorrectly validates that the input complies with the syntax.
Implementation · Input Validation
Assume all input is malicious. Use an "accept known good" input validation strategy, i.e., use a list of acceptable inputs that strictly conform to specifications. Reject any input that does not strictly conform to specifications, or transform it into something that does.
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)
Attack patterns
SQL Injection (opens in a new tab)
NoSQL Injection (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-1286

Technical details

References

Community reviews

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