Cyber Resilience

CVE-2026-21917

Juniper Junos 23.2 … 24.4

Published
15 January 2026
Modified
23 January 2026
Patch / advisory
View advisory
CVSS Score v4 8.7
Click a component to see what it means
Raw vectorCVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:L/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:Y/R:A/V:X/RE:M/U:X
EPSS Score 0.0037 30th percentile
Risk Priority 44 floored blend · peak EPSS

Summary

CVE-2026-21917 is a high-severity Improper Validation of Syntactic Correctness of Input (CWE-1286) vulnerability in Juniper Junos. Its CVSS base score is 8.7 (High).

Operationally, exploitation aligns with the MITRE ATT&CK technique Exploit Public-Facing Application (T1190); ranked at the 30th percentile by exploit likelihood (below the median); it is not currently listed in the CISA KEV catalog.

The strongest mitigations our analysis identified map to SI-10 (Information Input Validation) — see the control section below for these in your framework.

Deeper analysis AI-assisted summary

Synthesised by an AI model from the NVD description and linked references — a reading aid, not an authoritative source.

CVE-2026-21917 is an Improper Validation of Syntactic Correctness of Input vulnerability (CWE-1286) in the Web-Filtering module of Juniper Networks Junos OS on SRX Series devices. When an SRX device is configured for UTM Web-Filtering, it can be triggered by a specifically malformed SSL packet, causing an FPC crash and restart, resulting in a Denial-of-Service (DoS) condition. The vulnerability affects Junos OS versions 23.2 from 23.2R2-S2 before 23.2R2-S5, 23.4 from 23.4R2-S1 before 23.4R2-S5, 24.2 before 24.2R2-S2, and 24.4 before 24.4R1-S3 or 24.4R2; earlier versions are also affected but have no available fix. It has a CVSS v3.1 base score of 7.5 (AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H).

An unauthenticated, network-based attacker can exploit this vulnerability by sending a malformed SSL packet to an affected SRX device with UTM Web-Filtering enabled. Successful exploitation leads to an FPC crash and restart, disrupting network traffic processing on the device until recovery.

Juniper's security advisory JSA105996, detailed at https://kb.juniper.net/JSA105996 and https://supportportal.juniper.net/JSA105996, recommends upgrading to a fixed release as the primary mitigation: Junos OS 23.2R2-S5 or later, 23.4R2-S5 or later, 24.2R2-S2 or later, 24.4R1-S3 or later, or 24.4R2 or later. No workarounds are specified for affected versions lacking fixes.

EU & UK References

Vulnerability Data

An Improper Validation of Syntactic Correctness of Input vulnerability in the Web-Filtering module of Juniper Networks Junos OS on SRX Series allows an unauthenticated, network-based attacker to cause a Denial-of-Service (DoS). If an SRX device configured for UTM Web-Filtering receives…

more

a specifically malformed SSL packet, this will cause an FPC crash and restart. This issue affects Junos OS on SRX Series: * 23.2 versions from 23.2R2-S2 before 23.2R2-S5, * 23.4 versions from 23.4R2-S1 before 23.4R2-S5, * 24.2 versions before 24.2R2-S2, * 24.4 versions before 24.4R1-S3, 24.4R2. Earlier versions of Junos are also affected, but no fix is available.

CWE(s)
CWE-1286

Related Threats

MITRE ATT&CK Enterprise Techniques

T1190 Exploit Public-Facing Application Initial Access
Adversaries may attempt to exploit a weakness in an Internet-facing host or system to initially access a network.
attack.mitre.org →
Derived from this CVE’s CWE(s) via the direct CWE→ATT&CK cross-walk.

CVEs Like This One

CVE-2026-57026Same product: Juniper Junos
CVE-2026-33778Same product: Juniper Junos
CVE-2023-28985Same product: Juniper Junos
CVE-2025-30659Same product: Juniper Junos
CVE-2026-21920Same product: Juniper Junos
CVE-2025-6549Same product: Juniper Junos
CVE-2024-21616Same product: Juniper Junos
CVE-2024-39542Same product: Juniper Junos
CVE-2024-21598Same product: Juniper Junos
CVE-2024-39565Same product: Juniper Junos

Affected Assets

juniper
junos
23.2, 23.4, 24.2, 24.4

Mitigating Controls

Control response

Prevent
Stop it (NIST 800-53)

—

Detect
Catch it (NIST detect / respond)

—

Harden
Shrink the surface (DISA STIG)

—

Validate
Prove the fix (OWASP ASVS)
  • V1.1.1
  • V1.4.2
  • V2.1.1
  • V2.2.2

Mitigating Controls (NIST 800-53 r5) AI

SI-10 Information Input Validation good match

SI-10 requires validation of information inputs for correctness, directly stopping syntactic validation failures from being introduced or exploitable.

Mitigating Controls (NIST CSF 2.0) AI

Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→CSF cross-walk (authority under review) — links open the control.

PR.PS-06 mostly match
prevents

Secure SDLC practices directly require syntactic input validation to prevent malformed data handling.

Mitigating Controls (ISO/IEC 27001:2022 Annex A) AI

Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→ISO cross-walk (authority under review) — links open the control.

A.8.29 Security testing in development and acceptance mostly match
finds

Security testing in development catches missing or incorrect syntax validation.

A.8.25 Secure development life cycle partial match
prevents

Secure development life cycle mandates input validation to ensure syntactic correctness.

A.8.26 Application security requirements partial match
prevents

Application security requirements explicitly call for validation of input syntax.

A.8.27 Secure system architecture and engineering principles partial match
prevents

Secure architecture principles reduce the likelihood of missing syntactic checks.

A.8.28 Secure coding partial match
prevents

Secure coding standards directly require proper syntactic validation of all inputs.

References