JunosOperating system · Juniper

CVE-2026-21906

HIGH · 7.5 CVSS v3.1 Published 2026-01-15

Fix available

A fix is available. Upgrade to 21.4 or later.

See remediation →

84/100

Remediation priority · High

Remotely reachable No privileges Zero-click

Official description

NVD · unedited

An Improper Handling of Exceptional Conditions vulnerability in the packet forwarding engine (PFE) of Juniper Networks Junos OS on SRX Series allows an unauthenticated network-based attacker sending a specific ICMP packet through a GRE tunnel to cause the PFE to crash and restart. When PowerMode IPsec (PMI) and GRE performance acceleration are enabled and the device receives a specific ICMP packet, a crash occurs in the SRX PFE, resulting in traffic loss. PMI is enabled by default, and GRE performance acceleration can be enabled by running the configuration command shown below. PMI is a mode of operation that provides IPsec performance improvements using Vector Packet Processing. Note that PMI with GRE performance acceleration is only supported on specific SRX platforms. This issue affects Junos OS on the SRX Series: * all versions before 21.4R3-S12, * from 22.4 before 22.4R3-S8, * from 23.2 before 23.2R2-S5, * from 23.4 before 23.4R2-S5, * from 24.2 before 24.2R2-S3, * from 24.4 before 24.4R2-S1, * from 25.2 before 25.2R1-S1, 25.2R2.

Technical summary

dbcve analysis · high confidence

An improper handling of exceptional conditions in the packet forwarding engine (PFE) of Juniper Networks Junos OS on SRX Series allows an unauthenticated network-based attacker to cause a PFE crash by sending a specific ICMP packet through a GRE tunnel when PowerMode IPsec (PMI, enabled by default) and GRE performance acceleration are both enabled.

MitigationApply the vendor-supplied patches by upgrading to the fixed versions (21.4R3-S12, 22.4R3-S8, 23.2R2-S5, 23.4R2-S5, 24.2R2-S3, 24.4R2-S1, 25.2R1-S1, or 25.2R2 or later). Alternatively, if GRE performance acceleration is not required, disable it to eliminate the attack vector.

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

Affected products & versions

NVD · CPE data

JunosOperating system

Affected:< 21.4= 21.4= 22.4= 23.2= 23.4= 24.2= 24.4= 25.2

CVSS breakdown

From the vector

Attack vector: Network
Complexity: Low
Privileges: None
User interaction: None
Scope: Unchanged
Confidentiality: None
Integrity: None
Availability: High

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

Am I affected?

dbcve checks

Work through these to decide whether this CVE applies to you.

Confirm SRX Series device model

Run 'show chassis hardware' or 'show system information' to identify the device model

Affected if Device is not an SRX Series device (vulnerability only affects SRX Series)
Check installed Junos OS version

Run 'show version' and compare the Junos version to the affected list: <21.4, 21.4, 22.4, 23.2, 23.4, 24.2, 24.4, 25.2

Affected if Installed version matches any of the affected versions listed
Verify PowerMode IPsec (PMI) status

Run 'show security pmi' or 'show configuration security ipsec' to check if PMI is enabled (it is enabled by default)

Affected if PMI is enabled (vulnerability requires PMI to be active)
Verify GRE performance acceleration is enabled

Run 'show configuration interfaces' with 'gre' and check for performance acceleration parameters, or inspect the GRE tunnel configuration for acceleration-related settings

Affected if GRE performance acceleration is enabled (this is a required condition for the exploit)

A user is affected if they are running an affected Junos version on SRX Series with both PMI enabled (default) and GRE performance acceleration enabled, where GRE tunnels are configured.

Generated from the published advisory. Verify against your own configuration.

Remediation

dbcve · scoped

Upgrade available Upgrade to 21.4 or later

Fixed in 21.4

Interim mitigation

Apply the vendor-supplied patches by upgrading to the fixed versions (21.4R3-S12, 22.4R3-S8, 23.2R2-S5, 23.4R2-S5, 24.2R2-S3, 24.4R2-S1, 25.2R1-S1, or 25.2R2 or later). Alternatively, if GRE performance acceleration is not required, disable it to eliminate the attack vector.

Recommended fix High confidence

25.2R2 (or earliest fixed version for your current branch: 21.4R3-S12, 22.4R3-S8, 23.2R2-S5, 23.4R2-S5, 24.2R2-S3, 24.4R2-S1, or 25.2R2)

1. Identify the current Junos OS version on the SRX Series device using 'show version'
2. Determine if PowerMode IPsec (PMI) is enabled using 'show chassis pfe statistics traffic' or checking configuration under 'set security ipsec'
3. Determine if GRE performance acceleration is enabled by reviewing configuration for 'set interfaces gre-unit x unit x tunnel gre-options ipsec-acceleration'
4. If both PMI and GRE acceleration are enabled, the device is vulnerable
5. Schedule a maintenance window for the upgrade
6. Download the appropriate fixed Junos OS version based on current version: 21.4R3-S12 or later, 22.4R3-S8 or later, 23.2R2-S5 or later, 23.4R2-S5 or later, 24.2R2-S3 or later, 24.4R2-S1 or later, or 25.2R2
7. Upload the Junos OS upgrade package to the device
8. Install the upgrade using 'request system software add <package>' with appropriate options such as 'no-validate' if needed

Caveat Review Juniper release notes for your target version for any known compatibility issues; some platforms may have reduced functionality in newer branches

Generated from the published advisory — verify against the referenced sources before acting.

Fix this in Junos Scoped from the published advisory

Consultation4.0 h
Implementation6.0 h
Testing8.0 h
Review / QA4.0 h

22.0 hours of engineering $3,800

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References

Primary sources

kb.juniper.netVendor Advisory
supportportal.juniper.netVendor Advisory
www.juniper.netTechnical Description
nvd.nist.govNVD record

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2026-21906 in production — separate from our analysis above.

Notes0 Guidelines

Know something about CVE-2026-21906?

The advisory tells you what broke. It rarely tells you what actually worked. If you’ve dealt with this one, that detail is what the next engineer is searching for.

The version that genuinely resolved it — not the one the vendor claimed
A config change or rule that shut the vector down
A gotcha in the upgrade path that cost you an afternoon

No notes yet

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What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

What belongs here

Verified mitigations, workarounds, and config changes
Version or environment caveats, and links to real fixes
No weaponised exploit code, or anything meant to cause harm
No spam, self-promotion, credentials, or personal data

At a glance

A fix existsUpgrade to 21.4 or later

Severity 7.5 High

Exploit likelihood EPSS, next 30 days 0.5%

Affects: Junos

Remediation

A high-priority fix

Fix this in Junos 22 hrs · Moderate

Typical engagement from $3,800

Get a fix See the full breakdown ↓

A high-priority vulnerability. Send your environment and a specialist will scope CVE-2026-21906 and return a fixed quote and timeline.

You’ll deal with our team directly and deploy the reviewed fix to staging first — never straight to production.

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.

Check your environment

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

Scan for this in your stack

References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Practitioner notes

No notes yet

Official description

Technical summary

Affected products & versions

CVSS breakdown

Am I affected?

Remediation

References