dbcve.org
JunosOperating system · Juniper

CVE-2026-21905

HIGH · 7.5 CVSS v3.1 Published 2026-01-15
Fix available
A fix is available. Upgrade to 21.2 or later.
See remediation →
84/100
Remediation priority · High
Remotely reachable No privileges Zero-click

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.

NVD · unedited
A Loop with Unreachable Exit Condition ('Infinite Loop') vulnerability in the SIP application layer gateway (ALG) of Juniper Networks Junos OS on SRX Series and MX Series with MX-SPC3 or MS-MPC allows an unauthenticated network-based attacker sending specific SIP messages over TCP to crash the flow management process, leading to a Denial of Service (DoS). On SRX Series, and MX Series with MX-SPC3 or MS-MPC service cards, receipt of multiple SIP messages causes the SIP headers to be parsed incorrectly, eventually causing a continuous loop and leading to a watchdog timer expiration, crashing the flowd process on SRX Series and MX Series with MX-SPC3, or mspmand process on MX Series with MS-MPC. This issue only occurs over TCP. SIP messages sent over UDP cannot trigger this issue. This issue affects Junos OS on SRX Series and MX Series with MX-SPC3 and MS-MPC: * all versions before 21.2R3-S10,  * from 21.4 before 21.4R3-S12,  * from 22.4 before 22.4R3-S8,  * from 23.2 before 23.2R2-S5,  * from 23.4 before 23.4R2-S6,  * 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 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.

dbcve analysis · high confidence

An infinite loop vulnerability exists in the SIP Application Layer Gateway (ALG) in Juniper Junos OS on SRX Series and MX Series (with MX-SPC3 or MS-MPC). An unauthenticated attacker sending specially crafted SIP messages over TCP can cause SIP header parsing errors that trigger an infinite loop, eventually causing the watchdog timer to expire and crash the flowd (SRX/MX-SPC3) or mspmand (MS-MPC) processes, resulting in DoS.

MitigationApply the appropriate Junos OS software update for your version branch (21.2R3-S10, 21.4R3-S12, 22.4R3-S8, 23.2R2-S5, 23.4R2-S6, 24.2R2-S3, 24.4R2-S1, or 25.2R1-S1/25.2R2). Until patched, consider disabling SIP ALG or restricting TCP-based SIP traffic at network perimeter.

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

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.

NVD · CPE data
JunosOperating system
Affected:< 21.2= 21.2= 21.4= 22.4= 23.2= 23.4= 24.2= 24.4= 25.2

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.

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? 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.

dbcve checks

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

  1. Identify device platform
    Confirm the device is a Juniper SRX Series firewall or MX Series router with MX-SPC3 or MS-MPC line cards. Use 'show chassis hardware' or 'show version' to verify the model and SPC card presence.
    Affected if Device is not an SRX Series or MX Series with MX-SPC3/MS-MPC, then not affected by this specific vulnerability.
  2. Check Junos OS version
    Run 'show version' or 'show version detail' to obtain the installed Junos OS version. Compare against affected versions: any version < 21.2, or exactly 21.2, 21.4, 22.4, 23.2, 23.4, 24.2, 24.4, or 25.2.
    Affected if Installed version matches one of the affected versions listed in the CVE.
  3. Verify SIP ALG is enabled
    Run 'show security alg status' to check if SIP ALG is enabled. Alternatively, check the configuration with 'show configuration security alg sip' to see if SIP ALG is explicitly configured.
    Affected if SIP ALG is enabled on the device - this is required for the vulnerability to be exploitable.
  4. Confirm TCP-based SIP traffic handling
    Review the SIP ALG configuration for TCP transport settings. Check 'show configuration security alg sip' for TCP-related settings, and verify if the device is configured to process SIP over TCP from untrusted sources.
    Affected if SIP ALG is configured to process TCP-based SIP traffic, which is the attack vector for this vulnerability.

Device is affected if it is an SRX Series or MX Series with MX-SPC3/MS-MPC, runs a vulnerable Junos version, and has SIP ALG enabled with TCP-based SIP processing configured.

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

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

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.

dbcve · scoped
Upgrade available Upgrade to 21.2 or later
Fixed in 21.2
Interim mitigation

Apply the appropriate Junos OS software update for your version branch (21.2R3-S10, 21.4R3-S12, 22.4R3-S8, 23.2R2-S5, 23.4R2-S6, 24.2R2-S3, 24.4R2-S1, or 25.2R1-S1/25.2R2). Until patched, consider disabling SIP ALG or restricting TCP-based SIP traffic at network perimeter.

Recommended fix High confidence

Upgrade to 21.2R3-S10 / 21.4R3-S12 / 22.4R3-S8 / 23.2R2-S5 / 23.4R2-S6 / 24.2R2-S3 / 24.4R2-S1 / 25.2R1-S1 or 25.2R2 depending on current version branch

  1. 1. Identify the current Junos OS version on the affected device using 'show version' command.
  2. 2. Determine if the device is an SRX Series, or MX Series with MX-SPC3 or MS-MPC service cards.
  3. 3. Confirm SIP ALG is enabled (default) using 'show security alg sip' command.
  4. 4. Based on the current version, plan upgrade to the appropriate fixed release: For 21.2.x upgrade to 21.2R3-S10 or later; For 21.4.x upgrade to 21.4R3-S12 or later; For 22.4.x upgrade to 22.4R3-S8 or later; For 23.2.x upgrade to 23.2R2-S5 or later; For 23.4.x upgrade to 23.4R2-S6 or later; For 24.2.x upgrade to 24.2R2-S3 or later; For 24.4.x upgrade to 24.4R2-S1 or later; For 25.2.x upgrade to 25
  5. 5. Download the appropriate upgrade package from the Juniper Networks support portal.
  6. 6. Upload the upgrade package to the device and initiate the upgrade using 'request system software add <package> reboot' command.
  7. 7. After reboot, verify the new version is installed using 'show version'.
  8. 8. Verify SIP ALG functionality using 'show security alg sip' and confirm normal operation.
Caveat Standard Junos upgrade - review release notes for any configuration changes between versions; test in lab before production deployment

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

Fix this in Junos Scoped from the published advisory
  • Consultation4.0 h
  • Implementation8.0 h
  • Testing6.0 h
  • Review / QA2.0 h
20.0 hours of engineering $3,500
Get the upgrade done

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $5,600.

Scan for this in your stack

Free · runs locally
dbcve dependency scanner

Check whether your project pulls in CVE-2026-21905 — or any other known-vulnerable package — straight from your lock files. Free and open source; it runs locally and uploads nothing.

Get it on GitHub Scan in the browser

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

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2026-21905 in production — separate from our analysis above.

No notes yet

Be the first to add a field note for this CVE — a mitigation you’ve verified, a version caveat, or a link to a working fix. Sign in above to contribute.

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