CVE-2026-21905Network attack vector

Unauthenticated process crash via SIP header parsing loop

Published Jan 15, 2026 · Updated Jan 15, 2026

Infinite loop in the SIP ALG in Juniper Networks Junos OS allows remote attackers to crash flow management with crafted TCP SIP messages. Multiple SIP messages cause the ALG to parse SIP headers incorrectly and enter a continuous loop until the watchdog expires, crashing flowd on SRX and MX-SPC3 or mspmand on MS-MPC. Only TCP SIP traffic triggers the issue; UDP SIP messages do not, and the resulting impact is interruption of packet-flow processing.

CVSS severity8.7
High
EPSS probability0.42%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Infinite loop in the SIP ALG in Juniper Networks Junos OS allows remote attackers to crash flow management with crafted TCP SIP messages. Multiple SIP messages cause the ALG to parse SIP headers incorrectly and enter a continuous loop until the watchdog expires, crashing flowd on SRX and MX-SPC3 or mspmand on MS-MPC. Only TCP SIP traffic triggers the issue; UDP SIP messages do not, and the resulting impact is interruption of packet-flow processing.

The record

CVE
CVE-2026-21905
Published
Jan 15, 2026
Updated
Jan 15, 2026
Vendor
Juniper Networks
Product
Junos OS
Classifications
CWE-835, T1499.004
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Jan 15, 2026CVE publishedPublication date reported by the CVE source.
  2. Jan 15, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

Compare your product and version with the public record. A matching version still requires validation against your environment.

Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=0 <21.2R3-S10
  2. Affected versionversion=21.4 <21.4R3-S12
  3. Affected versionversion=22.4 <22.4R3-S8
  4. Affected versionversion=23.2 <23.2R2-S5
  5. Affected versionversion=23.4 <23.4R2-S6
  6. Affected versionversion=24.2 <24.2R2-S3
  7. Affected versionversion=24.4 <24.4R2-S1
  8. Affected versionversion=25.2 <25.2R1-S1, 25.2R2

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Juniper Networks · Junos OSversion=0 <21.2R3-S10; version=21.4 <21.4R3-S12; version=22.4 <22.4R3-S8; version=23.2 <23.2R2-S5; version=23.4 <23.4R2-S6; version=24.2 <24.2R2-S3; version=24.4 <24.4R2-S1; version=25.2 <25.2R1-S1, 25.2R2

Published CVSS scores

7.5Juniper NetworksCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
8.7Juniper NetworksCVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:L

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-835Loop with Unreachable Exit Condition ('Infinite Loop')
T1499.004Application or System Exploitation

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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