CyberSecBulletin
CVE-2026-21918NVD status: Analyzed

Junos OS: Double Free

Published Updated By the CyberSecBulletin desk

CVE-2026-21918 is a high-severity double free vulnerability in Junos OS from Juniper Networks, with a CVSS 3.1 base score of 7.5 out of 10. It can be exploited remotely over the network by an unauthenticated attacker, without any action from a user. The record lists versions 22.4R3-S7, 23.2R2-S3, 23.4R2-S4 as the first without the flaw, so updating Junos OS to one of those releases or later removes the exposure. There is no confirmed exploitation so far. EPSS estimates a 0.41% chance of exploitation within 30 days, which is low compared with other CVEs.

Next patch cycle

High severity with no evidence of exploitation so far.

What to do

  1. Update Junos OS to 22.4R3-S7 or 23.2R2-S3 or 23.4R2-S4 or 24.2R2, whichever matches your release line, or a later version.
  2. Update Junos to 22.4 or a later version.
  3. Read the advisory on kb.juniper.net for workarounds and exact fixed builds.
  4. Until you can update, restrict network access to the affected service so it is not reachable from untrusted networks.

Affected products and versions

VendorProductAffected versionsFixed in
Juniper NetworksJunos OSbefore 22.4R3-S7; 23.2 up to, but not including, 23.2R2-S3; 23.4 up to, but not including, 23.4R2-S4; 24.2 up to, but not including, 24.2R222.4R3-S7, 23.2R2-S3, 23.4R2-S4, 24.2R2
Juniper NetworksJunosbefore 22.4; 22.4; 23.2; 23.4; 24.222.4

Version data comes from the organisation that assigned the CVE. Confirm against the vendor advisory before deciding you are unaffected.

How the severity score breaks down

Attack vector
NetworkCan be exploited remotely over a network, including the internet.
Attack complexity
LowNo special conditions are needed; the attack works reliably.
Privileges required
NoneNo account or login is needed.
User interaction
NoneNo action by a victim is required.
Scope
UnchangedImpact stays within the vulnerable component.
Confidentiality
NoneNo data is disclosed.
Integrity
NoneNothing can be modified.
Availability
HighThe component can be taken down completely.

Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H. Scored by CNA (juniper.net).

Scoring systemScoreSeverityScored by
CVSS 4.08.7HighCNA (juniper.net)
CVSS 3.17.5HighCNA (juniper.net)

Official description

A Double Free vulnerability in the flow processing daemon (flowd) of Juniper Networks Junos OS on SRX and MX Series allows an unauthenticated, network-based attacker to cause a Denial-of-Service (DoS). On all SRX and MX Series platforms, when during TCP session establishment a specific sequence of packets is encountered a double free happens. This causes flowd to crash and the respective FPC to restart. This issue affects Junos OS on SRX and MX Series: * all versions before 22.4R3-S7, * 23.2 versions before 23.2R2-S3, * 23.4 versions before 23.4R2-S4, * 24.2 versions before 24.2R2.

Source: the CVE record as published by its numbering authority, via NVD and CVE.org.

The underlying weakness: Double Free

The product calls free() twice on the same memory address.

How developers prevent this class of flaw

  • Choose a language that provides automatic memory management.
  • Ensure that each allocation is freed only once. After freeing a chunk, set the pointer to NULL to ensure the pointer cannot be freed again. In complicated error conditions, be sure that clean-up routines respect the state of allocation properly. If the language is object oriented, ensure that object destructors delete each chunk of memory only once.
  • Use a static analysis tool to find double free instances.

More Double Free vulnerabilities (CWE-415)

Timeline

  1. CVE record published.
  2. CISA published its SSVC assessment (exploitation: none, automatable: yes, technical impact: partial).
  3. Record last changed materially (score, affected products, status or references).

References

Other vulnerabilities in Junos OS

All Junos OS vulnerabilities

Recent Double Free vulnerabilities

Spotted an error? Tell us.