Junos OS: Use After Free
CVE-2026-21908 is a high-severity use after free vulnerability in Junos OS from Juniper Networks, with a CVSS 3.1 base score of 7.1 out of 10. It can be exploited from the same local network by an attacker with a low-privileged account, without any action from a user. The record lists versions 23.2R2-S5, 23.4R2-S6, 24.2R2-S3 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 0.31% chance of exploitation within 30 days, which is low compared with other CVEs.
High severity with no evidence of exploitation so far.
What to do
- Update Junos OS to 23.2R2-S5 or 23.4R2-S6 or 24.2R2-S3 or 24.4R2-S1, whichever matches your release line, or a later version.
- Update Junos OS Evolved to 23.2R2-S5-EVO or 23.4R2-S6-EVO or 24.2R2-S3-EVO or 24.4R2-S1-EVO, whichever matches your release line, or a later version.
- Read the advisory on kb.juniper.net for workarounds and exact fixed builds.
Affected products and versions
| Vendor | Product | Affected versions | Fixed in |
|---|---|---|---|
| Juniper Networks | Junos OS | 23.2R2-S1 up to, but not including, 23.2R2-S5; 23.4R2 up to, but not including, 23.4R2-S6; 24.2 up to, but not including, 24.2R2-S3; 24.4 up to, but not including, 24.4R2-S1; 25.2 up to, but not including, 25.2R1-S2, 25.2R2 | 23.2R2-S5, 23.4R2-S6, 24.2R2-S3, 24.4R2-S1, 25.2R1-S2, 25.2R2 |
| Juniper Networks | Junos OS Evolved | 23.2R2-S1 up to, but not including, 23.2R2-S5-EVO; 23.4R2 up to, but not including, 23.4R2-S6-EVO; 24.2 up to, but not including, 24.2R2-S3-EVO; 24.4 up to, but not including, 24.4R2-S1-EVO; 25.2 up to, but not including, 25.2R1-S2-EVO, 25.2R2-EVO | 23.2R2-S5-EVO, 23.4R2-S6-EVO, 24.2R2-S3-EVO, 24.4R2-S1-EVO, 25.2R1-S2-EVO, 25.2R2-EVO |
| Juniper Networks | Junos | 23.2; 23.4; 24.2; 24.4; 25.2 | Not stated |
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
- AdjacentThe attacker must be on the same local network segment.
- Attack complexity
- HighSuccess depends on conditions outside the attacker's control.
- Privileges required
- LowA basic user account is enough.
- User interaction
- NoneNo action by a victim is required.
- Scope
- UnchangedImpact stays within the vulnerable component.
- Confidentiality
- HighAll data handled by the component can be read.
- Integrity
- HighData or code can be modified at will.
- Availability
- HighThe component can be taken down completely.
Vector: CVSS:3.1/AV:A/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H. Scored by CNA (juniper.net).
| Scoring system | Score | Severity | Scored by |
|---|---|---|---|
| CVSS 4.0 | 7.5 | High | CNA (juniper.net) |
| CVSS 3.1 | 7.1 | High | CNA (juniper.net) |
Official description
A Use After Free vulnerability was identified in the 802.1X authentication daemon (dot1xd) of Juniper Networks Junos OS and Junos OS Evolved that could allow an authenticated, network-adjacent attacker flapping a port to crash the dot1xd process, leading to a Denial of Service (DoS), or potentially execute arbitrary code within the context of the process running as root. The issue is specific to the processing of a change in authorization (CoA) when a port bounce occurs. A pointer is freed but was then referenced later in the same code path. Successful exploitation is outside the attacker's direct control due to the specific timing of the two events required to execute the vulnerable code path. This issue affects systems with 802.1X authentication port-based network access control (PNAC) enabled. This issue affects: Junos OS: * from 23.2R2-S1 before 23.2R2-S5, * from 23.4R2 before 23.4R2-S6, * from 24.2 before 24.2R2-S3, * from 24.4 before 24.4R2-S1, * from 25.2 before 25.2R1-S2, 25.2R2; Junos OS Evolved: * from 23.2R2-S1 before 23.2R2-S5-EVO, * from 23.4R2 before 23.4R2-S6-EVO, * from 24.2 before 24.2R2-S3-EVO, * from 24.4 before 24.4R2-S1-EVO, * from 25.2 before 25.2R1-S2-EVO, 25.2R2-EVO.
Source: the CVE record as published by its numbering authority, via NVD and CVE.org.
The underlying weakness: Use After Free
The product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory belongs to the code that operates on the new pointer.
How developers prevent this class of flaw
- Choose a language that provides automatic memory management.
- When freeing pointers, be sure to set them to NULL once they are freed. However, the utilization of multiple or complex data structures may lower the usefulness of this strategy.
Timeline
- CVE record published.
- CISA published its SSVC assessment (exploitation: none, automatable: no, technical impact: total).
- Record last changed materially (score, affected products, status or references).
References
Other vulnerabilities in Junos OS
- 7.5 High severity
- 9.8 Critical severity
- 9.8 Critical severity
- 8.8 High severity
- 8.8 High severity
- 7.5 High severity
Recent Use After Free vulnerabilities
- 7.8 High severity
- 9.8 Critical severity
- 7.8 High severity
- 7.8 High severity
- 7.8 High severity
- 8.8 High severity
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