CVE-2026-21909 Junos OS and Junos OS Evolved: Receipt of specific IS-IS update packet causes memory leak leading to RPD crash
An unauthenticated adjacent attacker can crash the routing daemon via IS-IS packets, causing a denial of service on Juniper devices.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Local or same network only
- Victim action needed
- Not needed
- Fix
- None listed
What is CVE-2026-21909?
CVE-2026-21909 is a high-severity denial of service vulnerability in Juniper Junos, published on 15 Jan 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Official description
A Missing Release of Memory after Effective Lifetime vulnerability in the routing protocol daemon (rpd) Juniper Networks Junos OS and Junos OS Evolved allows an unauthenticated attacker controlling an adjacent IS-IS neighbor to send a specific update packet causing a memory leak. Continued receipt and processing of these packets will exhaust all available memory, crashing rpd and creating a Denial of Service (DoS) condition.
Memory usage can be monitored through the use of the 'show task memory detail' command. For example:
user@junos> show task memory detail | match ted-infra TED-INFRA-COOKIE 25 1072 28 1184 229
user@junos>
show task memory detail | match ted-infra TED-INFRA-COOKIE 31 1360 34 1472 307
This issue affects:
Junos OS:
* from 23.2 before 23.2R2, * from 23.4 before 23.4R1-S2, 23.4R2, * from 24.1 before 24.1R2;
Junos OS Evolved:
* from 23.2 before 23.2R2-EVO, * from 23.4 before 23.4R1-S2-EVO, 23.4R2-EVO, * from 24.1 before 24.1R2-EVO.
This issue does not affect Junos OS versions before 23.2R1 or Junos OS Evolved versions before 23.2R1-EVO.
Weakness: CWE-401
Which versions are affected by CVE-2026-21909?
It affects 2 products.
Products include Juniper Junos, Juniper Junos Os Evolved.
How do I fix CVE-2026-21909?
No fix has been published for CVE-2026-21909 yet. Limit exposure of the affected software until one is available.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2026-21909 being exploited?
There is no known exploitation of CVE-2026-21909 in the wild and no public exploit code.
Other vulnerabilities in Juniper Junos
- CVE-2023-36845 Juniper Junos OS EX Series and SRX Series PHP External Variable Modification
- CVE-2020-1631 Juniper Junos OS Path Traversal
- CVE-2025-21590 Juniper Junos OS Improper Isolation or Compartmentalization
- CVE-2023-36851 Juniper Junos OS SRX Series Missing Authentication for Critical Function
- CVE-2023-36844 Juniper Junos OS EX Series PHP External Variable Modification
Sources
Also known as GHSA-cf3w-5gm7-wvp9.