CVE-2026-21918: Junos OS: SRX and MX Series: When TCP packets occur in a specific sequence flowd crashes

Severity: high (CVSS 7.5 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.4% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-01-15 · updated 2026-06-17
Weakness: CWE-415

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.

Affected products

Juniper Junos OS: before 22.4; 22.4, 22.4-r1, 22.4-r1-s1, 22.4-r1-s2, 22.4-r2, 22.4-r2-s1 (+27) — fixed in 22.4

How to fix

Upgrade to a fixed version: Juniper Junos OS 22.4. Check the vendor advisory for mitigations.

References

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