Junos OS (SRX firewalls)
CVE-2026-21918: SRX and MX Series: When TCP packets occur in a specific sequence flowd crashes
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).
Published
Automatic summary from the vendor's CVE record. We haven't written a full analysis of this vulnerability yet.
Affected and fixed versions
| Product / branch | Fixed in |
|---|---|
| Junos OS (SRX Series, MX Series) 0 | 22.4R3-S7 or later |
| Junos OS (SRX Series, MX Series) 23.2 | 23.2R2-S3 or later |
| Junos OS (SRX Series, MX Series) 23.4 | 23.4R2-S4 or later |
| Junos OS (SRX Series, MX Series) 24.2 | 24.2R2 or later |
Always confirm against the vendor advisory, which lists every fixed hotfix.
What it is
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.
Workarounds
There are no known workarounds for this issue.
Exploitation
Juniper SIRT is not aware of any malicious exploitation of this vulnerability.
Sources
KEV status, EPSS score and vendor data refreshed automatically, last on 13 Oct 2026.