Junos OS (SRX firewalls)
CVE-2024-39561: SRX4600, SRX5000 Series: TCP packets with SYN/FIN or SYN/RST are transferred after enabling no-syn-check with Express Path
An Improper Check for Unusual or Exceptional Conditions vulnerability in the flow daemon (flowd) of Juniper Networks Junos OS on SRX4600 and SRX5000 Series allows an attacker to send TCP packets with SYN/FIN or SYN/RST flags, bypassing the expected blocking of these packets.
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 (SRX4600, SRX5000 Series) 0 | 21.2R3-S8 or later |
| Junos OS (SRX4600, SRX5000 Series) 21.4 | 21.4R3-S7 or later |
| Junos OS (SRX4600, SRX5000 Series) 22.1 | 22.1R3-S6 or later |
| Junos OS (SRX4600, SRX5000 Series) 22.2 | 22.2R3-S4 or later |
| Junos OS (SRX4600, SRX5000 Series) 22.3 | 22.3R3-S3 or later |
| Junos OS (SRX4600, SRX5000 Series) 22.4 | 22.4R3-S2 or later |
| Junos OS (SRX4600, SRX5000 Series) 23.2 | 23.2R2 or later |
| Junos OS (SRX4600, SRX5000 Series) 23.4 | 23.4R1-S1, 23.4R2 or later |
Always confirm against the vendor advisory, which lists every fixed hotfix.
What it is
An Improper Check for Unusual or Exceptional Conditions vulnerability in the flow daemon (flowd) of Juniper Networks Junos OS on
SRX4600 and SRX5000 Series
allows an attacker to send TCP packets with
SYN/FIN or SYN/RST
flags, bypassing the expected blocking of these packets.
A TCP packet with SYN/FIN or SYN/RST should be dropped in flowd. However, when no-syn-check and Express Path are enabled, these TCP packets are unexpectedly transferred to the downstream network.
This issue affects Junos OS on SRX4600 and SRX5000 Series:
- All versions before 21.2R3-S8,
- from 21.4 before 21.4R3-S7,
- from 22.1 before 22.1R3-S6,
- from 22.2 before 22.2R3-S4,
- from 22.3 before 22.3R3-S3,
- from 22.4 before 22.4R3-S2,
- from 23.2 before 23.2R2,
- from 23.4 before 23.4R1-S1, 23.4R2.
Am I affected?
This issue only affects systems with both no-syn-check and Express Path enabled:
[set security flow tcp-session no-syn-check]
[set security forwarding-options services-offload enable] (This can also be enabled per security policy)
Note: Automated Express Path is enabled by default starting with Junos OS 21.2R1.
Workarounds
This issue can be mitigated by disabling Express Path:
[set security forwarding-options services-offload disable]
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.