Security readout for executives and security teams
Plain-English summary
CVE-2026-21918 can let an unauthenticated remote attacker crash Juniper SRX or MX packet-processing components by triggering a TCP session-establishment edge case. The result is denial of service: flowd crashes and the affected FPC restarts. For businesses relying on these devices for firewalling, routing, or perimeter connectivity, this can interrupt network availability.
Executive priority
Treat as a high-priority availability risk for Juniper SRX/MX environments. It is remotely reachable and unauthenticated, but current sources only support denial of service and do not show known active exploitation. Patch critical network paths promptly to reduce outage risk.
Technical view
Juniper describes a CWE-415 double-free flaw in Junos OS flowd on SRX and MX Series. A specific TCP packet sequence during session establishment causes flowd to crash and the respective FPC to restart. Affected releases are before 22.4R3-S7, 23.2R2-S3, 23.4R2-S4, and 24.2R2. CVSS v4.0 score is 8.7.
Likely exposure
Exposure is likely where affected SRX or MX Series devices process untrusted TCP traffic, especially internet-facing or perimeter deployments. Devices not running Junos OS, not in the SRX/MX families, or already on the fixed releases listed by Juniper are not indicated as affected by the provided sources.
Exploitation context
The sources describe unauthenticated, network-based exploitation with low attack complexity and no user interaction. They do not report active exploitation, and the CVE is not marked KEV in the provided bundle. The public information supports denial of service impact, not data theft or code execution.
Researcher notes
Key constraints: affected product scope is Junos OS on SRX and MX Series only. The described trigger is a TCP sequence during session establishment; avoid assuming broader protocol or device impact. No exploit publication or KEV evidence is provided. Validate remediation using Juniper’s fixed-release thresholds.
Mitigation direction
- Upgrade affected SRX and MX devices to Juniper’s fixed Junos OS releases or later.
- Prioritize internet-facing and critical-path SRX/MX devices first.
- Review Juniper advisory JSA106018 for any platform-specific guidance.
- If immediate upgrade is not possible, monitor affected devices for flowd crashes and FPC restarts.
Validation and detection
- Inventory Juniper SRX and MX devices running Junos OS.
- Compare installed Junos OS versions against the affected version ranges.
- Confirm upgraded devices run 22.4R3-S7, 23.2R2-S3, 23.4R2-S4, 24.2R2, or later as applicable.
- Review operational logs for flowd crash or FPC restart events.
- Track Juniper JSA106018 for updates or additional remediation details.
Public sources used
Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.
Conservative CVE-to-ATT&CK context
These mappings and lookup hints may be relevant to the vulnerability behavior, CWE, affected product, or exposure path. Glexia-inferred context is not an official MITRE, ATT&CK, CWE, or CVE Program mapping.
ATT&CK lookup starting points
Use these exact CWE pages and searches to review the Glexia ATT&CK library from this CVE's weakness and description context.
CWE-415: Exact CWE lookup
Use the exact CWE identifier as the starting point before reviewing related ATT&CK behavior. Open the exact CWE lookup page first, then review the ATT&CK searches from that MITRE weakness context. This is a Glexia lookup hint, not an official ATT&CK mapping.
Open ATT&CK lookupCVE-2026-21918 mapping review
Open the CVE-to-ATT&CK bridge for reviewed, inferred, or future official mappings tied to this CVE.
Open ATT&CK lookup- Severity
- High
- CVSS
- 8.7 (4.0)
- Known Exploited
- No
- Published
Vector: CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:L/AU:Y/R:A/RE:M
CNA and ADP enrichment extracted from CVE v5
These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.
CVSS vector scores
1 official scoreWe collect every scored CVSS vector available in the official CNA and ADP containers. When more than one version is present, the table keeps the source vectors side by side instead of collapsing them into the highest score.
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:L/AU:Y/R:A/RE:M——Primary CVE scoreVulnerability scoring details
Base CVSS 4.0 score
8.7HighVector: CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:L/AU:Y/R:A/RE:M
Source materials
- CVE List V5 sourceCVE List V5
- https://supportportal.juniper.net/JSA106018CVE reference · vendor-advisory
- https://kb.juniper.net/JSA106018CVE reference · vendor-advisory
Products and packages named in the record
CWE details
CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.
Double Free
Double Free represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.