Affects widely deployed Juniper SRX firewall DNS service; DoS could disrupt enterprise networks.
CVE-2026-21920
Unauthenticated DNS DoS on Junos SRX; crafted DNS requests crash flowd.
Is CVE-2026-21920 being exploited?
Not confirmed. CVE-2026-21920 does not appear in CISA's Known Exploited Vulnerabilities catalog, which records only exploitation that has been observed and reported publicly. That is evidence of absence of a report, not evidence the flaw is unattacked. EPSS currently estimates a 0.49% probability of exploitation in the next 30 days.
How severe is CVE-2026-21920?
CVE-2026-21920 is rated High with a CVSS score of 8.7. Severity describes how bad exploitation would be, not how likely it is: pair it with exploitation evidence before deciding what to patch first.
Is there a patch for CVE-2026-21920?
Yes. A fix has been recorded for CVE-2026-21920. The vendor advisory is the authority on the exact fixed version — apply it from there rather than from a summary.
What does CVE-2026-21920 affect?
CVE-2026-21920 affects Juniper Junos OS on SRX Series, Versions affected: 23.4 before 23.4R2-S5, Versions affected: 24.2 before 24.2R2-S1, Versions affected: 24.4 before 24.4R2. Confirm the exact affected versions against the vendor advisory before deciding you are exposed.
What should I do about CVE-2026-21920?
Upgrade to fixed releases: 23.4R2-S5+, 24.2R2-S1+, or 24.4R2+
Exploitation status reflects CISA's KEV catalog as we last synced it. Check the catalog directly.
Unauthenticated attacker can crash flowd via malformed DNS requests, causing DNS service DoS and restart.
Patch when possible
- affected>= 23.4 and < 23.4R2-S5
- affected>= 24.2 and < 24.2R2-S1
- affected>= 24.4 and < 24.4R2
As published in the CVE Program record. A version outside these ranges is not a statement that it is unaffected — vendors sometimes understate a range, and distribution-backported builds carry upstream numbers that do not reflect what was patched into them.