Security readout for executives and security teams
Plain-English summary
CVE-2026-21920 can let an unauthenticated network attacker interrupt affected Juniper SRX firewalls. A specially formatted DNS request can crash and restart the flowd process when DNS processing is enabled, causing temporary service disruption. This is a high availability risk for organizations relying on SRX devices for perimeter or internal traffic enforcement.
Executive priority
Prioritize remediation for SRX devices protecting critical connectivity, public-facing environments, or high-availability network paths. The main business risk is firewall service interruption, not data theft, but outage impact can be significant.
Technical view
The issue is an unchecked return value vulnerability, CWE-252, in the Junos OS DNS module on SRX Series. A crafted DNS request can crash flowd, which then restarts. Affected releases are Junos OS 23.4 before 23.4R2-S5, 24.2 before 24.2R2-S1, and 24.4 before 24.4R2. Versions before 23.4R1 are listed as unaffected.
Likely exposure
Exposure is limited to Juniper SRX Series devices running affected Junos OS versions and configured for DNS processing. The attacker is unauthenticated and network-based, so internet-facing or broadly reachable SRX services deserve priority review.
Exploitation context
The CVE record reports network attack vector, low complexity, no privileges, and no user interaction. The source bundle does not indicate known active exploitation, and CISA KEV status is false. Do not assume exploitation in the wild without additional evidence.
Researcher notes
The public record identifies availability impact through flowd crash and restart. It does not provide packet details, exploit artifacts, or workaround specifics in the supplied sources. Analysis should stay focused on version exposure, DNS processing configuration, and operational evidence of flowd instability.
Mitigation direction
- Upgrade affected Junos OS 23.4 deployments to 23.4R2-S5 or later.
- Upgrade affected Junos OS 24.2 deployments to 24.2R2-S1 or later.
- Upgrade affected Junos OS 24.4 deployments to 24.4R2 or later.
- Review Juniper advisory JSA106020 for deployment-specific guidance and any workarounds.
Validation and detection
- Inventory SRX Series devices and record Junos OS release versions.
- Identify SRX devices configured for DNS processing.
- Compare versions against the affected and fixed release ranges.
- Check device logs or monitoring for unexpected flowd crashes or restarts.
- After remediation, confirm devices run a fixed Junos OS release.
Public sources used
Automatically approved under Michael Williams's editorial policy. This source version was not individually reviewed.
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-252: 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-2026-21920 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/JSA106020CVE reference · vendor-advisory
- https://kb.juniper.net/JSA106020CVE 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.
Unchecked Return Value
Unchecked Return Value represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.