CVE-2026-21920 | Junos OS: SRX Series: If a specific request is processed by the DNS subsystem flowd will crash

An Unchecked Return Value vulnerability in the DNS module of Juniper Networks Junos OS on SRX Series allows an unauthenticated, network-based attacker to cause a Denial-of-Service (DoS). If an SRX Series device configured for DNS processing, receives a specifically formatted DNS request flowd will crash and restart, which causes a service interruption until the process has recovered. This issue affects Junos OS on SRX Series: * 23.4 versions before 23.4R2-S5, * 24.2 versions before 24.2R2-S1, * 24.4 versions before 24.4R2. This issue does not affect Junos OS versions before 23.4R1.

Published: 2026-01-15 Last update: 2026-01-23 Assigner: [email protected] Source: [email protected]
NVD Status：Analyzed，CVE State: published
View at NVD, CVE.org, EUVD

CVE-2026-21920 is rated Low Risk (36.1/100): CVSS High severity, with low exploitation likelihood (EPSS 0.02%). Monitor for updates and reassess as exploit intelligence or EPSS changes.

Risk is dynamic; we continuously reassess and refresh what is shown on this page as upstream context changes.

Exploit prediction scoring system (EPSS) score for CVE-2026-21920

EPSS lead: Daily EPSS estimates relative likelihood of exploitation; percentile ranks this CVE among scored vulnerabilities (higher = more severe relative rank).

# Date Old EPSS score New EPSS score Delta (New - Old)
1 2026-02-18 0.05% 0.02% -0.04%
2 2026-02-16 0.05% 0.05% +0.00%
3 2026-01-24 — 0.05% —

Full EPSS history (5 records total)

Common vulnerability scoring system (CVSS) metrics for CVE-2026-21920

CVSS metrics for this CVE.

Base score Version Severity Vector Exploitability Impact Score source
8.7 4.0 HIGH
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/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:Y/R:A/V:X/RE:M/U:X Click to expand
Attack vector (AV:N)
Could be attacked over the internet or any normal routed network.
Attack complexity (AC:L)
Exploitation conditions are straightforward and stable.
Attack requirements (AT:N)
No additional preconditions are required beyond normal reachability.
Privileges required (PR:N)
No privileges are required.
User interaction (UI:N)
No user interaction is required.
Vulnerable system confidentiality impact (VC:N)
No confidentiality impact on the vulnerable system.
Vulnerable system integrity impact (VI:N)
No integrity impact on the vulnerable system.
Vulnerable system availability impact (VA:H)
High availability impact on the vulnerable system.
Subsequent system confidentiality impact (SC:N)
No confidentiality impact on subsequent systems.
Subsequent system integrity impact (SI:N)
No integrity impact on subsequent systems.
Subsequent system availability impact (SA:L)
Limited availability impact on subsequent systems.
Exploit maturity (threat) (E:X)
Not defined: no reliable threat intelligence; scoring assumes the worst case (equivalent to Attacked).
Confidentiality requirement (CR:X)
Not defined: insufficient information; scoring treats this like High (worst case).
Integrity requirement (IR:X)
Not defined: insufficient information; scoring treats this like High (worst case).
Availability requirement (AR:X)
Not defined: insufficient information; scoring treats this like High (worst case).
Modified attack vector (MAV:X)
Not defined: scoring uses the Base Attack Vector (AV).
Modified attack complexity (MAC:X)
Not defined: scoring uses the Base Attack Complexity (AC).
Modified attack requirements (MAT:X)
Not defined: scoring uses the Base Attack Requirements (AT).
Modified privileges required (MPR:X)
Not defined: scoring uses the Base Privileges Required (PR).
Modified user interaction (MUI:X)
Not defined: scoring uses the Base User Interaction (UI).
Modified vulnerable system confidentiality impact (MVC:X)
Not defined: scoring uses the Base VC metric.
Modified vulnerable system integrity impact (MVI:X)
Not defined: scoring uses the Base VI metric.
Modified vulnerable system availability impact (MVA:X)
Not defined: scoring uses the Base VA metric.
Modified subsequent system confidentiality impact (MSC:X)
Not defined: scoring uses the Base SC metric.
Modified subsequent system integrity impact (MSI:X)
Not defined: scoring uses the Base SI metric.
Modified subsequent system availability impact (MSA:X)
Not defined: scoring uses the Base SA metric.
Safety (supplemental) (S:X)
Not evaluated.
Automatable (supplemental) (AU:Y)
Yes: all four kill-chain steps (reconnaissance, weaponization, delivery, exploitation) can be automated.
Recovery (supplemental) (R:A)
Automatic: services recover on their own after an attack.
Value density (supplemental) (V:X)
Not evaluated.
Vulnerability response effort (supplemental) (RE:M)
Moderate effort (remote update, disable subsystem, simple upgrade with minor service impact).
Provider urgency (supplemental) (U:X)
Not evaluated.
 — — [email protected]
7.5 3.1 HIGH
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H Click to expand
Attack vector (AV:N)
Could be attacked over the internet or any normal routed network—not just someone sitting at the machine.
Attack complexity (AC:L)
Once they can reach the bug, pulling it off is straightforward—no weird race conditions or rare setup.
Privileges required (PR:N)
No account or special rights needed—anonymous or random user is enough.
User interaction (UI:N)
Nobody has to click “OK” or open a trap file; it can work without a victim helping.
Scope (S:U)
Damage stays in the same “trust bubble” as the broken component—no big spill into unrelated systems.
Confidentiality (C:N)
Doesn’t really leak secrets in a meaningful way.
Integrity (I:N)
Data isn’t meaningfully altered or forged.
Availability (A:H)
Could take the service down hard or make it unusable for people who depend on it.
 3.9 3.6 [email protected]

Weakness enumeration for CVE-2026-21920

Affected software / configurations for CVE-2026-21920

Vendor Product Version Raw CPE
juniper junos 23.4 cpe:2.3:o:juniper:junos:23.4:-:*:*:*:*:*:*
juniper junos 23.4 cpe:2.3:o:juniper:junos:23.4:r1:*:*:*:*:*:*
juniper junos 23.4 cpe:2.3:o:juniper:junos:23.4:r1-s1:*:*:*:*:*:*
juniper junos 23.4 cpe:2.3:o:juniper:junos:23.4:r1-s2:*:*:*:*:*:*
juniper junos 23.4 cpe:2.3:o:juniper:junos:23.4:r2:*:*:*:*:*:*
juniper junos 23.4 cpe:2.3:o:juniper:junos:23.4:r2-s1:*:*:*:*:*:*
juniper junos 23.4 cpe:2.3:o:juniper:junos:23.4:r2-s2:*:*:*:*:*:*
juniper junos 23.4 cpe:2.3:o:juniper:junos:23.4:r2-s3:*:*:*:*:*:*
juniper junos 23.4 cpe:2.3:o:juniper:junos:23.4:r2-s4:*:*:*:*:*:*
juniper junos 24.2 cpe:2.3:o:juniper:junos:24.2:-:*:*:*:*:*:*
juniper junos 24.2 cpe:2.3:o:juniper:junos:24.2:r1:*:*:*:*:*:*
juniper junos 24.2 cpe:2.3:o:juniper:junos:24.2:r1-s1:*:*:*:*:*:*
juniper junos 24.2 cpe:2.3:o:juniper:junos:24.2:r1-s2:*:*:*:*:*:*
juniper junos 24.2 cpe:2.3:o:juniper:junos:24.2:r2:*:*:*:*:*:*
juniper junos 24.4 cpe:2.3:o:juniper:junos:24.4:-:*:*:*:*:*:*
juniper junos 24.4 cpe:2.3:o:juniper:junos:24.4:r1:*:*:*:*:*:*
juniper junos 24.4 cpe:2.3:o:juniper:junos:24.4:r1-s2:*:*:*:*:*:*
juniper junos 24.4 cpe:2.3:o:juniper:junos:24.4:r1-s3:*:*:*:*:*:*

References for CVE-2026-21920

cvelogic Threat Intelligence