Unchecked Return Value in Junos OS SRX DNS Module Causes DoS (23.4R2S5, 24.2R2S1, 24.4R2)
CVE-2026-21920 Published on January 15, 2026
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.
Vulnerability Analysis
CVE-2026-21920 is exploitable with network access, and does not require authorization privileges or user interaction. This vulnerability is considered to have a low attack complexity. The potential impact of an exploit of this vulnerability is considered to have no impact on confidentiality and integrity, and a high impact on availability.
Weakness Type
Unchecked Return Value
The software does not check the return value from a method or function, which can prevent it from detecting unexpected states and conditions. Two common programmer assumptions are "this function call can never fail" and "it doesn't matter if this function call fails". If an attacker can force the function to fail or otherwise return a value that is not expected, then the subsequent program logic could lead to a vulnerability, because the software is not in a state that the programmer assumes. For example, if the program calls a function to drop privileges but does not check the return code to ensure that privileges were successfully dropped, then the program will continue to operate with the higher privileges.
Products Associated with CVE-2026-21920
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Affected Versions
Juniper Networks Junos OS:- Version 23.4 and below 23.4R2-S5 is affected.
- Version 24.2 and below 24.2R2-S1 is affected.
- Version 24.4 and below 24.4R2 is affected.
- Before 23.4R1 is unaffected.
Exploit Probability
EPSS (Exploit Prediction Scoring System) scores estimate the probability that a vulnerability will be exploited in the wild within the next 30 days. The percentile shows you how this score compares to all other vulnerabilities.