CVE-2026-21920 Overview
An Unchecked Return Value vulnerability (CWE-252) has been identified in the DNS module of Juniper Networks Junos OS running on SRX Series firewalls. This vulnerability allows an unauthenticated, network-based attacker to cause a Denial-of-Service (DoS) condition by sending specially crafted DNS requests to affected devices.
When an SRX Series device configured for DNS processing receives a specifically formatted DNS request, the flowd process will crash and restart. This causes a service interruption that persists until the process has fully recovered, potentially disrupting critical network security functions.
Critical Impact
Unauthenticated attackers can remotely crash the flowd process on Juniper SRX Series firewalls, causing service disruption to DNS processing and potentially impacting network security operations.
Affected Products
- Juniper Networks Junos OS on SRX Series: 23.4 versions before 23.4R2-S5
- Juniper Networks Junos OS on SRX Series: 24.2 versions before 24.2R2-S1
- Juniper Networks Junos OS on SRX Series: 24.4 versions before 24.4R2
Discovery Timeline
- January 15, 2026 - CVE-2026-21920 published to NVD
- January 16, 2026 - Last updated in NVD database
Technical Details for CVE-2026-21920
Vulnerability Analysis
This vulnerability stems from an Unchecked Return Value condition (CWE-252) in the DNS processing module of Junos OS. The flowd daemon, which handles packet forwarding and security processing on SRX Series devices, fails to properly validate return values when processing certain DNS requests. This oversight allows malformed DNS packets to trigger an unhandled exception that crashes the process.
The attack can be executed remotely over the network without authentication, and no user interaction is required. The vulnerability specifically affects SRX Series firewalls that have DNS processing enabled, making it particularly concerning for organizations using these devices as DNS proxies or for DNS security inspection.
Importantly, this vulnerability does not affect Junos OS versions before 23.4R1, indicating the issue was introduced in more recent code changes to the DNS processing module.
Root Cause
The root cause is an Unchecked Return Value vulnerability (CWE-252) within the DNS module's packet processing logic. When the flowd process receives a specifically formatted DNS request, it fails to check the return value of a critical function call. This unchecked return value leads to subsequent operations being performed on invalid or unexpected data, ultimately causing the process to crash.
The vulnerability occurs because the code assumes successful execution of DNS parsing functions without verifying the actual result. When a malformed DNS packet is processed, the function returns an error condition that goes unhandled, leading to a null pointer dereference or similar fatal error condition.
Attack Vector
The attack is network-based and requires no authentication or user interaction. An attacker can exploit this vulnerability by:
- Identifying an SRX Series firewall with DNS processing enabled
- Crafting a malformed DNS request designed to trigger the unchecked return value condition
- Sending the malicious DNS packet to the target device
- The flowd process crashes and restarts, causing service disruption
The vulnerability affects devices configured for DNS processing, such as those acting as DNS proxies, performing DNS security inspection, or otherwise handling DNS traffic through the flowd process.
Since no verified proof-of-concept code is available, the specific packet structure required to exploit this vulnerability is not publicly documented. Refer to the Juniper Security Advisory JSA106020 for additional technical details.
Detection Methods for CVE-2026-21920
Indicators of Compromise
- Unexpected flowd process crashes and restarts observed in system logs
- Repeated service interruptions affecting DNS processing capabilities
- Unusual DNS traffic patterns or malformed DNS packets in network captures
- System log entries indicating process recovery cycles for the flow daemon
Detection Strategies
- Monitor SRX Series system logs for flowd crash events and automatic restarts
- Implement DNS traffic analysis to identify anomalous or malformed DNS requests
- Deploy network intrusion detection rules to flag suspicious DNS packet structures
- Review Junos OS system event logs for patterns consistent with DoS attacks
Monitoring Recommendations
- Configure SNMP traps or syslog alerts for flowd process restart events
- Establish baseline metrics for DNS processing performance to detect degradation
- Implement network flow analysis to identify potential DoS attack sources
- Monitor device availability and response times for early detection of service disruption
How to Mitigate CVE-2026-21920
Immediate Actions Required
- Identify all SRX Series devices running affected Junos OS versions (23.4 before 23.4R2-S5, 24.2 before 24.2R2-S1, 24.4 before 24.4R2)
- Review device configurations to determine if DNS processing is enabled
- Apply the appropriate firmware updates from Juniper Networks as soon as possible
- Implement network access controls to limit DNS traffic sources to trusted networks
Patch Information
Juniper Networks has released patched versions to address this vulnerability. Organizations should upgrade to the following fixed versions:
- Junos OS 23.4R2-S5 or later for the 23.4 release train
- Junos OS 24.2R2-S1 or later for the 24.2 release train
- Junos OS 24.4R2 or later for the 24.4 release train
For detailed patching instructions and download links, refer to the Juniper Security Advisory JSA106020 or the Juniper Support Portal.
Workarounds
- If DNS processing is not required, disable DNS-related features on affected SRX Series devices
- Implement strict access control lists (ACLs) to restrict DNS traffic to known trusted sources
- Deploy upstream DNS filtering or rate limiting to reduce exposure to malformed packets
- Consider using an alternative DNS proxy solution until patches can be applied
# Configuration example - Restrict DNS traffic to trusted sources only
# Apply on SRX Series to limit exposure until patching is complete
set security zones security-zone untrust host-inbound-traffic system-services dns exclude
set firewall filter DNS-PROTECT term ALLOW-TRUSTED from source-prefix-list TRUSTED-DNS-SOURCES
set firewall filter DNS-PROTECT term ALLOW-TRUSTED from protocol udp
set firewall filter DNS-PROTECT term ALLOW-TRUSTED from destination-port 53
set firewall filter DNS-PROTECT term ALLOW-TRUSTED then accept
set firewall filter DNS-PROTECT term DENY-ALL then discard
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
