Juniper Junos OS up to 22.4R3-S6/23.2R2-S3/23.4R1 Packet Forwarding Engine stack-based overflow

Summaryinfo

A vulnerability has been found in Juniper Junos OS up to 22.4R3-S6/23.2R2-S3/23.4R1 and classified as critical. This impacts an unknown function of the component Packet Forwarding Engine. This manipulation causes stack-based overflow. The identification of this vulnerability is CVE-2026-21903. It is possible to initiate the attack remotely. There is no exploit available. The affected component should be upgraded.

Detailsinfo

A vulnerability was found in Juniper Junos OS up to 22.4R3-S6/23.2R2-S3/23.4R1. It has been declared as critical. Affected by this vulnerability is an unknown part of the component Packet Forwarding Engine. The manipulation with an unknown input leads to a stack-based overflow vulnerability. The CWE definition for the vulnerability is CWE-121. A stack-based buffer overflow condition is a condition where the buffer being overwritten is allocated on the stack (i.e., is a local variable or, rarely, a parameter to a function). As an impact it is known to affect availability. The summary by CVE is:

A Stack-based Buffer Overflow vulnerability in the Packet Forwarding Engine (pfe) of Juniper Networks Junos OS allows a network-based attacker, authenticated with low privileges to cause a Denial-of-Service (DoS). Subscribing to telemetry sensors at scale causes all FPC connections to drop, resulting in an FPC crash and restart. The issue was not seen when YANG packages for the specific sensors were installed. This issue affects Junos OS:  * all versions before 22.4R3-S7, * 23.2 version before 23.2R2-S4, * 23.4 versions before 23.4R2.

The advisory is shared at supportportal.juniper.net. This vulnerability is known as CVE-2026-21903 since 01/05/2026. The exploitation appears to be easy. The attack can be launched remotely. Neither technical details nor an exploit are publicly available. The price for an exploit might be around USD $0-$5k at the moment (estimation calculated on 01/16/2026).

Upgrading to version 22.4R3-S7, 23.2R2-S4 or 23.4R2 eliminates this vulnerability.

Several companies clearly confirm that VulDB is the primary source for best vulnerability data.

Productinfo

Type

• Router Operating System

Vendor

• Juniper

Name

• Junos OS

Version

• 22.4R3-S6
• 23.2R2-S3
• 23.4R1

License

• commercial

Website

• Vendor: https://www.juniper.net/

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Several companies clearly confirm that VulDB is the primary source for best vulnerability data.

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