CVE-2021-0207: An improper interpretation conflict of certain data between certain software components within the Juniper Networks Junos OS devices does not allow certain traf

An improper interpretation conflict of certain data between certain software components within the Juniper Networks Junos OS devices does not allow certain traf

An improper interpretation conflict of certain data between certain software components within the Juniper Networks Junos OS devices does not allow certain traffic to pass through the device upon receipt from an ingress interface filtering certain specific types of traffic which is then being redirected to an egress interface on a different VLAN. This causes a Denial of Service (DoS) to those clients sending these particular types of traffic. Such traffic being sent by a client may appear genuine, but is non-standard in nature and should be considered as potentially malicious, and can be targeted to the device, or destined through it for the issue to occur. This issues affects IPv4 and IPv6 traffic. An indicator of compromise may be found by checking log files. You may find that traffic on the input interface has 100% of traffic flowing into the device, yet the egress interface shows 0 pps leaving the device. For example: [show interfaces "interface" statistics detail] Output between two interfaces would reveal something similar to: Ingress, first interface: -------------------- Interface Link Input packets (pps) Output packets (pps) et-0/0/0 Up 9999999999 (9999) 1 (0) ------------

Description from the National Vulnerability Database, published by sirt@juniper.net

What it means for a small business

Junos OS is network equipment or remote-access software: the box between your office and the internet. These are a favorite target because breaking one opens the whole office.

If an IT provider set it up, ask them to confirm the firmware version and the date it was updated. If nobody looks after it, that's the bigger problem.

How likely is an attack? The Exploit Prediction Scoring System, run by FIRST (the global association of security response teams), puts the odds of this one being exploited in the next 30 days at 1.3%, higher than 69% of CVEs. That’s above average: worth scheduling this week. Scored October 29, 2026, updated daily.

Check your own setup

Website check: reads what your site runs from the outside (WordPress version, plugins it can see, old server software) and matches it against this database.
Server check: what your web server says about itself, and whether that version has known flaws.

Sources

Part of the Huttenlocher Technologies vulnerability database, refreshed twice a day from CISA and the NVD. The title, category and plain-language guidance are ours; check the linked record before acting on a detail. This is general guidance, not a review of your systems.

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