The Linux kernel’s IPv4 FIB lookup incorrectly handles error routes when the CONFIG_IP_MULTIPLE_TABLES option is enabled and no rule is present. The logic flaw allows an error route in the merged local/main tables to be overwritten by a route from the default table, causing traffic that should be blocked to be routed incorrectly or failing to return a proper error. This inconsistent behavior can result in dropped or misrouted packets, degrading network reliability and exposing data to potential interception.

Systems running a Linux kernel with CONFIG_IP_MULTIPLE_TABLES enabled and that have multiple routing tables configured are affected. The vulnerability appears only when an error route is present in the merged tables; if no such route exists the bug does not manifest. Kernel developers have already incorporated a fix in later releases, so any deployment with an unpatched kernel in this configuration is vulnerable.

The CVSS score of 10 indicates a critical severity, and the EPSS score of <1% suggests a low exploitation probability, giving a more detailed risk assessment. The flaw requires the ability to add or modify routing rules, implying a local or privileged attacker. Once exploited, the impact is limited to network traffic handling, potentially causing denial of service or improper routing of user data. The vulnerability is not currently included in the CISA KEV catalog, but it warrants prompt remediation.