An anonymous reader quotes a report from Ars Technica: Researchers have analyzed a high-severity vulnerability in Linux that's able to escalate untrusted users to root by exploiting a bug you don't often see: a single errant character inside the kernel. The vulnerability, tracked as CVE-2026-23111, is located in nf_tables, a subsystem of the Linux kernel that provides packet filtering capabilities. It's used to manage firewall rules and replaces older subsystems such as iptables, ip6tables, arptables, and ebtables.

The presence of a single mis-issued exclamation point in code implementing nf_tables introduced a use-after-free, a class of vulnerability that corrupts memory by placing malicious code at memory addresses that haven't been properly freed of their previous contents. CVE-2026-23111 can be exploited by an unprivileged user or process to elevate system rights to root. The exploit works by disrupting the deletion of verdicts -- a determination within the nf_tables framework that determines if a packet matches a rule calling for a certain action to be performed. This process can use what are known as catchall elements, which act as a wildcard in the event a lookup doesn't match any other element in the set.

When a verdict map is deleted from memory, catchall elements are deactivated and a chain's reference counter is decremented. When errors occur the deletion can be reversed and the counter incremented. CVE-2026-53111 allows for that process to be altered. As a result, the exploit can decrement the variable an arbitrary number of times and then delete and free the chain when some objects still point to it. Although the kernel vulnerability was fixed in February, multiple proof-of-concept exploits have since emerged, including one from FuzzingLabs in April and another from Exodus Intelligence that works on Debian and Ubuntu.

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