Anthropic says Claude found a Linux kernel bug worth a root shell: Reveneau

Why it mattersPatch the kernel on any server that answers on an MPTCP port. A model can now find and chain real kernel bugs into working remote exploits.

A Linux server with a Multipath TCP port and no kernel patch is one connection from a root shell. Anthropic says its research model Claude found the race condition in the kernel's IPv4 TCP code. The company says Claude also wrote the exploit end to end. The bug is now CVE-2026-98432, patched upstream and backported to every stable branch.

What Anthropic says Claude did

Anthropic says Claude found a race condition in the Linux kernel's IPv4 TCP code. Claude then wrote an exploit for it. The exploit sends network packets to a server listening on Multipath TCP. The attacker needs no login. In Anthropic's own lab test, the proof of concept leaked a kernel pointer in 48 minutes. It then took five more minutes to reach a shell as root.

The bug is a use-after-free in the kernel's IP options memory. Anthropic says a failed attempt only fails the TCP handshake. It does not crash the server. The attacker can keep trying until it works.

The company says this is one of five attack chains Claude produced in one run. The others covered Chrome, Safari, Android and Windows. On Chrome, Anthropic says Google has assigned 62 CVEs so far. All are rated critical or high. Anthropic says 13 of them are sandbox escapes.

What the kernel teams did

The bug now has a public ID: CVE-2026-98432. Anthropic says it was patched in the upstream Linux kernel and backported to every stable branch that is still maintained. The fix is in the kernel, so an application patch does not help. A reboot into a fixed kernel does.

The Go security team sent an advisory to its users on 29 October. Neal Patel of the Go team wrote on the Google announce list. Go 1.24 and later turn on MPTCP by default for new listen sockets on Linux. A Go program on an affected kernel can open the bug without the author knowing.

The Go team's own words: upgrade the kernel. The team says a program-by-program fix is weak. One Go process left with MPTCP on is enough to make the host reachable. The team also says the bug may not be limited to MPTCP. Turning MPTCP off is not a full fix for Go.

What to do today

Three steps, from the openwall list and the kernel advisory. Check which Linux kernel every reachable server is running. Install the vendor's patched build. Reboot into the new kernel, so the running image carries the fix. The patched version is in Red Hat, Ubuntu, Debian and SUSE package trackers.

Servers with an MPTCP listener are first in line. That is the setup Anthropic says it tested. Internet-facing gateways, load balancers, Kubernetes nodes, VPN endpoints and reverse proxies that use Linux networking are next. The underlying race is not specific to MPTCP.

The point for a team that releases software is simple. A model now writes a chain of memory bugs into a working remote exploit on its own. The result is a real CVE on a widely used system. The next worm that uses this is unlikely to wait another 48 minutes.

Source

Anthropic, Discovering and exploiting vulnerabilities in critical, real-world software with Claude.
Go security team, Protecting against CVE-2026-98432 when using Go programs, forwarded to oss-security by Alan Coopersmith of Oracle.
CVE record: CVE-2026-98432 on cve.org.

This item was written by an AI system from the linked source. Reveneau is responsible for what it publishes.