A critical vulnerability in the Linux kernel allows unauthenticated remote attackers to execute arbitrary commands as root on servers running Multipath TCP (MPTCP) listeners. Tracked as CVE-2026-98432 and rated 9.8 out of 10 for severity by the Linux kernel CVE team, the flaw also affects ordinary TCP connections, though remote code execution has only been demonstrated on MPTCP-enabled systems.
Anthropic reported that an unreleased Claude AI model developed an exploit that bypassed kernel address space layout randomization (KASLR) and executed arbitrary commands as root on a Linux server with an MPTCP listener. According to Anthropic, the exploit took about 53 minutes to achieve remote root code execution. The flaw is a use-after-free vulnerability in the kernel's IPv4 TCP networking code, stemming from a race condition in how connection requests are handled.
The vulnerability affects Linux kernels dating back to version 4.4, including widely used distributions such as Ubuntu, Red Hat Enterprise Linux (RHEL), and Rocky Linux. “tcp_v4_syn_recv_sock() transfers ownership of ireq- ireq_opt to the child socket (newinet- inet_opt) without copying it,” the Linux kernel advisory explains. “Another cpu can concurrently process a retransmitted SYN for the same request socket, and send a SYNACK from tcp_check_req().”
Red Hat classifies the vulnerability as a potential denial-of-service issue that could cause a kernel crash, assigning it a lower severity score of 5.9 because exploitation is complex. However, Anthropic demonstrated that the vulnerability could be exploited for more than denial of service under specific conditions.
Multipath TCP (MPTCP) is a TCP extension that allows a single connection to use multiple network paths simultaneously, commonly in environments where devices have multiple network connections, such as Wi-Fi and cellular networks. MPTCP is not usually enabled by default in applications, but some software enables it automatically. Beginning in Go 1.24, the language enables MPTCP by default for TCP listeners on Linux if the kernel supports it, so some Go-based network services may enable MPTCP listeners without administrators' knowledge.
The vulnerability has been fixed in the upstream Linux kernel and backported to maintained branches, but not all distributions have released updates. As of October 30, Ubuntu 22.04 LTS, 24.04 LTS, 26.04 LTS, and 26.10 remain vulnerable, with fixes in progress. RHEL 8, 9, and 10 are affected, with fixes planned for future updates; RHEL 7 is not affected. Debian has fixed the issue in kernel package versions 6.1.190-1 for Debian 12 and 6.12.113-1 for Debian 13. Oracle Linux, Amazon Linux, and SUSE have also released fixes for some kernel versions.
In mitigation guidance, Rocky Linux support provider CIQ recommends temporarily blocking IPv4 TCP packets containing IP options before they reach the vulnerable TCP processing code and provides example nftables rules. CIQ cautions that disabling MPTCP alone is insufficient because the underlying vulnerability can still be triggered through ordinary TCP traffic. The vulnerability does not affect macOS. There are currently no confirmed reports of exploitation in the wild.
Analysis
Why This Matters
- The flaw allows unauthenticated remote attackers to execute code as root on Linux servers running MPTCP, a feature increasingly enabled by default in Go 1.24 and later.
- Many major Linux distributions have not yet released patches, leaving a window of exposure for affected servers.
- The demonstrated exploit bypasses KASLR, a key memory protection, showing that the vulnerability is more serious than the denial-of-service classification some vendors gave.
Background
MPTCP is an extension to the TCP protocol that improves performance and reliability by allowing a single connection to use multiple network paths simultaneously. It is often used in devices with both Wi-Fi and cellular connections. CVE-2026-98432 is a use-after-free vulnerability in the Linux kernel's IPv4 TCP stack that arises from a race condition in handling SYN packets. The Linux kernel CVE team assigned it a severity score of 9.8 (critical), while Red Hat assigned a lower score of 5.9 due to the complexity of exploitation.
Key Perspectives
Linux kernel CVE team: Rated the vulnerability 9.8 out of 10, reflecting its potential for remote, unauthenticated code execution.
Red Hat: Classified the flaw as a potential denial-of-service issue with a severity of 5.9, arguing that exploitation is complex.
Anthropic researchers: Demonstrated a working exploit that achieved remote root code execution in about 53 minutes, showing the flaw is exploitable beyond denial of service.
CIQ (Rocky Linux support provider): Warns that disabling MPTCP alone is insufficient and recommends blocking IPv4 TCP packets with IP options as a temporary mitigation.
What to Watch
- Whether exploits appear in the wild targeting unpatched systems, especially those running Go 1.24 services that may have MPTCP listeners enabled unknowingly.
- How quickly major distributions (Ubuntu, RHEL, Rocky Linux) release kernel updates for all affected versions.
- Whether additional attack vectors emerge for ordinary TCP connections now that the underlying race condition is public.
Checked
12 claims in this story checked against 1 source: 12 supported.
Show each claim
- SupportedThe headline says a critical Linux kernel flaw allows a remote root exploit via MPTCP.BleepingComputer: “Security researchers at Anthropic have developed an exploit for a critical Linux kernel vulnerability that allows unauthenticated attackers to remotely execute commands as root on servers running Multipath TCP (MPTCP) listeners.”
- SupportedAnthropic researchers demonstrated an exploit for the flaw.BleepingComputer: “However, researchers at Anthropic demonstrated that the vulnerability could be exploited for remote code execution under specific conditions.”
- SupportedSome distributions still lack fixes.BleepingComputer: “However, security updates have not yet been pushed out to all affected Linux distributions.”
- SupportedUnauthenticated remote attackers can execute commands as root on servers running MPTCP listeners.BleepingComputer: “allows unauthenticated attackers to remotely execute commands as root on servers running Multipath TCP (MPTCP) listeners.”
- SupportedThe flaw is tracked as CVE-2026-98432.BleepingComputer: “The flaw, tracked as CVE-2026-98432 , also affects ordinary TCP connections”
- SupportedThe Linux kernel CVE team rated the flaw 9.8 out of 10 for severity.BleepingComputer: “CVE-2026-98432 is a use-after-free vulnerability in the kernel's IPv4 TCP networking code that received a severity score of 9.8 out of 10 from the Linux kernel CVE team.”
- SupportedThe flaw also affects ordinary TCP connections.BleepingComputer: “The flaw, tracked as CVE-2026-98432 , also affects ordinary TCP connections”
- SupportedRemote code execution has not been publicly demonstrated on servers without an MPTCP listener.BleepingComputer: “although remote code execution has not been publicly demonstrated on servers without an MPTCP listener.”
- SupportedAnthropic said an unreleased Claude AI model developed an exploit that bypassed KASLR and executed arbitrary commands as root on a Linux server with an MPTCP listener.BleepingComputer: “An Anthropic report explained that an unreleased Claude AI model developed an exploit that bypassed kernel address space layout randomization (KASLR) and executed arbitrary commands as root on a Linux server running an MPTCP listener.”
- SupportedAnthropic said the exploit took about 53 minutes to achieve remote root code execution.BleepingComputer: “According to Anthropic, the exploit took about 53 minutes to achieve remote root code execution”
- SupportedThe story quotes the Linux kernel advisory: “tcp_v4_syn_recv_sock() transfers ownership of ireq- ireq_opt to the child socket (newinet- inet_opt) without copying it,”BleepingComputer: “"tcp_v4_syn_recv_sock() transfers ownership of ireq- ireq_opt to the child socket (newinet- inet_opt) without copying it," reads the advisory.”
- SupportedThe story quotes the Linux kernel advisory: “Another cpu can concurrently process a retransmitted SYN for the same request socket, and send a SYNACK from tcp_check_req().”BleepingComputer: “"Another cpu can concurrently process a retransmitted SYN for the same request socket, and send a SYNACK from tcp_check_req(). tcp_v4_send_synack()”