CVE-2026-63913
HighCVSS 8.2Exploitation Probability (EPSS)
Low risk46th percentile - higher than 46% of all known CVEs
Summary
A vulnerability in the Linux kernel's netfilter conntrack TCP module allows an RST packet with an invalid sequence number to force a connection into the CLOSE state without proper direction validation. This can prematurely terminate active NAT entries.
Risk Assessment
An attacker can send a crafted packet sequence (SYN followed by an invalid-seq RST) to close active NAT entries, potentially disrupting existing network connections and services.
Recommendation
Update the Linux kernel to a patched version that tightens TCP state transition logic in conntrack. Monitor security advisories from your Linux distribution for the appropriate fix.
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Original NVD description (English source)
In the Linux kernel, the following vulnerability has been resolved: netfilter: conntrack: tcp: do not force CLOSE on invalid-seq RST without direction check An unintended behavior in the TCP conntrack state machine allows a connection to be forced into the CLOSE state using an RST packet with an invalid sequence number. Specifically, after a SYN packet is observed, an RST with an invalid SEQ can transition the conntrack entry to TCP_CONNTRACK_CLOSE, regardless of whether the RST corresponds to the expected reply direction. The relevant code path assumes the RST is a response to an outgoing SYN, but does not validate packet direction or ensure that a matching SYN was actually sent in the opposite direction. As a result, a crafted packet sequence consisting of a SYN followed by an invalid-sequence RST can prematurely terminate an active NAT entry. This makes connection teardown easier than intended. So, tighten the state transition logic to ensure that RST-triggered CLOSE transitions only occur when the RST is a valid response to a previously observed SYN in the correct direction.