dbcve.org
VulnerabilityAwaiting classification

CVE-2026-63913

HIGH · 8.2 CVSS v3.1 Published 2026-07-19
Mitigation only
No fix yet — a mitigation exists. There is no fixed release. A documented workaround reduces exposure in the meantime.
See remediation →
91/100
Remediation priority · Urgent
Remotely reachable No privileges Zero-click 5 weeks old

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
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.

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis · high confidence

The Linux kernel's netfilter conntrack TCP state machine incorrectly forces connections to CLOSE state upon receiving an RST packet with an invalid sequence number after a SYN, without verifying the packet direction or that a valid matching SYN was sent in the opposite direction. This allows remote attackers to prematurely terminate active NAT entries using a crafted SYN + invalid-seq RST packet sequence.

MitigationApply the upstream Linux kernel patch that adds proper direction checking and validates that RST packets are legitimate responses before transitioning to CLOSE state. In the interim, monitor for anomalous RST patterns and consider rate-limiting or filtering suspicious TCP packets.

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Network
Complexity
Low
Privileges
None
User interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
Low
Availability
High

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:H

Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.

dbcve checks

Work through these to decide whether this CVE applies to you.

  1. Identify the running Linux kernel version
    Run `uname -r` or `cat /proc/version` to obtain the kernel version string
    Affected if The kernel version is older than the version that includes the CVE-2026-63913 fix (once patched)
  2. Verify netfilter conntrack module is loaded
    Run `lsmod | grep conntrack` or check if `/proc/net/nf_conntrack` exists and contains entries
    Affected if The conntrack module is loaded and tracking connections; the vulnerability only affects systems using netfilter connection tracking
  3. Check active conntrack connections
    Inspect `/proc/net/nf_conntrack` or run `conntrack -L` (if conntrack-tools is installed) to view current NAT/connection tracking entries
    Affected if Connections are being tracked; an attacker could force premature termination of these NAT entries
  4. Confirm NAT or firewall rules that rely on conntrack
    Run `iptables -L -n -v` or `nft list ruleset` to see if NAT or stateful firewall rules are active
    Affected if NAT or stateful filtering rules exist that depend on conntrack, as these would be impacted by premature connection termination

You are affected if your kernel version predates the fix for CVE-2026-63913 and your system uses netfilter conntrack for NAT or stateful firewalling.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

From vendor data
Mitigation available No clean upgrade yet — mitigate in the meantime
Mitigation

Apply the upstream Linux kernel patch that adds proper direction checking and validates that RST packets are legitimate responses before transitioning to CLOSE state. In the interim, monitor for anomalous RST patterns and consider rate-limiting or filtering suspicious TCP packets.

Have this fixed Scoped from the published advisory
  • Consultation4.0 h
  • Implementation12.0 h
  • Testing10.0 h
  • Review / QA6.0 h
32.0 hours of engineering $5,540
Get help mitigating

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $8,864.

Scan for this in your stack

Free · runs locally
dbcve dependency scanner

Check whether your project pulls in CVE-2026-63913 — or any other known-vulnerable package — straight from your lock files. Free and open source; it runs locally and uploads nothing.

Get it on GitHub Scan in the browser

References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2026-63913 in production — separate from our analysis above.

No notes yet

Be the first to add a field note for this CVE — a mitigation you’ve verified, a version caveat, or a link to a working fix. Sign in above to contribute.

What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

What belongs here
  • Verified mitigations, workarounds, and config changes
  • Version or environment caveats, and links to real fixes
  • No weaponised exploit code, or anything meant to cause harm
  • No spam, self-promotion, credentials, or personal data