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Linux KernelOperating system · Linux

CVE-2026-43037

CRITICAL · 9.8 CVSS v3.1 Published 2026-05-01
Fix available
A fix is available. Upgrade to 5.10.253 / 5.15.203 or later.
See remediation →
100/100
Remediation priority · Urgent
Remotely reachable No privileges Zero-click Patch available

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: ip6_tunnel: clear skb2->cb[] in ip4ip6_err() Oskar Kjos reported the following problem. ip4ip6_err() calls icmp_send() on a cloned skb whose cb[] was written by the IPv6 receive path as struct inet6_skb_parm. icmp_send() passes IPCB(skb2) to __ip_options_echo(), which interprets that cb[] region as struct inet_skb_parm (IPv4). The layouts differ: inet6_skb_parm.nhoff at offset 14 overlaps inet_skb_parm.opt.rr, producing a non-zero rr value. __ip_options_echo() then reads optlen from attacker-controlled packet data at sptr[rr+1] and copies that many bytes into dopt->__data, a fixed 40-byte stack buffer (IP_OPTIONS_DATA_FIXED_SIZE). To fix this we clear skb2->cb[], as suggested by Oskar Kjos. Also add minimal IPv4 header validation (version == 4, ihl >= 5).

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

Buffer overflow in Linux kernel's ip6_tunnel module where ip4ip6_err() mishandles skb control buffer: the IPv6-receive-path-written cb[] (struct inet6_skb_parm) is misinterpreted as IPv4 struct inet_skb_parm, causing inet6_skb_parm.nhoff offset to overlap inet_skb_parm.opt.rr and produce a non-zero rr value. This allows attacker-controlled optlen from packet data to trigger overflow into a fixed 40-byte stack buffer.

MitigationApply the kernel patch that clears skb2->cb[] before icmp_send() and adds IPv4 header validation (version==4, ihl>=5). Given the CVSS 9.8 and remote network exploitation vector, prioritize patching kernel TCB systems using ip6_tunnel.

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

Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.

NVD · CPE data
Linux KernelOperating system
Affected:>= 2.6.22, < 5.10.253>= 5.11, < 5.15.203>= 5.16, < 6.1.168>= 6.2, < 6.6.134>= 6.7, < 6.12.81>= 6.13, < 6.18.22>= 6.19, < 6.19.12= 7.0

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
High
Integrity
High
Availability
High

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/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 kernel version
    Run `uname -r` or check `/proc/version` to get the exact kernel version string
    Affected if The kernel version falls within any of these ranges: >= 2.6.22 and < 5.10.253; >= 5.11 and < 5.15.203; >= 5.16 and < 6.1.168; >= 6.2 and < 6.6.134; >= 6.7 and < 6.12.81; >= 6.13 and < 6.18.22; >= 6.19 and < 6.19.12; = 7.0
  2. Check if IPv6 and IP tunneling modules are loaded
    Run `lsmod | grep -E 'ipv6|ip6tnl|sit'` to see if IPv6 or IP-in-IPv6 tunnel modules are active
    Affected if IPv6 or IP tunneling (sit, ip6tnl) modules are loaded and the kernel is in an affected version range
  3. Determine if the system processes IP-in-IPv6 traffic
    Check network configuration with `ip -6 tunnel show` or `ip link show type sit` to list any configured SIT tunnels or IPv6 tunnels
    Affected if The system has active IPv6-in-IPv4 (SIT) or IPv6 tunnels configured, or acts as a gateway processing such tunneled traffic
  4. Verify if IP options processing is in use
    Check `/proc/sys/net/ipv4/ip_options` or monitor with `ss -A inet -p` to see if IPv4 options are being processed
    Affected if The system processes IPv4 packets with IP options and the kernel version is in the affected range

A system is affected if it runs a kernel version in the affected ranges AND processes IPv6 or IP-in-IPv6 tunneled traffic that could trigger the ip4ip6_err() error handling path.

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.

dbcve · scoped
Upgrade available Upgrade to 5.10.253 / 5.15.203 / 6.1.168 or later
Fixed in 5.10.2535.15.2036.1.168
Vendor patch git.kernel.org →
Interim mitigation

Apply the kernel patch that clears skb2->cb[] before icmp_send() and adds IPv4 header validation (version==4, ihl>=5). Given the CVSS 9.8 and remote network exploitation vector, prioritize patching kernel TCB systems using ip6_tunnel.

Recommended fix High confidence

Linux 5.10.253+, 5.15.203+, 6.1.168+, or 6.6.134+ (any stable release at or above these versions contains the fix)

  1. Identify the currently running Linux kernel version using 'uname -r'
  2. Determine which vulnerable version range your kernel falls into (>= 2.6.22, < 5.10.253; >= 5.11, < 5.15.203; >= 5.16, < 6.1.168; or >= 6.2, < 6.6.134)
  3. Upgrade to a stable kernel version with the fix: Linux 5.10.253 or later for the 5.10 branch, 5.15.203 or later for the 5.15 branch, 6.1.168 or later for the 6.1 branch, or 6.6.134 or later for the 6.6+ branch
  4. Apply the upgrade through your distribution's package manager (e.g., apt-get update && apt-get upgrade for Debian/Ubuntu, yum update for RHEL/CentOS, or dnf update for Fedora)
  5. Reboot the system to load the new kernel
  6. Verify the new kernel version is running with 'uname -r'
Caveat Kernel upgrades may require system reboot and could introduce compatibility issues with proprietary kernel modules or older system utilities; test in staging environment before production deployment

Generated from the published advisory — verify against the referenced sources before acting.

Fix this in Linux Kernel Scoped from the published advisory
  • Consultation2.0 h
  • Implementation4.0 h
  • Testing6.0 h
  • Review / QA3.0 h
15.0 hours of engineering $2,560
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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-2026-43037 in production — separate from our analysis above.

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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
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  • Version or environment caveats, and links to real fixes
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