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CVE-2026-43037

9.8 · CRITICAL

In the Linux kernel, the following vulnerability has been resolved: ip6_tunnel: clear skb2->cb[] in ip4ip6_err() Oskar Kjos reported the following problem.

Published May 1, 2026CWE-787EPSS 0.56%

Overview

CVE-2026-43037 is a critical-severity vulnerability. It was published on May 1, 2026 and has a CVSS 3.1 base score of 9.8 (CRITICAL).

This vulnerability has a CVSS 3.1 base score of 9.8, rated CRITICAL. It can be exploited remotely over the network. No authentication or special privileges are required for exploitation.

Technical description

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).

Remediation

Check the references section for vendor advisories, patches, and mitigation guidance. If immediate patching is not possible, review the CVSS vector to understand the attack surface and apply compensating controls such as network segmentation or access restrictions.

References

Refgit.kernel.org/stable/c/1063515ce15ff31065c4e7f8265f4c2fd3c54876Refgit.kernel.org/stable/c/2cc6e3b0fe0f0242d1f530a93a4924f48ab85ba5Refgit.kernel.org/stable/c/2edfa31769a4add828a7e604b21cb82aaaa05925Refgit.kernel.org/stable/c/4a622658f384b03560834cbe8ffcfe69a278f7c8Refgit.kernel.org/stable/c/590f622669b97eaf7b57a1de7b0a6e68c5d8b2c3Refgit.kernel.org/stable/c/a0c4ce9900a108eaf55d0f3b399cb55999647d39Refgit.kernel.org/stable/c/d6621f60192fe10c047a4487be42a6f4c150707fRefgit.kernel.org/stable/c/ea9f65b27c8404e164848ebff1443310fd187629Refaccess.redhat.com/errata/RHSA-2026:22900Refaccess.redhat.com/errata/RHSA-2026:22940

How CyberStrike helps

Frequently asked questions

What is CVE-2026-43037?

CVE-2026-43037 is a critical-severity vulnerability. It was published on May 1, 2026 and has a CVSS 3.1 base score of 9.8 (CRITICAL).

How severe is CVE-2026-43037?

This vulnerability has a CVSS 3.1 base score of 9.8, rated CRITICAL. It can be exploited remotely over the network. No authentication or special privileges are required for exploitation.

How do I fix or remediate CVE-2026-43037?

Check the references section for vendor advisories, patches, and mitigation guidance. If immediate patching is not possible, review the CVSS vector to understand the attack surface and apply compensating controls such as network segmentation or access restrictions.

How can CyberStrike help with CVE-2026-43037?

CyberStrike is an open-source, AI-augmented pentesting engine. Rather than flag a version number, it tests whether an issue like CVE-2026-43037 is actually reachable in your own applications and APIs, then verifies each finding before reporting it. That tells you if the weakness is a real, exploitable risk in your environment or a false positive you can safely deprioritize.

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