CVE-2026-80840
In the Linux kernel, the following vulnerability has been resolved: ipv6: seg6: clear IPv4 control block on IPIP decapsulation End.DX4 and End.DT4 decapsulate an IPv4 packet through decap_and_val…
Overview
CVE-2026-80840 is a known-severity vulnerability. It was published on September 4, 2026.
Technical description
In the Linux kernel, the following vulnerability has been resolved:
ipv6: seg6: clear IPv4 control block on IPIP decapsulation
End.DX4 and End.DT4 decapsulate an IPv4 packet through
decap_and_validate() and send it directly to IPv4 routing. The inner
packet therefore bypasses ip_rcv_core(), which normally clears IPCB
before IPv4 interprets skb->cb.
The skb instead retains IP6CB data from the outer packet. IP6CB and
IPCB use the same skb->cb storage, so IP6CB(skb)->lastopt overlaps
IPCB(skb)->opt.optlen and srr, while IP6CB(skb)->nhoff overlaps rr and
ts.
The sender can make the stale optlen byte nonzero with a valid outer
extension-header chain. The reproducers put an eight-byte Destination
Options header immediately after the 40-byte IPv6 header and before the
Segment Routing Header. ipv6_destopt_rcv() records the sender-controlled
Destination Options offset in both lastopt and nhoff, setting them to
40. On the reproduced little-endian x86-64 kernel, IPv4 therefore sees
optlen = 4
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
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Frequently asked questions
What is CVE-2026-80840?
CVE-2026-80840 is a known-severity vulnerability. It was published on September 4, 2026.
How severe is CVE-2026-80840?
CVSS score information is not yet available for this vulnerability. Check back as the CVE record is updated by NVD analysts.
How do I fix or remediate CVE-2026-80840?
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-2026-80840?
CyberStrike is an open-source, AI-augmented pentesting engine. Rather than flag a version number, it tests whether an issue like CVE-2026-80840 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.