Bug 2516662 (CVE-2026-72250)

Summary: CVE-2026-72250 kernel: netfilter: nf_conntrack_reasm: guard mac_header adjustment after IPv6 defrag
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security DevOps Team <prodsec-dev>
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Priority: medium    
Version: unspecifiedCC: rhel-process-autobot, watson-tool-maintainers
Target Milestone: ---Keywords: Security
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Hardware: All   
OS: Linux   
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A flaw was found in the Linux kernel's `nf_conntrack_reasm` module, specifically within the IPv6 defragmentation process. An attacker could exploit this by sending specially crafted IPv6 fragmented packets. The `nf_ct_frag6_reasm()` function incorrectly adjusts the `mac_header` offset, causing the system to wrongly identify a MAC header and point to an invalid memory location. This can lead to information disclosure, where sensitive data from kernel memory might be exposed, or a denial of service (DoS), which could make the system unavailable.
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Description OSIDB Bzimport 2026-08-15 06:24:38 UTC 
In the Linux kernel, the following vulnerability has been resolved:

netfilter: nf_conntrack_reasm: guard mac_header adjustment after IPv6 defrag

nf_ct_frag6_reasm() slides the packet head forward to drop the IPv6
fragment header and then unconditionally advances skb->mac_header:

	skb->mac_header += sizeof(struct frag_hdr);

On the NF_INET_LOCAL_OUT defrag path the skb has no link-layer header
yet, so skb->mac_header is still the "not set" sentinel (u16)~0U. Adding
sizeof(struct frag_hdr) wraps it to a small value (0xffff + 8 == 7),
after which skb_mac_header_was_set() wrongly reports a MAC header is
present and skb_mac_header() points into the headroom.

The reassembler has done this unconditional add since it was introduced;
it was harmless while mac_header was a bare pointer, but wrong once
mac_header became a u16 offset whose unset state is the ~0U sentinel
tested by skb_mac_header_was_set(). The sibling net/ipv6/reassembly.c
does the same relocation and does guard the adjustment; mirror the
guard here.
Comment 1 Keith Grant 2026-08-18 17:22:55 UTC 
Upstream advisory:
https://lore.kernel.org/linux-cve-announce/2026081553-CVE-2026-72250-1fa9@gregkh/T