Linux Kernel: CVE-2026-98852 IPv6 tunnel ext header mis-detection
CVE-2026-98852 Published on October 18, 2026
net: extend IPv6 exthdr detection of tunneled packets
In the Linux kernel, the following vulnerability has been resolved:
net: extend IPv6 exthdr detection of tunneled packets
Commit c4336a07eb6b ("net: correctly handle tunneled traffic on IPV6_CSUM
GSO fallback") split skb_gso_has_extension_hdr() into mutually exclusive
branches on skb->encapsulation. This did not yet address all paths:
1. With skb->encapsulation set, the outer header is not checked. IPv6
tunnels such as ip6_gre and ip6_tunnel add an outer Destination
Options header by default (encap_limit). Their GSO packets skip
software GSO, then skb_csum_hwoffload_help() sees the outer extension
header and calls skb_checksum_help() on the GSO skb, which warns and
drops it.
2. Tunnels over IPv6 without an outer transport header, such as
ip6_tunnel, leave skb->transport_header at the inner transport
header. skb_network_header_len() then spans the outer IPv6, tunnel
and inner IP headers, a false positive.
3. UDP tunnels without an inner network header, such as SCTP-in-UDP or
PSP, have no inner IPv6 header to check. Decide on the outer header
alone.
4. Directly dereferencing inner_ip_hdr(skb)->version without
skb_header_pointer() is unsafe.
Instead, check ipv6_ext_hdr(nexthdr) on the outer IPv6 header and, if
set, on the inner IPv6 header. Read the headers with skb_header_pointer().
Keep the skb_network_header_len() check when !skb->encapsulation to also
catch encapsulated packets without skb->encapsulation (e.g., virtio).
Use the same helper in skb_csum_hwoffload_help(). Its open coded test
has the false positive of (2) and ignores the inner header.
Background: checksum offload of tunneled packets invariants:
Non-GSO skb:
- If the inner packet is CHECKSUM_PARTIAL, Local Checksum Offload computes
the outer checksum in software and the device offloads only the inner L4
checksum.
- If the inner packet is CHECKSUM_NONE (e.g., SCTP-in-UDP, ESP-in-UDP,
Remote Checksum Offload), the device offloads the outer UDP or GRE
checksum instead.
GSO skb:
- A device with NETIF_F_GSO_UDP_TUNNEL_CSUM or NETIF_F_GSO_GRE_CSUM
computes both inner and outer checksums per segment.
The outer checksum is seeded with only the pseudo-header checksum.
A NETIF_F_IPV6_CSUM device must parse through the outer headers to
reach the inner ones.
Products Associated with CVE-2026-98852
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Affected Versions
Linux:- Version 33670f780e0120c3dacda188c512bbffe0b6044c and below 44ba9dc568c878b699d05e7cf699572dc9df4b59 is affected.
- Version a98b78116a27e2a57b696b569b2cb431c95cf9b6 and below aa480f05aed7a9eee87d4a6fc90647fe9679c670 is affected.
- Version c4336a07eb6b2526dc2b62928b5104b41a7f81f5 and below cabacdf98e09eb74599c4d97d7bd7fd659598fb7 is affected.
- Version c4336a07eb6b2526dc2b62928b5104b41a7f81f5 and below 5f0764cb1c81a9712bce605d38efe6d844a70b29 is affected.
- Version 2094a7cf91b71367b649f991aacc7b579f793d0b is affected.
- Version ed71cf465c75f5688b07a35d373cd1d6b589c8ea is affected.
- Version 732fdeb2987c94b439d51f5cb9addddc2fc48c42 is affected.
- Version 6.12.81 and below 6.12.113 is affected.
- Version 6.18.22 and below 6.18.56 is affected.
- Version 6.1.168 and below 6.2 is affected.
- Version 6.6.134 and below 6.7 is affected.
- Version 6.19.12 and below 6.20 is affected.
- Version 7.0 is affected.
- Before 7.0 is unaffected.
- Version 6.12.113, <= 6.12.* is unaffected.
- Version 6.18.56, <= 6.18.* is unaffected.
- Version 7.2.10, <= 7.2.* is unaffected.
- Version 7.3-rc6, <= * is unaffected.