HIGH
net/ipv6 GSO Fallback Bypass
CVE-2026-43057
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
KernelScan AI7.5HIGH
01Description
In the Linux kernel, the following vulnerability has been resolved: net: correctly handle tunneled traffic on IPV6_CSUM GSO fallback NETIF_F_IPV6_CSUM only advertises support for checksum offload of packets without IPv6 extension headers. Packets with extension headers must fall back onto software checksumming. Since TSO depends on checksum offload, those must revert to GSO. The below commit introduces that fallback. It always checks network header length. For tunneled packets, the inner header length must be checked instead. Extend the check accordingly. A special case is tunneled packets without inner IP protocol. Such as RFC 6951 SCTP in UDP. Those are not standard IPv6 followed by transport header either, so also must revert to the software GSO path.
02KernelScan AI Analysis
Risk summary
Network devices that support IPv6 and process tunneled traffic (such as GREoIPv6 or SCTP-in-UDP encapsulation) can be forced into an unsupported checksum offload path when IPv6 extension headers are present, causing software fallback, kernel warnings, and severe network throughput degradation. This affects any system transmitting IPv6 tunneled packets via devices advertising NETIF_F_IPV6_CSUM.
Vulnerability analysis
The root cause is that the GSO fallback logic introduced in commit 864e3396976e only checked the outer network header length to detect IPv6 extension headers, but failed to check the inner header length for encapsulated (tunneled) packets. This violates the NETIF_F_IPV6_CSUM contract, which only supports plain IPv6 without extension headers. The fix adds skb_gso_has_extension_hdr() to properly evaluate both encapsulated and non-encapsulated packets, checking inner headers for tunneled traffic and correctly handling special cases like SCTP-in-UDP where no inner IP header exists. The vulnerability is remotely reachable because an attacker can induce or forward IPv6 tunneled packets with extension headers through an affected system.
03Fix Versions
| Branch | Fixed in | Patch commit |
|---|---|---|
| 6.1 | 6.1.168 | 2094a7cf91b7 |
| 6.12 | 6.12.81 | 33670f780e01 |
| 6.17 | 6.17 | a98b78116a27 |
| 6.18 | 6.18.22 | 732fdeb2987c |
| 6.19 | 6.19.12 | c4336a07eb6b |
| 6.6 | 6.6.134 | ed71cf465c75 |
| mainline | 7.0 | — |