High [CVE-2026-80844] validate routing header segments_left

In the Linux kernel, the following vulnerability has been resolved: xfrm: ah6: validate routing header segments_left AH6 rearranges routing-header addresses before computing or verifying the ICV. ipv6_rearrange_rthdr() assumes that segments_left is not larger than the number of addresses described by the routing header's hdrlen field.

That assumption does not hold for raw IPv6 HDRINCL packets. A packet with hdrlen equal to 2 describes one address, but can carry an arbitrary segments_left value.

With segments_left equal to 255, the function moves its address pointer 4,064 bytes backwards and passes a 4,064-byte length to memmove(), resulting in an out-of-bounds access.

Validate the invariant locally before modifying the routing header or performing any address-pointer arithmetic, and propagate malformed-header errors to the existing AH6 input and output error paths. A flaw was found in the Linux kernel's xfrm subsystem, specifically within the AH6 (Authentication Header for IPv6) module.

This oversight allows an attacker to craft a malicious packet that can cause an out-of-bounds memory access, potentially leading to memory corruption. This issue is classified as Important severity because exploitation can be achieved by a local attacker with unprivileged user access.