CVE-2025-71098: 
Linux Kernel vulnerability analysis and mitigation

CVE-2025-71098 is a buffer underflow vulnerability in the Linux kernel's IPv6 Generic Routing Encapsulation (ip6_gre) module, specifically in the ip6gre_header() function. It affects Linux kernel versions from 3.7 through 6.19-rc8, including stable branches up to 5.10.247, 5.15.197, 6.1.159, 6.6.119, 6.12.63, and 6.18.3. The vulnerability was published on January 13, 2026, with patches released through January–March 2026. It carries a CVSS v3.1 base score of 5.5 (Medium) (Microsoft MSRC).

The root cause is classified as CWE-476 (NULL Pointer Dereference) and involves improper handling of socket buffer headroom in ip6gre_header(). Team or bonding drivers can dynamically change dev->needed_headroom and/or dev->hard_header_len at runtime; when IPv6 multicast packets (e.g., MLD sendpack operations) are processed concurrently, the allocated socket buffer may have insufficient reserved headroom. This causes skb_push() to trigger a skb_under_panic kernel BUG at net/core/skbuff.c:213, crashing the system. The attack vector is local with low privilege requirements and no user interaction needed, as a local user can manipulate network interface configurations using team/bonding drivers to trigger the race condition (Microsoft MSRC).

Successful exploitation results in a kernel panic and complete system crash, causing a total loss of availability for all users and services on the affected system. There is no confidentiality or integrity impact — the vulnerability is purely a denial-of-service condition. Because the crash is triggered at the kernel level via a BUG condition in skbuff.c, recovery requires a system reboot, making this particularly disruptive in production or multi-tenant environments (Microsoft MSRC).

• Logs: Kernel logs (dmesg, /var/log/kern.log) showing skbuff: skb_under_panic messages referencing ip6gre_header in net/ipv6/ip6_gre.c; kernel BUG traces at net/core/skbuff.c:213.
• Logs: Stack traces involving mld_sendpack, ip6_finish_output2, neigh_connected_output, and ip6gre_header in sequence.
• Network: Unexpected system reboots or kernel panics on hosts running team or bonding network interfaces with IPv6 multicast (MLD) traffic.
• Process: Sudden termination of all processes due to kernel panic, with crash dump files (if kdump is configured) referencing ip6_gre.ko or skbuff.c.

Apply the appropriate kernel patch for your version branch: upgrade to 5.10.248, 5.15.198, 6.1.160, 6.6.120, 6.12.64, or 6.18.4 from kernel.org stable branches. Microsoft released a security update for Azure Linux 3 (kernel 6.6.119.3-3) on January 15, 2026. Debian, Ubuntu (USN-8177-1, USN-8177-2, USN-8183-1, USN-8183-2, USN-8245-1, USN-8257-1), Amazon Linux 2, and Oracle Linux have also issued advisories with patched packages. As a temporary workaround, limit or avoid dynamic reconfiguration of team/bonding network interfaces on systems processing IPv6 multicast traffic until patches are applied (Microsoft MSRC).