CVE-2026-63994 tunnels: load network headers after skb_cow() in iptunnel_pmtud_build_icmp[v6]()

What is CVE-2026-63994?

CVE-2026-63994 is a critical use after free vulnerability in Linux kernel, published on 19 Jul 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

Official description

In the Linux kernel, the following vulnerability has been resolved:

tunnels: load network headers after skb_cow() in iptunnel_pmtud_build_icmp[v6]()

Sashiko found that iptunnel_pmtud_build_icmp() and iptunnel_pmtud_build_icmpv6() were caching ip_hdr() and ipv6_hdr() before an skb_cow() call which can reallocate skb->head.

Fix this possible UAF by initializing the local variables after the skb_cow() call.

Remove skb_reset_network_header() calls which were not needed.

Weakness: CWE-416

Which versions are affected by CVE-2026-63994?

It affects 1 product.

Products include Linux kernel.

How do I fix CVE-2026-63994?

Apply the vendor's patch.

Upgrade to the fixed version. These bugs are found with fuzzing and sanitisers and avoided by memory-safe languages. More about Use after free.

Is CVE-2026-63994 being exploited?

There is no known exploitation of CVE-2026-63994 in the wild and no public exploit code.

Other vulnerabilities in Linux kernel

• CVE-2026-53266 Linux Kernel Out-of-Bounds Write Exploited High 8.8
• CVE-2013-6282 Linux Kernel Improper Input Validation Exploited High 8.8
• CVE-2022-0185 Linux Kernel Heap-Based Buffer Overflow Exploited High 8.4
• CVE-2013-2094 Linux Kernel Privilege Escalation Exploited High 8.4
• CVE-2026-53362 Linux Kernel Unspecified Exploited High 7.8

All Linux kernel vulnerabilities

Sources

• CVE record
• NVD
• OSV
• GitHub advisory
• How this page is made

Also known as BELL-CVE-2026-63994, DEBIAN-CVE-2026-63994, GHSA-hvgq-ch9f-27rq, ROOT-OS-UBUNTU-2204-CVE-2026-63994, UBUNTU-CVE-2026-63994.