CVE-2026-89563
HighCVSS 7.8Exploitation Probability (EPSS)
Low risk4th percentile - higher than 4% of all known CVEs
Summary
In the Linux kernel, ip6_tnl_xmit() uses skb_realloc_headroom(), which replaces the caller-owned skb and keeps the replacement only in a local variable. If an error occurs after reallocation (e.g., non-NONE encap rejection in collect_md tunnels or ip6_tnl_encap() failure), the caller still holds the original skb pointer, leading to a double free of the skb.
Risk Assessment
A double free of the skb can cause kernel crashes or potential code execution, threatening the stability and security of systems using IPv6 tunnels.
Recommendation
Update the Linux kernel to a version where ip6_tnl_xmit() uses skb_cow_head() instead of skb_realloc_headroom().
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Original NVD description (English source)
In the Linux kernel, the following vulnerability has been resolved: ip6_tunnel: use skb_cow_head() in ip6_tnl_xmit() ip6_tnl_xmit() may need to expand headroom before it can push the outer IPv6 and optional encap headers. It currently does that with skb_realloc_headroom(), copies skb->sk ownership, consumes the original skb, and then continues processing with the replacement skb kept only in its local variable. That is safe only if the helper cannot fail afterwards. But this helper still has post-reallocation error exits. collect_md tunnels reject non-NONE encap after the replacement, and ip6_tnl_encap() can also fail later. In those cases the helper returns an error to its callers while the caller still only has the original skb pointer. Both ip6_tnl_start_xmit() and the IPv6 GRE paths free the caller skb on error, so they can end up freeing an skb that ip6_tnl_xmit() already consumed. Use skb_cow_head() instead. It provides the required headroom and writability without privately replacing the caller-owned skb, so later error returns cannot leave callers with a stale pointer. The Ethernet users, ip6gretap and ip6erspan, clear IFF_TX_SKB_SHARING and already call skb_cow_head() before entering ip6_tnl_xmit(). They do not rely on the removed skb_shared() reallocation. This also makes the IPv6 tunnel path consistent with ip_tunnel_xmit().
Vulnerability data from NVD (NIST) · CISA KEV · EPSS