ipv4: raw: reject IP_HDRINCL packets with ihl < 5

Summary

In the Linux kernel, the following vulnerability has been resolved: ipv4: raw: reject IP_HDRINCL packets with ihl < 5 raw_send_hdrinc() validates that the caller-supplied IPv4 header fits within the message length: iphlen = iph->ihl * 4; err = -EINVAL; if (iphlen > length) goto error_free; if (iphlen >= sizeof(*iph)) { /* fix up saddr, tot_len, id, csum, transport_header */ } It does not, however, reject ihl < 5. For such a packet the "if (iphlen >= sizeof(*iph))" branch is skipped, leaving the crafted iphdr untouched, but the packet is still handed to __ip_local_out() and onward. Downstream consumers that read iph->ihl assume a sane value: net/ipv4/ah4.c:ah_output() in particular subtracts sizeof(struct iphdr) from top_iph->ihl * 4 and passes the (signed-int-negative, then cast to size_t) result to memcpy(), producing an OOB access of length close to SIZE_MAX and a host kernel panic. An IPv4 header with ihl < 5 is malformed by definition (RFC 791: "Internet Header Length is the length of the internet header in 32 bit words ... Note that the minimum value for a correct header is 5."). The kernel should not be willing to inject such a packet into its own output path. Reject "iphlen < sizeof(*iph)" alongside the existing "iphlen > length" check. This matches the principle that locally constructed packets that re-enter the IP stack must pass the same basic sanity tests that a foreign packet would be subjected to. Once this lands, the "if (iphlen >= sizeof(*iph))" wrapper around the fixup branch becomes redundant; left in place to keep the patch minimal and backport-friendly. A follow-up can unwrap it. Note that commit 86f4c90a1c5c ("ipv4, ipv6: ensure raw socket message is big enough to hold an IP header") ensures the message buffer is large enough to hold an iphdr, but does not constrain the self-reported iph->ihl. Reachability: the malformed packet source is any caller with CAP_NET_RAW, including an unprivileged process in a user+net namespace on a kernel with CONFIG_USER_NS=y. The reproduced AH crash also requires a matching xfrm AH policy on the outgoing route; a container granted CAP_NET_ADMIN can install that state and policy in its netns. Loopback bypasses xfrm_output, so the trigger uses a real netdev. Reproduced on UML + KASAN: kernel-mode fault at addr 0x0 with memcpy_orig at the crash site. Same shape reproduces inside a rootless Docker container with --cap-add NET_ADMIN on a stock distro kernel.

Timeline

1. Jul 19, 2026, 3:40 PM UTCCVE published
2. Oct 16, 2026, 3:36 PM UTCCISA SSVC assessment

   Exploitation: none · Automatable: no · Technical impact: total

3. Oct 19, 2026, 8:28 PM UTCRecord last updated

Affected versions

• Linux Linux

  From 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 · fixed in 1065b9efa4126df559b03a849c139ecfae92cd25; From 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 · fixed in bc20dbd48c26e743f7e0845020c11ed2ce8b15ee; From 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 · fixed in 5a564f737ec54d63e8ee221d3ff396d07586d464; From 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 · fixed in 3c5411fa4944ed99af3d9d1de750ea8169b6dac9; From 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 · fixed in 145e9afa5b905229b4788bb72c3255f5a5f77508; From 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 · fixed in 7725cd3b471740fd23d25ed1da722c671fb2a5d3; From 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 · fixed in bbe0be67de296176e7243c76e3d9f02f6ae9ff0b; From 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 · fixed in 915fab69823a14c170dbaa3b41978768e0fe62fc

• Linux Linux

  2.6.12

• KernelLinux

  From 2.6.12 · fixed in 5.10.258; From 5.11.0 · fixed in 5.15.209; From 5.16.0 · fixed in 6.1.175; From 6.2.0 · fixed in 6.6.142; From 6.7.0 · fixed in 6.12.92; From 6.13.0 · fixed in 6.18.34; From 6.19.0 · fixed in 7.0.11

References 8