# CVE-2025-21629: net: reenable NETIF_F_IPV6_CSUM offload for BIG TCP packets

> A kernel bug causes excessive warnings and potential performance degradation for BIG TCP packets, requiring a patch to restore proper hardware offload handling.

- **Severity:** High 8.2 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 15 Jan 2025
- **Updated:** 5 Aug 2026
- **Also known as:** AZL-68552, BELL-CVE-2025-21629, DEBIAN-CVE-2025-21629, GHSA-qj58-65cx-j37v, ROOT-OS-DEBIAN-11-CVE-2025-21629, ROOT-OS-UBUNTU-2204-CVE-2025-21629, UBUNTU-CVE-2025-21629

## What is CVE-2025-21629?

CVE-2025-21629 is a high-severity denial of service vulnerability in Linux kernel, published on 15 Jan 2025. 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:

net: reenable NETIF_F_IPV6_CSUM offload for BIG TCP packets

The blamed commit disabled hardware offoad of IPv6 packets with
extension headers on devices that advertise NETIF_F_IPV6_CSUM,
based on the definition of that feature in skbuff.h:

 *   * - %NETIF_F_IPV6_CSUM
 *     - Driver (device) is only able to checksum plain
 *       TCP or UDP packets over IPv6. These are specifically
 *       unencapsulated packets of the form IPv6|TCP or
 *       IPv6|UDP where the Next Header field in the IPv6
 *       header is either TCP or UDP. IPv6 extension headers
 *       are not supported with this feature. This feature
 *       cannot be set in features for a device with
 *       NETIF_F_HW_CSUM also set. This feature is being
 *       DEPRECATED (see below).

The change causes skb_warn_bad_offload to fire for BIG TCP
packets.

[  496.310233] WARNING: CPU: 13 PID: 23472 at net/core/dev.c:3129 skb_warn_bad_offload+0xc4/0xe0

[  496.310297]  ? skb_warn_bad_offload+0xc4/0xe0
[  496.310300]  skb_checksum_help+0x129/0x1f0
[  496.310303]  skb_csum_hwoffload_help+0x150/0x1b0
[  496.310306]  validate_xmit_skb+0x159/0x270
[  496.310309]  validate_xmit_skb_list+0x41/0x70
[  496.310312]  sch_direct_xmit+0x5c/0x250
[  496.310317]  __qdisc_run+0x388/0x620

BIG TCP introduced an IPV6_TLV_JUMBO IPv6 extension header to
communicate packet length, as this is an IPv6 jumbogram. But, the
feature is only enabled on devices that support BIG TCP TSO. The
header is only present for PF_PACKET taps like tcpdump, and not
transmitted by physical devices.

For this specific case of extension headers that are not
transmitted, return to the situation before the blamed commit
and support hardware offload.

ipv6_has_hopopt_jumbo() tests not only whether this header is present,
but also that it is the only extension header before a terminal (L4)
header.

## Which versions are affected by CVE-2025-21629?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2025-21629?

Apply the vendor's patch.

## Is CVE-2025-21629 being exploited?

There is no known exploitation of CVE-2025-21629 in the wild and no public exploit code.

## Other vulnerabilities in Linux kernel

- [CVE-2026-53266](https://insights.sbomify.com/CVE-2026-53266/): Linux Kernel Out-of-Bounds Write (high 8.8, exploited in the wild)
- [CVE-2013-6282](https://insights.sbomify.com/CVE-2013-6282/): Linux Kernel Improper Input Validation (high 8.8, exploited in the wild)
- [CVE-2022-0185](https://insights.sbomify.com/CVE-2022-0185/): Linux Kernel Heap-Based Buffer Overflow (high 8.4, exploited in the wild)
- [CVE-2013-2094](https://insights.sbomify.com/CVE-2013-2094/): Linux Kernel Privilege Escalation (high 8.4, exploited in the wild)
- [CVE-2026-53362](https://insights.sbomify.com/CVE-2026-53362/): Linux Kernel Unspecified (high 7.8, exploited in the wild)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2025-21629 and fix it if it is affected.

What it is: CVE-2025-21629 is a high-severity denial of service vulnerability in Linux kernel, published on 15 Jan 2025. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2025-21629 in the wild and no public exploit code.

Affected product: Linux kernel. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2025-21629/ (Markdown: https://insights.sbomify.com/CVE-2025-21629/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2025-21629
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2025-21629
- OSV: https://osv.dev/vulnerability/CVE-2025-21629

---
From sbomify Insights: https://insights.sbomify.com/CVE-2025-21629/
Summaries are generated and may contain mistakes; the official description above is the record.