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

Severity: high (CVSS 7.8 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.3% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2024-02-28 · updated 2026-06-17
Weakness: CWE-400

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

Affected products

Linux kernel: from 4.15 before 4.19.191; from 4.20 before 5.4.119; from 5.5 before 5.10.37; from 5.11 before 5.11.21; from 5.12 before 5.12.4 — fixed in 4.19.191, 5.10.37, 5.11.21, 5.12.4, 5.4.119

How to fix

Upgrade to a fixed version: Linux kernel 4.19.191, 5.10.37, 5.11.21, 5.12.4. Check the vendor advisory for mitigations.

References

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