In the Linux kernel, the following vulnerability has been resolved:

ppp: require CAP_NET_ADMIN in target netns for unattached ioctls

/dev/ppp open is currently authorized against file->f_cred->user_ns,
while unattached administrative ioctls operate on current->nsproxy->net_ns.

As a result, a local unprivileged user can create a new user namespace
with CLONE_NEWUSER, gain CAP_NET_ADMIN only in that new user namespace,
and still issue PPPIOCNEWUNIT, PPPIOCATTACH, or PPPIOCATTCHAN against
an inherited network namespace.

Require CAP_NET_ADMIN in the user namespace that owns the target network
namespace before handling unattached PPP administrative ioctls.

This preserves normal pppd operation in the network namespace it is
actually privileged in, while rejecting the userns-only inherited-netns
case.

https://git.kernel.org/stable/c/1a8a51ce85075a56a743b6f142606dd2696a391c

https://git.kernel.org/stable/c/2bb6379416fd19f44c3423a00bfd8626259f6067

https://git.kernel.org/stable/c/3b2c2157dc2afc5c17cd7238afefca92f1ef330e

https://git.kernel.org/stable/c/5013be175c7ffd8b39efbc3c9c4db5b10b85fea8

https://git.kernel.org/stable/c/5080e188c914110034bbc569d5cfa2f06204681d

https://git.kernel.org/stable/c/67e901e28d177ac9a9bed76d69ce3471e704a89e

https://git.kernel.org/stable/c/954745d0223e7caec917c0b2d1a889ff56fa6e54

https://git.kernel.org/stable/c/c9edd90c57ae23692fff6b049fdfa4572a9fd532

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