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Date: Wed, 10 May 2023 16:21:31 -0300
From: Thadeu Lima de Souza Cascardo <>
Cc: Turritopsis Dohrnii Teo En Ming <>,, Piotr Krysiuk <>
Subject: Re: New Linux kernel NetFilter flaw gives attackers
 root privileges

On Wed, May 10, 2023 at 06:55:46PM +0200, Solar Designer wrote:
> Hi,
> On Wed, May 10, 2023 at 11:52:58PM +0800, Turritopsis Dohrnii Teo En Ming wrote:
> > I have just come across this article. Thought of sharing it.
> > 
> > Article: New Linux kernel NetFilter flaw gives attackers root privileges
> > Link:
> We don't normally want in here links to news articles on something that
> was already brought up in here in more detail.  However, as a moderator,
> I reluctantly approved this posting so that we can use the resulting
> thread to discuss whether this issue got blown out of proportion and if
> so what we can do to avoid that going forward.  Here's the original
> posting this refers to:
> Another Linux kernel issue, in io_uring subsystem, was also disclosed in
> here on the same day, but I think didn't gain such tech media attention:
> Is the netfilter issue really worse than the io_uring issue?  I doubt
> it.  So _maybe_ it was something in the wording that tripped someone
> writing for one of those tech news websites, then others picked it up?
> Piotr's posting about the netfilter issue mentions intent to disclose an
> exploit later (like it should have, thank you Piotr!)
> Tobias' posting directly links to an exploit (which is also fine).
> Is intent to disclose an exploit later more newsworthy than having done
> so right away?  I doubt it.
> So maybe it's just random, and there's nothing to see here, after all.
> Now as to the actual issue and its description, I think we should
> clarify what exactly is meant by "unprivileged local users."  Piotr, I
> guess you actually meant not literally unprivileged, but users with
> CAP_NET_ADMIN, which can be had via unprivileged user/net namespaces if
> enabled in the distro / on the system, or when already in a container
> with such capability granted to container root.  Correct?  I think going
> forward we should always make this clear right away.  Here's a former
> netfilter core team leader also bringing this up:
> LaForge - @LaF0rge:
> > Really curious to see how CVS-223-32233 for #linux #netfilter nf_tables
> > can be exploted fom
> > "unprivileged local users".  AFAICT, nf_tables_api  goes through
> > nfnetlink, and nfnetlink_rcv() checks for CAP_NET_ADMIN way  before the
> > code in nf_tables_api.
> and a reply:
> Alex Plaskett @alexjplaskett:
> > Didn't look in depth at this one but you can trigger nf_tables_api
> > operations from a user / network namespace and distros such as Ubuntu
> > have unpriv user namespaces enabled.

If users don't need user namespaces, they can disable it on Ubuntu kernels as a
mitigation by doing:

sysctl -w kernel.unprivileged_userns_clone=0

Or persisting the option by adding a .conf file at /etc/sysctl.d/ with the
following line:



> As expected.  Now, from a typical distro user's standpoint,
> "unprivileged local users" may be just right.  However, not all distros
> have unprivileged user namespaces enabled by default.
> Alexander

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