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Message-ID: <ZFvuu+PpS3BO2T/t@quatroqueijos.cascardo.eti.br>
Date: Wed, 10 May 2023 16:21:31 -0300
From: Thadeu Lima de Souza Cascardo <cascardo@...onical.com>
To: oss-security@...ts.openwall.com
Cc: Turritopsis Dohrnii Teo En Ming <tdtemccnp@...il.com>,
ceo@...-en-ming-corp.com, Piotr Krysiuk <piotras@...il.com>
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: https://www.bleepingcomputer.com/news/security/new-linux-kernel-netfilter-flaw-gives-attackers-root-privileges/
>
> 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:
>
> https://www.openwall.com/lists/oss-security/2023/05/08/4
>
> 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:
>
> https://www.openwall.com/lists/oss-security/2023/05/08/3
>
> 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:
>
> https://twitter.com/LaF0rge/status/1655867494152667140
>
> LaForge - @LaF0rge@...os.social @LaF0rge:
> > Really curious to see how CVS-223-32233 for #linux #netfilter nf_tables
> > https://seclists.org/oss-sec/2023/q2/133 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:
kernel.unprivileged_userns_clone=0
Cascardo.
>
> 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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