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Date: Sat, 30 Mar 2024 01:08:22 +0100
From: "Rein Fernhout (Levitating)" <me@...itati.ng>
To: oss-security@...ts.openwall.com
Cc: Solar Designer <solar@...nwall.com>
Subject: Re: backdoor in upstream xz/liblzma leading to ssh
server compromise
> Andres, maybe you (or Florian or someone else) can post the .o file
> from
> 5.61 as well (gzipped just like the previous one, please)?
I think the attached liblzma_la-crc64-fast.o is taken from 5.6.1.
I compiled 5.6.1 and ended up with a nearly identical object file.
When I compiled 5.6.0 I got a larger object file with additional symbols
crc64_generic, crc64_arch_optimized and crc64_resolve.
Attached is that object file.
On 2024-03-29 22:46, Solar Designer wrote:
> On Fri, Mar 29, 2024 at 12:19:26PM -0700, Andres Freund wrote:
>> On 2024-03-29 19:44:05 +0100, Matthias Weckbecker wrote:
>> > I've attached a yara rule to detect the *.o droplet you attached in the
>> > email (liblzma_la-crc64-fast.o.gz).
>>
>> Unfortunately xz 5.61 added further obfuscations, making it harder to
>> detect. Should have made it clearer that the attached .o was from
>> 5.60. Among
>> others 5.61 removed the two symbols you're checking against here.
>> That's why
>> Vegard's script looks for a specific instructions sequence, but
>> obviously is
>> also more obscure :/
>
> Andres, maybe you (or Florian or someone else) can post the .o file
> from
> 5.61 as well (gzipped just like the previous one, please)?
>
> On Fri, Mar 29, 2024 at 08:51:26AM -0700, Andres Freund wrote:
>> openssh does not directly use liblzma. However debian and several
>> other
>> distributions patch openssh to support systemd notification, and
>> libsystemd
>> does depend on lzma.
>
> It is indeed a security risk that sshd on major distros brings in so
> many libraries. For example, on RHEL 9.x and its rebuilds, "ldd sshd"
> is 28 lines. In the Rocky Linux SIG/Security override package, we've
> so
> far reduced this to 13 lines, which is still a lot:
>
> https://sig-security.rocky.page/packages/openssh
>
> For systemd notification, I patched it (half a year ago, so not in
> response to these new findings) to dlopen() libsystemd into a new sshd
> child process that's briefly spawned on sshd service startup or
> restart,
> notifies systemd, and exits. I could probably also drop privileges in
> that child process, but so far I didn't bother. I just didn't want
> those libraries to stay in the process address space after startup.
>
> Luckily, RHEL is not affected by the xz backdoor anyway, but if it were
> I think these changes would just happen to have prevented the backdoor
> from working. Indeed, it's still bad code that could run as root (and
> even if not in sshd, then in other services that use libsystemd), so it
> could have as well e.g. modified sshd on disk, but its current way of
> dynamically plugging into sshd authentication wouldn't work.
>
> I've attached the patch, which applies on top of Red Hat's patches. If
> using it in a package, explicit dependency on libsystemd (or the
> package
> that provides it) should be added to the (sub)package with sshd, e.g.:
>
> Requires: systemd-libs
>
> That's because the package manager would no longer automatically detect
> the dependency, which is now a soft one.
>
> I took this approach back then in order not to drop functionality, but
> I'd re-think it now. Perhaps systemd notification isn't worth even the
> reduced risk, and should be dropped completely. For the latter, an
> edit
> to the systemd unit file is needed, changing "Type=notify" to
> "Type=simple", which should fit "sshd -D".
>
> Not only Red Hat'ish distros, but also Debian and Ubuntu are similar in
> this respect, and I think should want to make similar changes.
>
> Alexander
Download attachment "liblzma_la-crc64_fast.o" of type "application/x-object" (126360 bytes)
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