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Date: Sat, 30 Mar 2024 22:47:56 +0100
From: Fay Stegerman <flx@...usk.net>
To: oss-security@...ts.openwall.com
Subject: Re: Re: backdoor in upstream xz/liblzma leading to
ssh server compromise
* Andres Freund <andres@...razel.de> [2024-03-30 20:48]:
> On 2024-03-29 08:51:26 -0700, Andres Freund wrote:
> > The prior section explains that RSA_public_decrypt@....plt was redirected to
> > point into the backdoor code. The trace I was analyzing indeed shows that
> > during a pubkey login the exploit code is invoked:
> >
> > sshd 1736357 [010] 714318.734008: 1 branches:uH: 5555555ded8c ssh_rsa_verify+0x49c (/usr/sbin/sshd) => 5555555612d0 RSA_public_decrypt@...+0x0 (/usr/sbin/sshd)
> >
> > The backdoor then calls back into libcrypto, presumably to perform normal authentication
> >
> > sshd 1736357 [010] 714318.734009: 1 branches:uH: 7ffff7c137cd [unknown] (/usr/lib/x86_64-linux-gnu/liblzma.so.5.6.0) => 7ffff792a2b0 RSA_get0_key+0x0 (/usr/lib/x86_64-linux-gnu/libcrypto.so.3)
> >
> >
> > I have not yet analyzed precisely what is being checked for in the injected
> > code, to allow unauthorized access. Since this is running in a
> > pre-authentication context, it seems likely to allow some form of access or
> > other form of remote code execution.
>
> FWIW, RSA_public_decrypt is reachable, regardless of server configuration,
> when using certificate based authentication.
Does "regardless of server configuration" mean the server doesn't have to
actually *enable* certificate-based authentication for it to be reachable, just
the client providing a certificate?
> Whether the key type is allowed is only verified after already verifying it
> (sshkey_from_blob_internal()->cert_parse()->sshkey_verify()). Whereas for
> normal pubkey auth, first the key type is checked, then the key id is checked
> against authorized_keys, and only then the key is verified (which also reaches
> sshkey_verify()).
- Fay
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