Attachment 16251 Details for Bug 14497 – advisory v4

advisory v4

CVE-2020-1472-v4.txt (text/plain), 6.43 KB, created by Karolin Seeger on 2020-09-18 11:10:19 UTC

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Creator: Karolin Seeger

Created: 2020-09-18 11:10:19 UTC

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patch

obsolete

>===========================================================
>== Subject:     Unauthenticated domain takeover via netlogon ("ZeroLogon")
>==              
>==
>== CVE ID#:     CVE-2020-1472
>==
>== Versions:    Samba 4.0 and later
>==
>== Summary:     An unauthenticated attacker on the network can gain
>==              administrator access by exploiting a netlogon
>==              protocol flaw.
>===========================================================
>
>===========
>Description
>===========
>
>The following applies to Samba used as domain controller only (most
>seriously the Active Directory DC, but also the classic/NT4-style DC).
>
>Installations running Samba as a file server only are not directly
>affected by this flaw, though they may need configuration changes to
>continue to talk to domain controllers (see "file servers and domain
>members" below).
>
>The netlogon protocol contains a flaw that allows an authentication
>bypass. This was reported and patched by Microsoft as CVE-2020-1472.
>Since the bug is a protocol level flaw, and Samba implements the
>protocol, Samba is also vulnerable.
>
>However, since version 4.8 (released in March 2018), the default
>behaviour of Samba has been to insist on a secure netlogon channel,
>which is a sufficient fix against the known exploits. This default is
>equivalent to having 'server schannel = yes' in the smb.conf.
>
>Therefore versions 4.8 and above are not vulnerable unless they have
>the smb.conf lines 'server schannel = no' or 'server schannel = auto'.
>
>Samba versions 4.7 and below are vulnerable unless they have 'server
>schannel = yes' in the smb.conf.
>
>Note each domain controller needs the correct settings in its smb.conf.
>
>Vendors supporting Samba 4.7 and below are advised to patch their
>installations and packages to add this line to the [global] section if
>their smb.conf file.
>
>The 'server schannel = yes' smb.conf line is equivalent to Microsoft's
>'FullSecureChannelProtection=1' registry key, the introduction of
>which we understand forms the core of Microsoft's fix.
>
>Consequences
>============
>
>The exploitation of this issue is by changing the a server password.
>In an AD domain changing a DC password allows full password database
>disclosure including the krbtgt password, unsalted MD4 password hash
>(the 'NT Hash') for each user, and the LM password hash if stored.
>(Via DRS replication).
>
>The krbtgt password allows the attacker to issue a 'golden ticket' to
>themselves and return to take over the domain at any point in the
>future.
>
>Other consequences includes disclosure of session keys, as well as
>general denial of service to the trust account selected.
>
>Samba NT4-like / classic domains
>================================
>
>In NT4-like domains Samba does not provide a replication service (this
>is done at lower layers, like OpenLDAP), but changing machine account
>passwords can allow the attacker limited rights, similar to any other
>member server or trusted domain. This includes disclosure of session
>keys and inter-domain trust passwords (only), as well as general
>denial of service to the domain member selected.
>
>Therefore while still real, the risk is lower in these domains than
>for the AD DC.
>
>File servers and domain members
>===============================
>
>File servers and domain members do not run the NETLOGON service in
>supported Samba versions and only need to ensure that they have not
>set 'client schannel = no' for continued operation against secured DCs
>such as Samba 4.8 and later and Windows DCs in 2021. Users running
>Samba as a file server should still patch to ensure the server-side
>mitigations (banning certain un-random values) do not very rarely
>impact service.
>
>Allow listed exceptions
>=======================
>
>Some domains employ third-party software that will not work with a
>'server schannel = yes'. For these cases patches are available that
>allow specific machines to use insecure netlogon. For example, the
>following smb.conf:
>
>   server schannel = yes
>   server require schannel:triceratops$ = no
>   server require schannel:greywacke$ = no
>
>will allow only "triceratops$" and "greywacke$" to avoid schannel.
>
>Exploitability of Samba despite 'server schannel = yes'
>=======================================================
>
>The published proof of concept exploit for this issue only attempts to
>authenticate to the NetLogon service but does not attempt a takeover of
>the domain.
>
>On domains with 'server schannel = yes', these tests claim to show a
>vulnerability against Samba despite being unable to access any
>privileged functionality.
>
>This Samba release adds additional server checks for the protocol
>attack in the client-specified challenge that provides some protection
>when 'server schannel = no/auto' and avoids this false-positive
>result.
>
>These server checks are identical to the server logic added by
>Microsoft for their patch for the Windows server code for
>CVE-2020-1472. The Samba Team would like to thank Microsoft for their
>disclosure of the method used to prevent the proof of concept exploit
>code from working against such a hardened server.
>
>
>==================
>Patch Availability
>==================
>
>Patches addressing this defect are available at:
>
>    https://www.samba.org/samba/security/
>
>Additionally, Samba 4.10.18, 4.11.13, and 4.12.7 have been issued as
>security releases to correct the defect. Samba administrators are
>advised to upgrade to these releases or apply the patch as soon as
>possible.
>
>==================
>CVSSv3 calculation
>==================
>
>CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H (9.8)
>
>==========
>Workaround
>==========
>
>Users of versions of Samba before 4.8 should set
>
>    server schannel = yes
>
>in their smb.conf and restart Samba (on all domain controllers!)
>
>Users of versions 4.8 and above should ensure their smb.conf either
>a) has the "server schannel = yes" line, or
>b) has no "server schannel" line.
>
>If in doubt, add "server schannel = yes" to your smb.conf.
>
>=======
>Credits
>=======
>
>This problem was originally discovered by Tom Tervoort of Secura,
>though it was not successfully reported to the Samba team before its
>public disclosure.
>
>Stefan Metzmacher made the changes to Samba 4.8 that preemptively
>dodge this bug in default installs.
>
>Andrew Bartlett, Gary Lockyer, GÃ¼nther Deschner, Jeremy Allison, and
>Stefan Metzmacher have triaged the bug and written patches and tests.
>
>This advisory written by Andrew Bartlett and Douglas Bagnall.
>
>==========================================================
>== Our Code, Our Bugs, Our Responsibility.
>== The Samba Team
>==========================================================

Flags:

metze: review+
dbagnall: review+
jra: review+

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