Pierluigi Paganini September 23, 2020

Samba team has released a security patch to address the Zerologon issue in the Microsoft Windows Netlogon Remote Protocol (MS-NRPC).

Samba team has released a security patch to address the Zerologon (CVE-2020-1472) issue in the Microsoft Windows Netlogon Remote Protocol (MS-NRPC).

The CVE-2020-1472 flaw is an elevation of privilege that resides in the Netlogon. The Netlogon service is an Authentication Mechanism used in the Windows Client Authentication Architecture which verifies logon requests, and it registers, authenticates, and locates Domain Controllers.

“An elevation of privilege vulnerability exists when an attacker establishes a vulnerable Netlogon secure channel connection to a domain controller, using the Netlogon Remote Protocol (MS-NRPC). An attacker who successfully exploited the vulnerability could run a specially crafted application on a device on the network.” reads the advisory published by Microsoft.

“To exploit the vulnerability, an unauthenticated attacker would be required to use MS-NRPC to connect to a domain controller to obtain domain administrator access.”

An attacker could exploit the vulnerability to impersonate any computer, including the domain controller itself, and execute remote procedure calls on their behalf.

An attacker could also exploit the flaw to disable security features in the Netlogon authentication process and change a computer’s password on the domain controller’s Active Directory.

Microsoft addressed the vulnerability with the release of August 2020 Patch Tuesday, last week the Department of Homeland Security’s CISA issued an emergency directive to order government agencies to address the Zerologon vulnerability (CVE-2020-1472) by this Monday.

Experts pointed out that the vulnerability also impacted Samba that relies on Netlogon for file sharing between Linux and Windows systems.

Samba is also vulnerable to the NetLogon flaw only when used as domain controller, according to the advisory Active Directory DC installations are the most affected. Default installations running Samba as a file server only are not directly impacted.

“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.” reads the advisory. “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 impacted by the vulnerability unless they have ‘server schannel = yes’ in the smb.conf.

“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. ” continues the advisory.

An attacker can exploit this vulnerability to completely domain takeover (on Active Directory DC domains), or disclosure of session keys or denial of service (on NT4-like domains).

Vendors supporting Samba 4.7 and below have to patch their installations and packages installing the available patches as soon as possible.

Pierluigi Paganini

(SecurityAffairs – hacking, ZeroLogon)

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