The Microsoft Windows Netlogon Remote Protocol (MS-NRPC) reuses a known, static, zero-value initialization vector (IV) in AES-CFB8 mode. This allows an unauthenticated attacker to impersonate a domain-joined computer, including a domain controller, and potentially obtain domain administrator privileges.
The Microsoft Windows Netlogon Remote Protocol (MS-NRPC) is a core authentication component of Active Directory that provides authentication for user and computer accounts. MS-NRPC uses an initialization vector (IV) of 0 (zero) in AES-CFB8 mode when authenticating computer accounts.
Zerologon: Unauthenticated domain controller compromise by subverting Netlogon cryptography (CVE-2020-1472) describes how this cryptographic failure allows a trivial statistical attack on the MS-NRPC authentication handshake:
The ComputeNetlogonCredential function, however, defines that this IV is fixed and should always consist of 16 zero bytes. This violates the requirements for using AES-CFB8 securely: its security properties only hold when IVs are random.
When encrypting a message consisting only of zeroes, with an all-zero IV, there is a 1 in 256 chance that the output will only contain zeroes as well.
By choosing a client challenge and ClientCredential of all zeros, an attacker has a 1 in 256 chance of successfully authenticating as any domain-joined computer. By impersonating a domain controller, an attacker can take additional steps to change a computer's Active Directory password (Exploit step 4: changing a computer’s AD password) and potentially gain domain administrator privileges (Exploit step 5: from password change to domain admin).