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Encryption Oracle Remediation

This `REG_DWORD` value selects the protection and compatibility level for applications and services that use the Credential Security Support Provider protocol (CredSSP), such as Remote Desktop Connection.

since 0.9.8.11

Purpose

Show registry keys (1)
  • HKLM\Software\Microsoft\Windows\CurrentVersion\Policies\System\CredSSP\Parameters\AllowEncryptionOracle

This REG_DWORD value selects the protection and compatibility level for applications and services that use the Credential Security Support Provider protocol (CredSSP), such as Remote Desktop Connection. It determines whether updated CredSSP clients may fall back to insecure protocol versions and whether CredSSP services accept unpatched clients.

The supported value data has the following meaning:

  • 0 – Force Updated Clients: Clients cannot fall back to insecure CredSSP versions. Services reject unpatched clients.
  • 1 – Mitigated: Clients cannot fall back to insecure CredSSP versions. Services continue to accept unpatched clients.
  • 2 – Vulnerable: Clients support fallback to insecure CredSSP versions. Services accept unpatched clients.

Notes

Security

The setting forms part of the mitigation for CVE-2018-0886. With vulnerable CredSSP versions, an attacker can relay user credentials during authentication and use them to execute code on a target system.

A value of 2 permits insecure protocol fallback and exposes remote servers to attacks involving vulnerable clients. A value of 1 prevents such fallback by the client but still allows services to accept unpatched clients. A value of 0 also rejects unpatched clients and therefore provides the strictest protection of the three levels.

Possible side effects

Stricter protection levels can block CredSSP communication between systems with different patch levels. In particular, updated clients that prohibit insecure fallback cannot establish a CredSSP connection to an unpatched server. With value 0, CredSSP services also refuse connections from unpatched clients.

This can affect Remote Desktop Connection and other applications that rely on CredSSP authentication. Windows may report an authentication error and record system event 6041 when a client and remote host cannot negotiate an allowed protocol version. Results can differ when a Remote Desktop server uses a TLS authentication fallback, because Microsoft's interoperability matrix describes CredSSP behavior only.

Notes

Microsoft changed the default behavior of updated systems from Vulnerable to Mitigated with the security updates released on May 8, 2018. Microsoft also documents that a change to AllowEncryptionOracle requires a restart.

The value controls the behavior of appropriately updated CredSSP components. It does not replace the security updates that correct the underlying vulnerability.

Microsoft documentation

Microsoft Learn: ADMX_CredSsp Policy CSP

Microsoft Support: CredSSP updates for CVE-2018-0886

See also