CVE-2016-2125: Samba - Security Announcement Archive

CVE-2016-2125.html:

=============================================================================== == Subject: Unconditional privilege delegation to Kerberos servers in trusted realms == == CVE ID#: CVE-2016-2125 == == Versions: Samba 3.0.25 to 4.5.2 == == Summary: Samba client code always requests a forwardable ticket == when using Kerberos authentication. This means the == target server, which must be in the current or trusted == domain/realm, is given a valid general purpose Kerberos == “Ticket Granting Ticket” (TGT), which can be used to == fully impersonate the authenticated user or service. == ================================================================================

=========== Description ===========

The Samba client code always requests a forwardable Kerberos ticket when performing Kerberos authentication by passing the GSS_C_DELEG_FLAG to the gss_init_sec_context() GSSAPI function.

The use of GSS_C_DELEG_FLAG, if accepted by the Kerberos KDC, results in passing the forwardable TGT to the remote target service via Kerberos AP-REQ. The target service by design must be part of the same or a trusted Kerberos realm (a domain in the same or trusted Active Directory forest).

The service that accepts the AP-REQ from the client can thus do whatever the client is also able to achieve with the Kerberos TGT.

The risks of impersonation of the client are similar to the well known risks from forwarding of NTLM credentials, with two important differences:

• NTLM forwarding can and should be mitigated with packet signing
• Kerberos forwarding can only be attempted after the trusted destination server decrypts the ticket.

Finally, it should be noted that typically the connections involved are either explicitly requested, or are between or to Domain Controllers already of ultimate privilege.

Controlling Kerberos forwarding

In the Active Directory world it’s possible for administrators to limit the delegation. User and computer objects can both act as Kerberos users and also as Kerberos services. Both types of objects have an attribute called ‘userAccountControl’ which is a bitmask that controls the behavior of the account. The following three values have impact on possible delegation:

0x00100000: UF_NOT_DELEGATED:

The UF_NOT_DELEGATED can be used to disable the ability to get forwardable TGT for the account. It means the KDC will respond with an error if the client asks for the forwardable ticket. The client typically gives up and removes the GSS_C_DELEG_FLAG flag and continues without passing delegated credentials. Administrators can use this to disable possible delegation for the most privileged accounts (e.g. administrator accounts).

0x00080000: UF_TRUSTED_FOR_DELEGATION

If the UF_TRUSTED_FOR_DELEGATION is set on an account a KDC will include the OK_AS_DELEGATE flag in a granted service ticket. If the client application uses just GSS_C_DELEG_POLICY_FLAG (instead of GSS_C_DELEG_FLAG) gssapi/Kerberos libraries typically only include delegated credentials when the service ticket includes the OK_AS_DELEGATE flag. Administrators can use this to control which services will get delegated credentials, for example if the service runs in a trusted environment and actually requires the presence of delegated credentials.

0x01000000: UF_TRUSTED_TO_AUTHENTICATE_FOR_DELEGATION

The UF_TRUSTED_TO_AUTHENTICATE_FOR_DELEGATION is not really relevant for this CVE and just listed here for completeness. This flag is relevant for the S4U2Proxy feature, where a service can ask the KDC for a proxied service ticket which can impersonate users to other services.

The patched Samba version will still use GSS_C_DELEG_POLICY_FLAG and provide delegated credentials to services which have the UF_TRUSTED_FOR_DELEGATION bitmask set on the user account.

================== Patch Availability ==================

A patch addressing this defect has been posted to

https://www.samba.org/samba/security/

Additionally, Samba 4.5.3, 4.4.8 and 4.3.13 have been issued as security releases to correct the defect. Samba vendors and administrators running affected versions are advised to upgrade or apply the patch as soon as possible.

========== Workaround ==========

See the description about UF_NOT_DELEGATED above.

The samba-tool command and the AD DC mode honours the undocumented “gensec_gssapi:delegation=no” option in the [global] section of the smb.conf file.

======= Credits =======

This vulnerability was discovered by Simo Sorce of Red Hat and researched by Stefan Metzmacher of SerNet (https://samba.plus) both members of the Samba Team (https://www.samba.org). Stefan Metzmacher also provided the fixes.