Projects/ConstrainedDelegation

This is an early stage project for MIT Kerberos. It is being fleshed out by its proponents. Feel free to help flesh out the details of this project. After the project is ready, it will be presented for review and approval.

Background

Whilst we've had constrained delegation support in the KDC since 1.7, it has required a proprietary backend that supports validating the Windows PAC. This project provides a constrained delegation implementation that works without this requirement.

Architecture

In constrained delegation, the service passes the KDC a ticket from a user it has authenticated. This "evidence" ticket is placed in the additional tickets field of the TGS-REQ: there is nothing preventing the service from forging this, because it knows its long-term key. To avoid this attack, Windows takes advantage of the fact the PAC is signed with the TGS long-term key: in the constrained delegation case, this is verified before proceeding.

Heimdal implements PAC-less constrained delegation by including an authorization data element containing a checksum over the ticket and delegation path with the TGS key. The ASN.1 encoding of this element is described below:

KRB5-AUTHDATA-SIGNTICKET INTEGER ::= -17

KRB5SignedPathPrincipals ::= SEQUENCE OF Principal

-- never encoded on the wire, just used to checksum over
KRB5SignedPathData ::= SEQUENCE {
        encticket[0]    EncTicketPart,
        delegated[1]    KRB5SignedPathPrincipals OPTIONAL
}

KRB5SignedPath ::= SEQUENCE {
        -- DERcoded KRB5SignedPathData
        -- krbtgt key (etype), KeyUsage = XXX 
        etype[0]        ENCTYPE,
        cksum[1]        Checksum,
        -- srvs delegated though
        delegated[2]    KRB5SignedPathPrincipals OPTIONAL
}

The checksum is over KRB5SignedPathData, where encticket excludes the KRB5SignedPath authorization data. (Unlike Heimdal, we do not require that this be the last element in the ticket authorization data.) KRB5SignedPath is wrapped in AD-IF-RELEVANT, because it does not need to be understood by application servers.

Implementation

libkrb5

New types and encoders are added for the delegation path. Unfortunately, the ASN.1 library appears to have forced the use of a container type for a SEQUENCE of principals.

typedef struct _krb5_transited_service {
    krb5_principal principal;
} krb5_transited_service;

typedef struct _krb5_ad_signedpath_data {
    krb5_enc_tkt_part enc_tkt_part;
    krb5_transited_service **delegated;
} krb5_ad_signedpath_data;

typedef struct _krb5_ad_signedpath {
    krb5_enctype enctype;
    krb5_checksum checksum;
    krb5_transited_service **delegated;
} krb5_ad_signedpath;

Note: krb5_ad_signedpath_data represents data to be checksummed, krb5_ad_signedpath is what is actually encoded and sent on the wire. The enctype field in the latter is a hint to which TGS key to use, but presently this is unused in our implementation.

Constants are added for the signed path AD type and the checksum key usage:

#define KRB5_KEYUSAGE_AD_SIGNEDPATH            -21
#define KRB5_AUTHDATA_SIGNTICKET    -17

I have kept this aligned with Heimdal so that, when using the HDB bridge, a mixed realm of Heimdal and MIT KDCs should interoperate with constrained delegation.

The new API krb5int_get_authdata_containee_types() is added to return the authorization data types within an authdata container without completely decoding it.

KDC

Note: some changes were imported from the HDBBridge branch to speed implementation.

Changes are briefly summarised as follows:

• We now correctly filter out known KDC issued AD elements when copying authorization data from the TGT. (This uses the krb5int_get_authdata_containee_types() API above.)
• Because of the above fix, we can now separate the copying of TGT authdata from the propagation of KDB authdata. We previously relied on the backend to do this.
• A new authdata handler, handle_signedpath_authdata(), is added, which supports the verification and generation of the signed data path.
• The authorization data handlers are now called immediately before the ticket is encrypted, for the purpose of checksumming.
• The merge_authdata() internal API now takes an additional argument denoting whether KDC issued authdata we recognise should be omitted.
• The greet_server authdata plugin has been updated.

KDB

CHECK_ALLOWED_TO_DELEGATE

We provide a CHECK_ALLOWED_TO_DELEGATE db_invoke callback for the LDAP backend that authorizes that target service against the krbAllowedToDelegateTo attribute. There is no support for administrating this attribute via kadmin, or for the DB2 backend.

Open issues

• What is the interaction of the ticket checksumming with FAST?

Status

Code is in the users/lhoward/s4u2proxy branch.