Projects/Audit

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.

Purpose

Create an Audit infrastructure within MIT Kerberos to monitor security related events on the KDC. In future expand Kerberos Audit facility to the application servers, kadmin if it remains desirable.

Requirements

The new audit system should be:

• build-time enabled;
• run-time pluggable;
• simple and flexible, so it could be easily replaced with the OS specific and third-parties implementations;

Events

(Common Criteria Class FIA)

This section details the categories of the auditable events and the associated data.

1. Startup and shutdown of the KDC must be recorded by audit system;

2. AS_REQ and TGS_REQ:

Phase	Data to be logged	AS_REQ	TGS_REQ
Authenticate request content and client	client’s address and port	✔	✔
	original KDC request and request ID	✔	✔
	primary ticket ID	✗	(S4U:front-end server's) TGT
Determine service principal	modified KDC request and request ID	✔	✔
	cross-realm referral	✗	service principal, TGS
	user-to-user: client in the 2nd ticket	✗	✔
Validate policies	local policy violation	✔	✔
	protocol constraints	✗	S4U2Proxy, S4U2Self
Issue ticket	ticket renewed	✗	✔
	ticket validated	✗	✔
	session key enctype (short-term)	✔	✔
	enctype of the service's long-term key	✗	✔
	derived ticket ID	TGT	service or referral TGT
Encrypt reply	KDC reply	✔	✔
	Reply-encrypting key enctype (long-term)	✔	✔

Note, that audit plugin implementors will be able to extract the following auditable information:

KDC request:

requested service principal;

client’s principal;

KDC options;

requested ticket start, end and renew_till times;

list of requested addresses;

requested enctypes;

preauth types

KDC reply:

preauth types;

TGT, referral TGT or service ticket with the following level of details:

client and server principals;

flags;

start, end and renew_till times;

authtime;

authentication path - encoded list of transited realms for service ticket or referral TGT (for TGS only);

Other events to consider for the future development:

Policy

Policies violation - what went wrong and how to prevent it;

Secrets (Common Criteria FCS_CKM.1, FCS_CKM.4);

long- and short-term keys creation, manipulation, cleaning.

Design details

The following are highlights of this new feature:

Ticket ID

Ticket ID is recorded as part of audit messages. This allows to link tickets to their initial TGT at any stage of the Kerberos exchange.

For the purpose of this project we will create a private to KDC ticket ID: each successfully created ticket will be hashed and recorded into audit log. The administrators will correlate the primary and derived ticket IDs after the fact.

For the future, however, we need a more complex system that would allow to tie the tickets from a successful AS_REQ all the way to the application server. It is marked as an action item in this section.

Request ID

Request ID - hash of the request is recorded as part of audit messages. Using this piece of information an administrator can easily correlate multiple audit events related to a single request.

KDC facing API

/* Audit plugin loaded/unloaded */
krb5_error_code 
load_audit_plugin(krb5_context context);
krb5_error_code 
unload_audit_plugin(krb5_context context);
/* event specific functions */
krb5_error_code 
kau_kdc_start(krb5_context context, struct server_handle shdl, int status);
krb5_error_code 
kau_kdc_stop(krb5_context context, krb5_error_code status);
krb5_error_code 
kau_as_req(krb5_context context, struct as_req_state *state, int status);
krb5_error_code 
kau_tgs(krb5_context context, struct tgs_req_audit_state *state, int status);

Pluggable interface

/* Audit plugin vtable */
typedef struct krb5_audit_vtable_st {
   /* Mandatory: name of module. */
   char             *name;
   kau_open_fn       open;
   kau_close_fn      close;
   kau_generic_fn    generic;
   kau_kdc_start_fn  kdc_start;
   kau_kdc_stop_fn   kdc_stop;
   kau_as_req_fn     as_req;
   kau_tgs_req_fn    tgs_req;
} *krb5_audit_vtable;

typedef krb5_error_code
(*kau_open_fn)(kau_ctx *au_ctx);

typedef krb5_error_code
(*kau_close_fn)(kau_ctx au_ctx);

/* general purpose interface to pass unspecified number of 
 *  key-type-value triplets to a plugable interface.
 */
typedef krb5_error_code
(*kau_generic_fn)(kau_ctx au_ctx, const int event_id, const int status, ... );

/* one-API-per-event surrogate */
typedef krb5_error_code
(*kau_kdc_start_fn)(kau_ctx au_ctx, const int event_id, const int status,
                    struct server_handle_san shdl);
typedef krb5_error_code
(*kau_kdc_stop_fn)(kau_ctx au_ctx, const int event_id, const int status);
typedef krb5_error_code
(*kau_as_req_fn)(kau_ctx au_ctx, const int event_id, const int status,
                 struct as_req_state_san *state);
typedef krb5_error_code
(*kau_tgs_fn)(kau_ctx au_ctx, const int event_id, const int status,
              struct tgs_req_state_san *state);

where new types server_handle_san, as_req_state_san and tgs_req_state_san are sanitized variants of server_handle, as_req_state and tgs_req_state structures respectively.

Dictionary

The possible basic field names are:

Key	Type	Comments
tkt_id_in	STR	primary (TGT) ticket ID
tkt_id_out	STR	derived (service or referral TGT) ticket ID
client	STR	client’s principal
service	STR	requested service principal
kdc_status	TR	KDC status (“ISSUE” on success)
full_address	STR	Alternative to "fromport"/"fromaddr"
sess_etype	NUM	enctype of session key
rep_etype	NUM	enctype of reply-encrypting key
srv_etype	NUM	enctype of long-term key of the service key
tkt_renewed	BOOL	was ticket renewed
tkt_validated	BOOL	was ticket validated
req.addresses	STR	requested addresses
req.avail_etypes	STR	requested/available enc types
req.kdc_options	NUM	KDC options (forwardable, allow_postdate etc)
req.pa_type	STR	preauth types
req.tkt_start	NUM	requested ticket start time
req.tkt_end	NUM	requested ticket end time
req.tkt_renew_till	NUM	requested ticket renew-till time
req.tkt_authtime	NUM	requested ticket authtime
req.sectkt_cname	STR	client principal in the second ticket (U2U etc)
req.sectkt_sname	STR	service principal in the second ticket
req.sectkt_flags	NUM	second ticket flags
req.sectkt_start	NUM	second ticket start time
req.sectkt_end	NUM	second ticket end time
req.sectkt_authtime	NUM	second ticket authtime
req.sectkt_etype	NUM	second ticket key type
req.sname	STR	requested service principal
req.cname	STR	client's principal
rep.sname	STR	service principal in ticket
rep.cname	STR	client principal in ticket
rep.pa_type	STR	reply preauth types
rep.rep_flags	NUM	ticket flags
rep.rep_authtime	NUM	ticket authtime
rep.tkt_start	NUM	ticket start time
rep.tkt_end	NUM	ticket end time
rep.tkt_renew_till	NUM	ticket renewed-till time
rep.tr_contents	STR	ticket transited-realms list

Configuration

The following ./configure option to be added:

--with-audit-plugin=simple

(For demo and testing purposes) Build the audit plugin "simple" and enable audit plugin.

Future work

1. Standardize a Ticket_ID;
2. Make reporting auditable events configurable. For example, one can choose to report TGS_REQ, but not AS_REQ etc;
3. Define and make configurable the DETAILED and BASIC levels of the events;
4. Sanitize KDC request and reply before passing them to audit implementation: security sensitive information should not leave KDC boundaries;
5. Develop Audit system for Preauth and Authdata mechanisms.

Test implementation

We will use libaudit module available on Fedora, Debian, Suse for the first round.

Some "simple" audit plugin will be implemented and Python test system will become aware of its existence. New ./configure --with-audit-plugin option will be introduced to build "simple" audit plugin for testing purpose. If audit is enabled and audit plugin is available, "make check" will store audit messages into audit log file.

References

1. Common Criteria for Information Technology Security Evaluation http://www.commoncriteriaportal.org/files/ccfiles/CCPART2V3.1R4.pdf
2. Oracle Solaris Auditing http://docs.oracle.com/cd/E19963-01/html/821-1456/auditov-1.html
3. Understanding Linux Audit http://doc.opensuse.org/products/draft/SLES/SLES-security_sd_draft/cha.audit.comp.html
4. Advanced Security Audit Policy Settings http://technet.microsoft.com/en-us/library/dd772712(v=ws.10).aspx
5. Events Classification in Log Audit http://airccse.org/journal/nsa/0410ijnsa5.pdf
6. CEE Log Syntax (CLS) Encoding http://cee.mitre.org/language/1.0-beta1/cls.html