CMPE208 Presentation Terminal Access Controller Access Control System Plus (TACACS+) By MARVEL (Libing, Bhavana, Ramya, Maggie, Nitin)

Agenda  Introduction – TACACS+  Format and Header Values  TACACS+ Operations (AAA)  Vulnerabilities  Conclusion

Introduction – TACACS+  TACACS+ (terminal access controller access-control system plus), pronounced as “tack-acks plus”  TACACS+ originated from the TACACS and extended TACACS protocols. But it is not compatible with them.  A Cisco proprietary protocol.  provides the access control for routers, network access servers, and many other networked computing devices through one or more centralized servers.  Provides separate Authentication, Authorization, and Accounting services for server access.

Format and Header Values  Every TACACS+ packet has the 12 byte header.  Always sent in clear text format.

TACACS+ Authentication  who is allowed to gain access to the network.  Using usernames and passwords, or “one time” passwords.  takes place when the user first logs in to a machine or requests a service of it.  packet types: START, CONTINUE and REPLY.  START and CONTINUE are always sent by the client;  REPLY is always sent by the daemon.

TACACS+ Authentication Process  Authentication Process  1. connection request from the user;  2. START packet is sent to the AAA server;  3. REPLY packet is sent back, requesting user name;  4. sends a CONTINUE packet to AAA server with username;  5. REPLY packet is sent back, asking password;  6. sends a CONTINUE packet to AAA server with password;  7. REPLY packet is sent back to indicate a pass/fail of authentication;

TACACS+ Authorization  what a user is allowed to do.  what services the user has access to.  customize the service for the particular user.  An authorization session is defined as a single pair of messages, a REQUEST followed by a RESPONSE.

TACACS+ Authorization Process  Authorization Process  1. resource request from the user; (assuming authentication has already taken place)  2. A REQUEST packet is sent to AAA server for certain service;  3. A REPONSE packet is sent back, indicating a pass or fail;  4. user access is granted or denied;

TACACS+ Accounting  follows the processes of authentication & authorization  maintains complete accounting information  used either for billing purposes of the services or for security reasons.  TACACS+ accounting also uses the two message types: a REQUEST and a REPONSE

TACACS+ Accounting Process  Accounting Process  1. A resource request from user;  2. A REQUEST packet, including START, STOP, or CONTINUE, is sent to AAA server;  3. A RESPONSE packet is sent back, including SUCCESS, ERROR, or FOLLOW;

Vulnerabilities of TACACS+

Lack of Integrity Checking  No integrity checking exists in TACACS+  ‘MD5- encryption stream cipher’ mechanism is used.  Make changes to accounting packets. Example: Modifying elapsed time from 8000 to 1000

2) Vulnerability to replay attacks  No protection against replay attacks.  TACACS+ session starts with sequence number 1.  Accounting sessions have only one packet.  Duplicate accounting records can be produced, with forged task_id fields.

3) Session ID collision  More unique IDs the more stronger the encryption is.  Multiple sessions get the same ID, leading to frequency analysis attack.  Plaintext of one session know leads to decrypt the other session with the same sequence and session ID.  TACACS+ server encrypts reply packet with own session ID.(Mostly when the sequence number of the packet is 2).

4) The Birthday paradox  Session IDs are too small.  Leads to less unique IDs. Example: 100, 000 TACAS+ sessions  20, 000 dial up sessions  1000 matches per moth  Few 100 user passwords.

5) Lack of Padding  No padding in any fields or end of the packet.  Variable size data fields determined from the packet sizes.  Reveals the length of the user passwords.

6) MD5 Context Leak  Theoretical vulnerability  MD5- like hashes(16 bytes long) should be avoided.  TACACS+ packets encrypted by XOR’ing the MD5 hashes.  MD5_1=MD5(session_id, key, version, seq_no)  MD5_2=MD5(session_id, key, version, seq_no, MD5_1)

7) DoS and/or Overflow  Denial of Service – attempt to make resources unavailable to intended users.  Unlike others, it is an implementation defect.  No sanity check  Not check for an integer overflow in calculating the total memory size to allocate.

FIXES  Apply Packet Filtering  Choose Strong Encryption Keys  Avoid running tac_plus as root.

Conclusion  TACACS+ is a Cisco Proprietary protocol facilitating AAA model in Cisco Devices.  It provides AAA communication between AAA client and AAA server.  It improves on TACACS and XTACACS by separating the authentication, authorization and accounting.  More preferred since it uses TCP- reliable protocol.

Reference  [1] “The TACACS+ Protocol”, Version by D. Carrel, Lol Grant, Cisco Systems, January,  [2] “TACACS+ Protocol”, Version by D. Carrel, Lol Grant, Cisco Systems, October, [3] Cisco Access Control Security: AAA Administrative Services, by Brandon Carroll. May 27,  [4] “TACACS+ Authentication for HTTP Server Users”  [5] “TACACS+ and RADIUS Comparison”.

Q & A?  Thanks!