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Presentation Layer Security

Lecture 7Supakorn Kungpisdan

supakorn@mut.ac.th

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Roadmap

• Introduction to Presentation Layer• NetBIOS and SMB• NetBIOS and Enumeration• Sniffing Encrypted Traffic

Introduction to Presentation Layer

• Presentation layer deals primarily with data representation

• If one host supports Extended Binary-coded Decimal Interchange Code (EBCDIC) for character sets and another supports ASCII, the presentation layer converts data according to each hosts’ needs

• Other functionalities include data compression, data encryption, and manipulating XML objects

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CIFS (or SMB)

• Common Internet File System (CIFS), aka Server Message Block (SMB), is a network protocol for file sharing on a LAN, running at app/presentation layer

• A client-server protocol• Rely on other protocols for transport

– Most commonly used is NetBIOS over TCP (NBT)

• MS Oses use CIFS for remote file operations (mapping networtk drives), browsing (via Network Neighborhood), authentication (NT, Win2000, 2003), and remote printer services

• Unix Oses also implement CIFS via the Samba program

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NetBIOS and SMB VS OSI

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CIFS History

• In 1984, IBM wrote NetBIOS, an API allowing basic network communications between hosts on a small subnet, but it required a transport protocol

• The following year, IBM released a transport protocol that makes NetBIOS come to life

• Both of them are merged into NetBIOS Enhanced User Interface (NetBEUI)

• Later, NetBIOS was implemented using various transport protocols e.g. DECnet, IPX/SPF, and TCP/IP

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CIFS History (cont.)

• Soon after, MS and Intel created the 1st rendition of the SMB/CIFS file sharing protocol titled “Core Protocol”

• MS and Intel chose NetBIOS to deliver the upper protocol CIFS packets

• CIFS using NetBIOS over TCP became standard network file sharing mechanism for MS Oses

• Several operations have been added to CIFS overtime:– File access– File and record locking– Safe file caching– File change notification

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- Protocol negotiation

- Extended file attribute handling

- Batched requests

- Unicode support

NetBIOS

• NetBIOS runs over many transport protocols, but recently TCP/IP is the most common transport protocol used

• Main NetBIOS services:– Name service– Session service– Datagram service

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NetBIOS Name Service

• NetBIOS names are human readable names assigned to computers on a network– Commonly seen in Windows in Network Neighborhood

• Serve the same purpose as the DNS system in TCP/IP, but different in the method

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NetBIOS Naming Properties

• Broadcast and/or server based– Name registration and lookups can be accomplished

by broadcasting to the LAN and/or using a central NetBIOS name server (NBNS or WINS)

• Broadcast only (b-node)• NBNS only (p-node)• Broadcast first then NBNS (m-node)• NBNS first then broadcast (h-node)

• Dynamic registration– With NetBIOS, when a computer boots, it registers its

name/IP combination dynamically

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NetBIOS Common Procedure

• Two most common NetBIOS name service are name registration and name query

• Name registration associates a NetBIOS name with an IP

• Name query determines the IP address associated with an given name

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NetBIOS and SMB

• The original NetBIOS protocol was developed to become the NetBIOS Frames Protocol (NFB) often referred to as NetBEUI or just NetBIOS.

• NBF or NetBEUI provides a datagram delivery and session service that can be used for a variety of network applications.

• The above protocol is often encapsulated in other (routable) protocols such as IPX/SPX (which Microsoft refers to as NBIPX) or TCP/IP (which Microsoft refers to as NBT).

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NetBIOS and SMB (cont.)

• Although NBF (either in encapsulated form or "on the wire") can be used for a variety of applications, it is often used as a foundation for the Server Message Block (SMB) protocol.

• One of the most widely used network configurations is SMB running over NetBIOS over TCP/IP.

• SMB has been developed to become the Common Internet File System (CIFS).

• Recently CIFS has been implemented directly on TCP/IP without requiring the NetBIOS over TCP/IP layer.

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SMB and SAMBA

• SAMBA is a project has been established to provide free implementations of the SMB protocol and file and printing sharing facilities for various platforms.

• The SAMBA project has had to "reverse engineer" the protocols and continues to work in this manner in order to keep the software free.

• Despite having released a version of SMB to the X-Open organization, Microsoft continues to develop the protocol as a proprietary protocol and details of some of the more recent versions have not been made freely available.

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Name Registration (b-node)

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21. Broadcast name registration

packet containing name and IP over UDP port 137 three times (waiting 250 ms each)

2. If a host has already registered the name, it sends back a defense packet back

Name Registration (p-node)

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21. Unicast name registration packet

over UDP port 137 directly to NBNS three times (waiting 250 ms each)

2. If the name is already registered, NBNS sends back a defense packet back

WINS server

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Name Query (b-node)

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21. Broadcast name query request

containing name over UDP port 137 three times (waiting 5 s each)

2. If a host has already registered the name, it sends back a defense packet back

Name Query (p-node)

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21. Unicast name query request

over UDP port 137 directly to NBNS three times (waiting 250 ms each)

2. If the name is already registered, NBNS sends back a defense packet back

WINS server

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Session Service

• RFC1001: “A session is a reliable message exchange, conducted between pair of NetBIOS applications. Session is full-duplex, sequenced, and reliable”

• CIFS uses the session service to send and receive all upper layer commands, including file and printer operations

• The first step in any CIFS communications is to establish a NetBIOS session between client and server

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Session Service Primitives

• Call: Initiate a NetBIOS session– Create TCP connection and send a NetBIOS call packet

containing the client’s NetBIOS name and the server’s NetBIOS name

• Listen: Wait for a NetBIOS call command– Mapped a server waiting on TCP port 139

• Hang up: end a NetBIOS session– Mapped into TCP by initiating a TCP teardown sequence

• Send: send a message over a NetBIOS session• Receive: receive a message from a NetBIOS session• Session status: obtain information about session

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Datagram Service

• RFC1001: “The datagram service is unreliable, non-sequenced, connectionless service”

• Used by NetBIOS application as a fast, broadcast-capable, low-overhead method of transferring data

• Implemented on UDP port 138• All NetBIOS datagram packets adds a header containing

NetBIOS name and whether or not the NetBIOS datagram was fragmented to be sent via UDP

• CIFS protocol standard does not implement NetBIOS datagram server, but CIFS implementations typically use it for browsing

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CIFS Internals

• CIFS allows file sharing between network nodes• Client-server design• Each packet contains a command field indicating the

packet purpose e.g. Login, open a file, read from a file, or write to a file

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CIFS Properties

• Client/server + request/response– Allow multiple simultaneous requests be assigning

multiplex id (MID) to each request– The server replies with the same MID

• Command based– CIFS packet contains a 1-byte command field

allowing 100+ commands available

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CIFS Properties

• Protocol dialects/negotiation– Allow different variants (dialects) of CIFS to

communicate– When client wishes to access files on a remote

server, client sends a negotiate protocol packet containing a list of dialect strings that it is capable of understanding. The server sends back the response containing which dialect it wishes to communicate

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SMB Variants

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Ref: http://samba.anu.edu.au/cifs/docs/what-is-smb.html

CIFS Properties (cont.)

• User/share level security– User level security

• A client wishing to access the share must provide a username and a password

• Used in windows 2000 onwards

– Share level security• The share itself requires a password, but no username is

required• Used in windows 95 and 98

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CIFS Properties (cont.)

• Encryption– Password entered is sent in an encrypted format to

server– NTLM and LM

• Command batching– Many CIFS packets are capable of piggybacking

other CIFS packets to reduce latency and better utilizing network bandwidth

– This technique is referred as ANDX batching

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CIFS Properties (cont.)

• Opportunitistic locking (oplock)– When a CIFS packet specifies to open file, an oplock

can be requested– If granted by server, no other entities can access the

file

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CIFS Packet Header

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CIFS Operations

• Two common CIFS client/server packet exchanges– Client initiates contact with a server– Client requires to open a file and read from it

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Initial Contact, Login, and Tree Connect

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Establish NetBIOS session

Positive NetBIOS session acknowledgement

Negotiate CIFS dialect

Choose CIFS dialect from request list

User login

Indicate User IF or returns error if bad password

Connect to particular resource

Indicate Tree ID or error if share name does not exist

Client Server

File Open and Read

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Open a file

Indicate File ID, or error code if problem

Read from a file

Return file data requested

Client Server

Null User Overview

• At the core of the Windows authentication scheme lies the 'User' administrative unit.

• When we create users, we assign them rights, privileges, and policy restrictions to define what they can and can't do on our networks.

• But in addition to the standard User, Windows supports a special type of user called the 'Null' user, which is basically a pseudo-account that has no username or password, but is allowed to access certain information on the network

• Some MS Oses and services used to require that you utilize the null user in order to operate properly

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Null User Overview (cont.)

• Null user can enumerate account names and shares on domain controllers, member servers, and workstations alike.

• Therefore this Null user, a user with no credentials, can be used to glean a tremendous amount of information from your network without raising any eyebrows

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Why would we ever support null user?

• Null user account is used by IPC (Interprocess Communications) all the time.

• For instance, in a multi-domain model where one-way trusts are created, the Null user is used when permissions for resources in the trusting domain need to be granted to user accounts in the trusted domain.

• After all, the users needing to choose from a list of available accounts are not trusted, so someone has to be able to enumerate the users

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Why would we ever support null user? (cont.)

• Microsoft's System Management Server uses the Null user to enumerate shares during discovery.

• Even the mundane action of starting a NT workstation or server creates an initial Null user logon to one of your domain controllers after its NetBIOS name is successfully registered (before you get the Ctrl + Alt + Delete logon prompt)

• System account uses the null user to get to remote resources

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NetBIOS Enumeration

• Null sessions allow users to communicate via NetBIOS in order to query any server as the null user

• If this occurs, user’s can enumerate shares, users, groups, permissions, policies. They can probably brute force their way into the network

• Many tools can be used to enumerate different aspects of a Windows server

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enum

• enum is a console-based Win32 information enumeration utility.

• Using null sessions, enum can retrieve userlists, machine lists, sharelists, namelists, group and member lists, password and LSA policy information.

• enum is also capable of a rudimentary brute force dictionary attack on individual accounts.

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Ref: http://www.cotse.com/tools/netbios.htm

enum (cont.)

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No limit number of logon attempts

enum (cont.)

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Exploiting the IPC$ Share

• Windows have hidden administrative shares that have $ at the end (e.g. C$, ADMIN$, IPC$). The system tells itself to omit this share from any request for the enumeration of shares

• Interprocess Communication (IPC$) share is necessary to make systems work properly

• IPC is used for data sharing between processes

• In Old versions of Windows (esp NT), users can exploit the use of IPC$ and other hidden shares through a null session

• Once connected, the user may utilize the net view and net use command to browse and connect to the hidden shares on a machine

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Exploiting the IPC$ Share (cont.)

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winfo

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C:\winfo\winfo 192.168.1.1 –v -n

Establish a null session before dump data

verbose

Other NetBIOS Issues

• An attacker can generate a fake packet to deny a host joining the network

• Many vulnerabilities have been fixed in new Windows OSes, but several organizations still have legacy systems

• Thus, these types of NetBIOS vulnerability still exist

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Default NetBIOS Options

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Sniffing Encrypted Traffic

• Obfuscation– Some utilize encryption and others use one-way

function– Make it difficult to intercept a transmission of a

message and retrieve it content

• Base64 encoding – Not suitable for data protection (confidentiality)– No shared key between encode and decode functions

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Sniffing Encrypted Traffic (cont.)

• XOR encryption– Use shared key, provide more protection

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Sniffing Encrypted Traffic (cont.)

• Message Digest– MD5 and SHA-1– Not an encryption algorithm– The premise for using digests and hashes is to make

it difficult for a collision to occur, computationally infeasible to reproduce identical digests/hashes from different input

– Most commonly used to obfuscate passwords– Vulnerable to brute-force and rainbow table attacks

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Attacking Kerberos

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Attacking Kerberos (cont.)

• Communications between C and AS is derived from user’s password

• Windows computes a one-way function (OWF) against the password to generate the key

• If the hash value is known, we can run a dictionary attack against the key

• Known ciphertext attack if the ciphertext and algorithm are known, we can perform computations with known plaintext until the ciphertext is matched

• Security of Kerberos depends on how strong the password is

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Attacking Kerberos (cont.)

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Hashed password

Attacking Kerberos (cont.)

• Run Kerbsniff, a command-line utility to capture AS-REQ value to a text file

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Attacking Kerberos (cont.)

• Run Kerbcrack on the hashed password to recover the password

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Dictionary attackInput file

Attacking Weak Protection Schemes

• Many people said that data protection requires strong cryptographic algorithms that can resist a dictionary or brute-force attack

• However, much of the problems lies in selecting weak password

• Weak protection scheme reduces the effort an attacker must take once the hash value is known

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John the Ripper

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Question?

Next week

Presentation Layer Security