CS390S week 13: RandomnessPascal Meunier, Ph.D., M.Sc., CISSPNovember 15, 2006Developed thanks to the support of Symantec Corporation,NSF SFS Capacity Building Program (Award Number 0113725) and the Purdue e-Enterprise CenterCopyright (2004) Purdue Research Foundation. All rights reserved.

Learning objectives

Understand why creating files in insecure directories like /tmp is difficult but useful

Learn why OS-provided function calls help tremendously

Understand the need for good randomness Learn which OS-provided function calls help provide

good random numbers Learn how to create random file names Learn a randomness visualization technique Understand the need for randomness in networking

protocols such as TCP

Temporary Files

Space for temporary files is found in directories such as /tmp, /var/tmp or C:\TEMP, where everyone can write

Space may be purged regularly (e.g., "every night, files older than 5 days are deleted") and during reboot

Space used by many UNIX or Windows utilities, installers and programs

UNIX systems are often configured so that this space is not counted as part of user quota– Allow large, temporary jobs

Shared vs Secured Directories

Secured Directories– Solution discussed in previous slides (Part 5)

e.g., temporary directory in user directories in Windows– %userprofile%\Local Settings\Temp

– Avoid the problems of shared directories Most of the problems discussed in these slides don't apply to

secured directories

– Prefered solution

What if you need or want to use a shared directory?– What are the dangers of using shared directories?– What do you need to do to avoid these problems?– If that's what you want to know keep reading these slides

Race Conditions In Shared Directories

There's a race condition between testing if a file already exists and creating it– Need a unique and unpredictable name to avoid a

collision between links and your files or directories

There's a race condition between creating a file and changing its permissions– Permissions initially set by OS based on:

umask (UNIX) ACLs of parent (Windows)

– Are they the desired and correct permissions?

Name Collisions Attacks

What if the name of your temporary file (lock file or other) in /tmp is constant or predictable?– Your program using a lock file may never run or do what

it's supposed to! Run the lock.c example from part A, but this time, create a

lock file beforehand... Your program will never get past the lock file test (obviously)

Lock files need to be put where other users can't create files

– It's easy to make a symlink pointing to a sensitive file

Symlink attacks are easier if the name of the temporary file is predictable

How Not to Choose a Random Name

Use the process ID Use the user ID Use the time of day Use a counter Use a bad random number generator etc...

OS Support for Temporary Files

The following take a filename “template” as input– mktemp - generate temporary file name (unique)– mkstemp - also create the file– mkstemps - generate temporary file name with suffix– mkdtemp - create a directory

Overwrite part of a template to create a unique name

Some of these functions used to create names using parts of the date or process ID, etc... and were insecure

mktemp (1) (3)

Section (1): command line (shell scripts)– BSD/MacOS X:– creates file with mode 0600

unique name

Section (3): C programs– Race condition between getting the name and creating the

file!– The program must use "open" with the O_CREAT |

O_EXCL flags, and loop until the file is successfully created, or use a different function

Command Line Example

% mktemp "testXXXX"testpnbE% ls -al-rw------- pascal staff testpnbE

mkstemp

Creates name Creates file open for reading and writing with mode

0600 Returns a file descriptor No race condition! Recommended function Usage for extremely paranoid people:

“Unlink” the hard link pointing to the descriptor immediately afterwards (this is a race condition)

The file still exists but nobody else (except with difficulty, the superuser) can access it

Mini Lab

Take the previous lock.c example Modify it to use mkstemp to generate a temporary

file with a unique name Of course, the temporary file created that way is not

a lock file anymore, and would be used to store temporary data instead

Windows Shared Directories

No equivalent to mkstemp() GetTempFileName

– Creates names by incrementing a counter!– Predictable file name

Race condition between getting the name and creating the file– Attacker could create the file to prevent you from using it– If you use the CREATE_ALWAYS flag, see next slide

Under Windows, you have no choice but to write your own function

Still a race condition, limitation due to lack of OS support (use secured directories instead)

Windows CreateFile Problems

Recommended use with the "CREATE_ALWAYS" flag is dangerous– "CREATE_ALWAYS" flag recommended by MSDN,

Howard and Leblanc 2003 Overwrites the file Does not set the security descriptor specified by the

SECURITY_ATTRIBUTES structure– Do the SECURITY_ATTRIBUTES matter to your application?

Perfect opportunity to trick you into overwriting a sensitive file– e.g., with a hard link– Can't use the flag to not follow reparse points– Note that links being uncommonly used in Windows FS

won't prevent an attack from succeeding

Windows CreateFile

TRUNCATE_EXISTING will follow a hard link and could truncate something else than intended

Use "CREATE_NEW"– "The function fails if the specified file already exists. "

(MSDN) – You need to check for errors and loop until the file is

successfully created

GetTempPath

MSDN recommends that software use the GetTempPath function to get the location of the temp dir, but this is dangerous

Checks for the existence of environment variables in the following order and uses the first path found:1. The path specified by the TMP environment variable.

2. The path specified by the TEMP environment variable.

3. The path specified by the USERPROFILE environment variable.

4. The Windows directory."

Are the environment variables safe to use?– Probably not unless you set them yourself

Exercise (Windows): Creating Temporary Files

Go to http://msdn.microsoft.com/library/default.asp?url=/library/en-us/fileio/base/creating_and_using_a_temporary_file.asp

Discuss things that you would do differently, compared to the example, when creating a temporary file in Windows– Find the race condition (hint: MoveFileEx)

Exercise Answers

Possible answers:– They used the CREATE_ALWAYS flag instead of

CREATE_NEW Add a loop until success

– Use randomly generated file names How to do that on Windows? (see next slides)

The Need for Random Numbers

Unique file or directory names Session IDs that carry proof of authentication

(nonces), passwords Games (data, behavior, opponent generation,

character generation) Encryption Cryptographic protocols

How Random Numbers Are Generated

Linear Congruential Generators– Simple way to generate pseudo-random numbers– Easily cracked– Produce finite sequences of numbers– Each number is tied to the others– Some sequences of numbers will not ever be generated

Cryptographic random number generators Entropy sensors (i.e., extracted randomness)

Seeded Random Number Generators

Pseudo-random generators depend solely on a seed, which determines the entire sequence of numbers returned

How random is the seed?– Process ID, UserID: Bad Idea– Current time: if you’re running NTP (Network Time

Protocol) all systems are synchronized up to some precision. If you use the time, maybe I can guess which seed you used (microsecond part might be difficult to guess, but is limited)

How to Cheat At Random Number Generation

Find a seed that will produce the numbers you want Seed the generator with it Convince someone: "it's random, see?"

– RPG Character generation, etc...

Roll Your Own Generator?

What matters is not only the average and the variance of the numbers generated

All sequences of numbers must be possible LCGs travel definite, limited “paths” through the

universe of possible sequences Need to incorporate entropy as it becomes available Need to avoid betraying the internal state of the

generator... It's difficult to do correctly

Which Generator to use?

Read description, avoid Linear Congruential Generators such as these:– “C” rand(3)– rand (Windows CE, Visual C++, Visual Basic, etc...)– Perl rand – C# Random– PHP rand

Good Generators

Hardware-based– Noise

Cryptographical quality software, entropy-seeded– Fast, secure

Pure Entropy– Random timing of events

Packets Mouse movement, clicks Keyboard

– Slow

Linux/UNIX Devices

/dev/random:– MacOS X: same as urandom– Linux: this is a blocking call that returns only when

sufficient entropy has been captured– Good for seeding pseudo-random number generators

/dev/urandom:– Implements a fairly complex algorithm that varies between

“random” and a well-seeded LCG depending on the availability of entropy

– Non-blocking call– Try "cat /dev/urandom"

Portability

FreeBSD, OpenBSD, NetBSD compatible Several projects ported the functionality to Solaris,

HP-UX, AIX, IRIX MacOS X implements Yarrow for both random and

urandom (so the behavior of “random” is unexpected).

Windows

Windows developers must use the function CryptGenRandom(), which uses the same idea as /dev/urandom

There is no directly accessible entropy collector provided by the OS– Reference: "Secure Programming Cookbook", section

11.4 (Viega et al.)

Randomness Visualization

Strange attractors–Zalewski 2001, 2002 "Strange Attractors and TCP/IP

Sequence Number Analysis"

Given a sequence of numbers s[n] compute:–x[n] = s[n-2] - s[n-3]–y[n] = s[n-1] - s[n-2]–z[n] = s[n] - s [n-1]

These are the x,y,z coordinates of a point–Plot them to see hidden dependencies

Cisco IOS 12.2 (Zalewski 2002)

IRIX (Zalewski 2002)

Windows XP (Zalewski 2002)

ISN Vulnerabilities

Predictable–Symantec Raptor Firewall 6.5 and 6.5.3, Enterprise

Firewall 6.5.2 and 7.0, VelociRaptor Models 500/700/1000 and 1100/1200/1300, and Gateway Security 5110/5200/5300 generate easily predictable initial sequence numbers (ISN), which allows remote attackers to spoof connections.

CAN-2002-1463

–Cisco switches and routers running IOS 12.1 and earlier produce predictable TCP Initial Sequence Numbers (ISNs), which allows remote attackers to spoof or hijack TCP connections.

CVE-2001-0288

–etc...

TCP RST Flag

TCP reset (RST) flag is used to abort TCP connections, usually to signify an irrecoverable error

–Receiver deletes the connection, frees data structures

RST messages are accepted only if they fit inside the sequence number window

–Prevents delayed RST messages from previous connections to affect the current connection

TCP RST Attack

Send a RST (TCP RESET flag) packet with a spoofed IP address to either side of a valid connection

–Need to guess a sequence number inside the appropriate window

Or sniff traffic to know which number to use

–The range can be guessed fairly efficiently for RST attacks–Sequence numbers: 32 bits–Window size: up to 16 bits–Number of guesses 32-16 = 16 bit address space

65535 RST attempts, ~ 4 min on DSL connectionFaster connection or zombies, faster RSTThis is the brute force RST attack

TCP Session Hijacking

Idea: all that’s required to mess up someone else’s TCP session is guessing or knowing the sequence numbers for their connection.

–Only need to fall within the needed range, exact guess not needed

Send a spoofed IP packet, with a TCP payload that inserts dataBlast the legitimate client off the net

–Replies are still sent to client but client is incapacitated–You do not get to see replies: “blind” hijacking

Unless you can sniff traffic, in which case the sequence numbers to use are also known

Questions or Comments?

About These Slides

You are free to copy, distribute, display, and perform the work; and to

make derivative works, under the following conditions.

– You must give the original author and other contributors credit

– The work will be used for personal or non-commercial educational uses

only, and not for commercial activities and purposes

– For any reuse or distribution, you must make clear to others the terms of

use for this work

– Derivative works must retain and be subject to the same conditions, and

contain a note identifying the new contributor(s) and date of modification

– For other uses please contact the Purdue Office of Technology

Commercialization.

Developed thanks to the support of Symantec Corporation

Pascal Meunierpmeunier@purdue.eduContributors:Jared Robinson, Alan Krassowski, Craig Ozancin, Tim Brown, Wes Higaki, Melissa Dark, Chris Clifton, Gustavo Rodriguez-Rivera, Michael Howard