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Foundations of Network and Computer Security. J ohn Black Lecture #32 Nov 18 th 2009. CSCI 6268/TLEN 5550, Fall 2009. StackGuard. Idea (1996): Change the compiler to insert a “canary” on to the stack just after the return address

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Foundations of network and computer security l.jpg
Foundations of Network and Computer Security

John Black

Lecture #32

Nov 18th 2009

CSCI 6268/TLEN 5550, Fall 2009

Stackguard l.jpg

  • Idea (1996):

    • Change the compiler to insert a “canary” on to the stack just after the return address

    • The canary is a random value assigned by the compiler that the attacker cannot predict

    • If the canary is clobbered, we assume the return address was altered and we terminate the program

    • Built in to Windows 2003 Server and provided by Visual C++ .NET

      • Use the /GS flag; on by default (slight performance hit)

Sample stack with canary l.jpg
Sample Stack with Canary





Return address

4 bytes





4 bytes


4 bytes


Canaries can be defeated l.jpg
Canaries can be Defeated

  • A nice idea, but depending on the code near a buffer overflow, they can be defeated

    • Example: if a pointer (int *a) is a local and we copy another local (int *b) to it somewhere in the function, we can still over-write the return address

      • Not too far fetched since we commonly copy ptrs around

Avoiding canaries l.jpg
Avoiding Canaries



int *a

Address of ret

int *b

Address of i

Address of buffer

int i




Address of buffer

Return address

First, overflow the buffer as shown above.

Then when executing *a = *b we will copy code start addr into ret

Moral if overruns exist high probability of an exploit l.jpg
Moral: If Overruns Exist, High Probability of an Exploit

  • There have been plenty of documented buffer overruns which were deemed unexploitable

  • But plenty of them are exploitable, even when using canaries

  • Canaries are a hack, and of limited use

Non executing stacks and return to libc l.jpg
Non-Executing Stacks and Return to LibC

  • Suppose the stack is marked as non-executable

    • Some hardware can enforce bounded regions for executable code

    • This is not the case on generic Linux, however, since all our example programs for stack overruns work just fine, but there is a Linux version which supports this

      • Has to do all kinds of special stuff to accommodate programs which need an executable stack

      • Linux uses executable stacks for signal handling

      • Some functional languages use an executable stack for dynamic code generation

      • The special version of Linux has to detect this and allow executable stacks for these processes

Return to libc getting around the non executing stack problem l.jpg
Return to LibC: Getting around the Non-Executing Stack Problem

  • Assume we can still over-write the stack

    • 1) Set return address to system() in LibC

      • Use address of dynamically-linked entry point

    • 2) Write any sfp

    • 3) Write address of exit() as new ret addr

    • 4) Write pointer to “/bin/sh”

    • 5) Write string “/bin/sh”

Return to libc stack configuration l.jpg
Return to LibC: Stack Configuration Problem


Garbage -- Unimportant




Address of system()


Address of exit()

ptr to s



First, overflow the buffer as shown above.

When function returns, we go to system(“/bin/sh”) which spawns a shell

Automated source code analysis l.jpg
Automated Source Code Analysis Problem

  • Advantages:

    • Can be used as a development tool (pre-release tool)

    • Can be used long after release (legacy applications)

    • Method is proactive rather than reactive

      • Avoid vulnerabilities rather than trying to detect them at run-time

  • In order to conduct the analysis, we need to build a model of the program

    • The model will highlight features most important for security

Password crackers l.jpg
Password Crackers Problem

  • Unix approach: store one-way hash of password in a public file

    • Since hash is one-way, there is no risk in showing the digest, right?

    • This assumes there are enough inputs to make exhaustive search impossible (recall IP example from the midterm)

    • There are enough 10-char passwords, but they are NOT equally likely to be used

      • HelloThere is more likely than H7%$$a3#.4 because we’re human

Password crackers cont l.jpg
Password Crackers (cont) Problem

  • Idea is simple: try hashing all common words and scan for matching digest

    • Original Unix algorithm for hash is to iterate DES 25 times using the password to derive the DES key

      • DES25(pass, 064) = digest

      • Note: this was proved secure by noticing that this is the CBCMAC of (064)25 under key ‘pass’ and then appealing to known CBCMAC results

      • Why is DES iterated so many times?

Password crackers cont13 l.jpg
Password Crackers (cont) Problem

  • Note: Actually uses a variant of DES to defeat hardware-based approaches

  • Note: Modern implementations often use md5 instead of this DES-based hash

  • But we can still launch a ‘dictionary attack’

    • Take large list of words, names, birthdays, and variants and hash them

    • If your password is in this list, it will be cracked

Password crackers example l.jpg
Password Crackers: example Problem

Pasword file




[email protected]











knuth:ih*22882h*[email protected]*8haa





Making things harder salt l.jpg
Making Things Harder: Salt Problem

  • In reality, Unix systems always add a two-character “salt” before hashing your password

    • There are 4096 possible salts

    • One is randomly chosen, appended to your password, then the whole thing is hashed

    • Password file contains the digest and the salt (in the clear)

    • This prevents attacking all passwords in /etc/passwd in parallel

Password crackers with salt l.jpg
Password Crackers: with Salt Problem

Table for Salt Value: A6

Pasword file




[email protected]










no match


knuth:ih*22882h*[email protected]*8haaU%





Fighting the salt 4096 tables l.jpg
Fighting the Salt: 4096 Tables Problem

  • Crackers build 4096 tables, one for each salt value

    • Build massive databases, on-line, for each salt

      • 100’s of GB was a lot of storage a few years ago, but not any longer!

      • Indexed for fast look-up

      • Most any common password is found quickly by such a program

      • Used by miscreants, but also by sysadmins to find weak passwords on their system

Getting the etc passwd file l.jpg
Getting the /etc/passwd File Problem

  • Public file, but only if you have an acct

    • There have been tricks for remotely fetching the /etc/passwd file using ftp and other vulnerabilities

    • Often this is all an attacker is after

      • Very likely to find weak passwords and get on the machine

    • Of course if you are a local user, no problem

    • Removing the /etc/passwd from global view creates too many problems

Shadowed passwords l.jpg
Shadowed Passwords Problem

  • One common approach is to put just the password digests into /etc/shadow

    • /etc/passwd still has username, userid, groupid, home dir, shell, etc., but the digests are missing

    • /etc/shadow has only the username and digests (and a couple of other things)

    • /etc/shadow is readable and writeable for root only

      • Makes it a bit harder to get a hold of

      • Breaks some software that wants to authenticate users with their passwords

        • One might argue that non-root software shouldn’t be asking for user passwords anyhow

    • BSD does things a little differently

Last example ingres authorization strings l.jpg
Last Example: Ingres Authorization Strings Problem

  • Ingres, 1990

    • 2nd largest database company behind Oracle

  • Authorization Strings

    • Encoded what products and privileges the user had purchased

      • Easier to maintain this way: ship entire product

      • Easier to sell upgrades: just change the string

  • Documentation guys

    • Needed an example auth string for the manual

Moral l.jpg
Moral Problem

  • There’s no defending against stupidity

  • Social engineering is almost always the easiest way to break in

    • Doesn’t work on savvy types or sys admins, but VERY effective on the common user

    • I can almost guarantee I could get the password of most CU students easily

      • “Hi this is Jack Stevens from ITS and we need to change your password for security reasons; can you give me your current password?”

Social engineering phishing l.jpg
Social Engineering: Phishing Problem

  • Sending authentic looking email saying “need you to confirm your PayPal account information”

    • Email looks authentic

    • URL is often disguised

    • Rolling over the link might even pop-up a valid URL in a yellow box!

    • Clicking takes you to attacker’s site, however

      • This site wants your login info

Disguising urls l.jpg
Disguising URLs Problem

  • URI spec

    • [email protected]:// is supposed to send you to

      • Can be used to disguise a URL:

        •>jd7788<AccountMaintenace-495[email protected]

      • Notice feel-good words

      • Length of URI exceeds width of browser, so you may not see the end

      • could be hex encoded for more deception

        • %77%77%77%2e%65%76%69%6c%2e%63%6f%6d =

Disguising url s cont l.jpg
Disguising URL’s (cont) Problem

  • This no longer works on IE

  • Still works on Mozilla

  • In IE 5.x and older, there was another trick where you could get the toolbar and URL window to show “” even though you had been sent to a different site

    • Very scary

  • Moral: don’t click on email links; type in URL manually

Digression character encodings l.jpg
Digression: Character Encodings Problem

  • Normally web servers don’t allow things like this:


      • The “..” is filtered out

    • Character encodings can sometimes bypass the filter

      • Unicode is a code for representing various alphabets

      • . = C0 AE

      • / = C0 AF

      • \ = C1 9C

    • In Oct 2000, a hacker revealed that IIS failed to filter these encodings

      • …/~jrblack/%C0AE/%C0AE/%C0AE/%C0AE/etc/passwd

Segue to web security l.jpg
Segue to Web Security Problem

  • The web started out simple, but now is vastly complex

    • Mostly because of client-side scripting

      • Javascript, Java applets, Flash, Shockwave, VBScript, and more

    • And server-side scripting

      • CGIs (sh, C, perl, python, almost anything), Java servlets, PHP, ASP, JSP

    • All of these are security-sensitive

      • Ugh

We can t hope to survey all possible web security issues l.jpg
We Can’t Hope to Survey all Possible Web Security Issues Problem

  • Too many to count

  • Goal: look at a few thematic ones

  • Cataloguing all of them would not be very instructive, most likely

Typical server side vulnerability l.jpg
Typical Server-Side Vulnerability Problem

  • PHP: Personal HomePage (?!)

    • An easy-to-use and Perl-like server-side scripting language

    • A “study in poor security” – Gary McGraw

    • Variables are dynamically declared, initialized, and global by default; this can be dangerous:

      • if(isset($page)) {   include($page); }

      • Now we call this script with:

        • script.php?page=/etc/passwd

Javascript l.jpg
Javascript Problem

  • Javascript (and VBScript) can do bad things

    • Get your cookies, for one, which may include sensitive information

  • You don’t want to run scripts unless the site you are visiting is “trustworthy”

    • Javascript has had a large number of security problems in the past; dubious sites might take advantage of these

    • If you set your security level high in IE, it turns off Javascript; that should tell you something

Javascript cont l.jpg
Javascript (cont) Problem

  • Turning it off in your browser is one solution

    • But often you lose a bunch of functionality

  • How can an attacker get you to run his malicious Javascript code?

    • Gets you to visit his website

      • But you might know better

    • Old trick: post something to a bulletin board with <script>…</script> in it

    • When you view his post, you run his script!

Filtering l.jpg
Filtering Problem

  • To prevent this, a correct bulletin board implementation always filters stuff that others have posted

  • You can post <b>YES!</b> but not <script> evil stuff… </script>

  • But until recently we didn’t worry about filtering stuff from you to yourself! 

Xss attacks l.jpg
XSS Attacks Problem

  • XSS: Cross Server Scripting

    • Not CSS (Cascading Style Sheets)

    • Idea: you open a website, passing a value, and the site echoes back that value

      • What if that value has a script embedded?!

    • Example: 404 Not Found

      • Suppose you see a link (in email, on IRC, on the web) saying, “Click here to search Google”

        • The link really does go to google, so what the heck…

        • However the link is

          • Above contains an embedded, hidden script

        • Google says, “badurl%0a%5C…” not found

        • Just displaying this to you, executes the script

Xss vulnerabilities l.jpg
XSS Vulnerabilities Problem

  • They’ve been found all over the web

    • Fairly new problem

    • Lots of examples still exist in the wild

    • Very tricky to find them all

  • Solution is to filter, of course

    • Need to filter inputs from users that server will be echoing back to the user

Phishing revisited l.jpg
Phishing Revisited Problem

Dear Amazon User,

During our regular update and verification of the accounts, we could not verify your current information. Either your information has changed or it is incomplete.

As a result, your access to buy on Amazon has been restricted. To continue using your Amazon account again, please update and verify your information by clicking the link below :

http://[email protected]/exec/obidos/subst/home/?EnterConfirm&UsingSSL=0&pUserId=&us=445&ap=0&dz=1&Lis=10&ref=br_bx_c_2_2

Thank you very much for your cooperation!

Amazon Customer Support

Please note: This e-mail message was sent from a notification-only address

that cannot accept incoming e-mail. Please do not reply to this message.

Earth's Biggest Selection

Where does the info go l.jpg
Where does the info go? Problem

  • maps to IP

    % whois

    OrgName: Yahoo!

    OrgID: YAOO

    Address: 701 First Avenue

    City: Sunnyvale

    StateProv: CA

    NetRange: -

Defenses against phishing l.jpg
Defenses Against Phishing Problem

  • Spoofguard

    • Product out of Stanford

    • Doesn’t work for me (ugh!)

    • Detects various suspicious behaviors and flags them

      • Red light, green light depending on threshold

  • There are others as well

  • Bottom line:

    • Don’t believe emails from “legitimate companies”

    • This is frustrating for companies!

Wireless security l.jpg
Wireless Security Problem

  • Why is wireless security essentially different from wired security?

    • Almost impossible to achieve physical security on the network

    • You can no longer assume that restricting access to a building restricts access to a network

      • The “parking lot attack”

Wireless security challenges l.jpg
Wireless Security Challenges Problem

  • Further challenges:

    • Many wireless devices are resource-constrained

      • Laptops are pretty powerful these days but PDAs are not

      • Sensors are even more constrained

      • RFIDs are ridiculously constrained

    • Paradox: the more resource-constrained we get, the more ambitious our security goals tend to get

Ieee 802 11a b g l.jpg
IEEE 802.11a/b/g Problem

  • A standard ratified by IEEE and the most widely-used in the world

    • Ok, PCS might be a close contender

    • Also called “Wi-Fi”

      • 802.11 products certified by WECA (Wireless Ethernet Compatibility Alliance)

    • Bluetooth is fairly commonplace but not really used for LANs

      • More for PANs (the size of a cubicle)

      • Connect PDA to Cell Phone to MP3, etc.

Wireless network architecture l.jpg
Wireless Network Architecture Problem

  • Ad Hoc

    • Several computers form a LAN

  • Infrastructure

    • An access point (AP) acts as a gateway for wireless clients

    • This is the model we’re most used to

    • Available all through the EC, for example

War driving l.jpg
War Driving Problem

  • The inherent physical insecurity of wireless networks has led to the “sport” of war-driving

    • Get in your car, drive around, look for open access points with you laptop

    • Name comes from the movie “War Games”

    • Some people get obsessed with this stuff

    • You can buy “war driving kits” on line

      • Special antennas, GPS units to hook to you laptop, mapping software

More war driving l.jpg
More War Driving Problem

  • People use special antennas on their cars

    • It used to be Pringles cans, but we’ve moved up in the world

  • People distribute AP maps

  • War driving contest at BlackHat each year

What s the big deal l.jpg
What’s the Big Deal? Problem

  • My home access point is wide-open

    • People could steal bandwidth

    • I’m not that worried about it

    • People could see what I’m doing

    • I’m not that worried about it

  • There are ways to lock-down your access point

    • MAC filtering

    • Non-signalling APs and non-default SSIDs

    • Wired Equivalent Privacy (WEP)

Mac filtering l.jpg
MAC Filtering Problem

  • Allow only certain MACs to associate

    • Idea: you must get permission before joining the LAN

    • Pain: doesn’t scale well, but for home users not a big deal

    • Drawback: people can sniff traffic, figure out what MACs are being used on your AP, then spoof their MAC address to get on

Non signalling aps l.jpg
Non-Signalling APs Problem

  • 802.11 APs typically send a “beacon” advertising their existence (and some other stuff)

    • Without this, you don’t know they’re there

    • Can be turned off

    • If SSID is default, war drivers might find you anyway

      • SSID is the “name” of the LAN

      • Defaults are “LinkSYS”, NETGEAR, D-Link, etc

      • Savvy people change the SSID and turn off beacons

    • SSID’s can still be sniffed when the LAN is active however, so once again doesn’t help much

Let s use crypto l.jpg
Let’s Use Crypto! Problem

  • WEP (Wired Equivalent Privacy)

    • A modern study in how not to do things

    • The good news: it provides a wonderful pedagogical example for us

  • A familiar theme:

    • WEP was designed by non-cryptographers who knew the basics only

      • That’s enough to blow it

Wep protocol l.jpg
WEP Protocol Problem

  • One shared key k, per LAN

    • All clients and APs have a copy of k

    • We are therefore in the symmetric key setting

      • Very convenient: no public key complexities needed

      • Has drawbacks, as we’ll see later

    • In the symmetric key model, what do we do (minimally) for data security?

      • Authentication and Privacy!

      • (MAC and encrypt)

Wep protocol52 l.jpg
WEP Protocol Problem

  • For message M, P = (M, c(M))

    • c() is an unkeyed CRC (cyclic redundancy check)

  • Compute C = P © RC4(v, k)

    • RC4 is a stream cipher

      • Think of a stream cipher as a “randomness stretcher”: give it n random bits and it produces (essentially) infinite pseudo-random bits

      • The input is variously called the “seed” or the “key”

      • Seems a lot like a pseudo-random number generator!

      • We will look at RC4 in more detail later

    • v is an IV

      • As usual, the IV should never be repeated over the life of the key

  • Sender transmits (v, C)

Wep decryption l.jpg
WEP Decryption Problem

  • Receiver obtains (v’, C’) and knows k

    • Computes C’ © RC4(v’, k) = (P’ © RC4(v’,k)) © RC4(v’,k) = P’

    • Then checks integrity with P’ = (M’, c’) and asking whether c’ = c(M’)

      • If not, reject the frame as inauthentic

    • Looks familiar, but we should be suspicious: a keyless function is not a MAC!

Goals l.jpg
Goals Problem

  • Security Goals of WEP:

    • Privacy

    • Integrity

      • What we also have called “authenticity”

      • It should be “hard” to tamper with ciphertexts without being detected

      • It should be “hard” to forge packets

    • Access Control

      • Discard all packets not properly encrypted with WEP (optional part of the 802.11 standard)

  • WEP Document:

    • Security “relies on the difficult of discovering the secret key through a brute-force attack”

Wep keys l.jpg
WEP Keys Problem

  • 802.11 was drafted when 40 bits were all we could export

    • This restriction was lifted in 1998, but the standard was already in draft form

    • Some manufacturers extended the key to an optional 128-bit form

      • This is misleading: the 128 form uses a 104 bit key because the IV is 24 bits

Wep keys56 l.jpg
WEP Keys Problem

  • Two forms: the 40 bit key

  • The “128” bit key



24 bits

40 bits



24 bits

104 bits

Recall: IV is public, so shouldn’t count as “key”

Entering wep keys l.jpg
Entering WEP Keys Problem

Note: Four keys allowed to encourage key-rotation, but this has to all be synchronized

among all users of the WLAN.

Goals achieved l.jpg
Goals Achieved: Problem;

  • Let’s start with the Privacy goal

    • WEP is using an encryption pad; what is the cardinal rule of encryption pads?

    • So how might a pad be re-used?

      • If the IV repeats, the pad will repeat:

        • Pad is RC4(v, k)

        • k is fixed for all communications

      • Since IV is public, an attack sees when the IV repeats

Iv repeats l.jpg
IV repeats Problem

  • It’s bad:

    • Some cards fix IV=0, end of story

      • (This is 802.11 compliant, by the way!)

    • Some cards re-initialize IV to 0 each time they are powered up

      • So each time you insert a PCMCIA card into your laptop, or power up your laptop

      • IV repeats in the lower range far more likely here

    • The IV is only 24 bits, so eventually it will wrap around

      • 1500-byte packets, 5Mbps, IV wraps in less than 12 hours

      • With random IVs, the birthday effect says we expect a repeat within 5000 packets (a few mins in the scenario above)

What to do with repeated ivs l.jpg
What to do with repeated IVs? Problem

  • Build a “decryption dictionary”

    • Once we figure out the plaintext

      • Because it’s broadcast in the clear and encrypted

      • Because it’s part of a standard transmission

      • Because you injected the message from the outside

    • …then we know the keystream

    • Put keystream and IV into a table for later use

      • Allows quick decryption of any ciphertext where we know the keystream of its IV

      • About 24 GB to store 1500 bytes for each of the possible 224 IVs

  • Note: it would probably be easier to brute-force the 40-bit key

    • But this approach works against the 104-bit key as well

Authentication l.jpg
Authentication Problem

  • Recall c() was a CRC

    • CRC’s are polynomials over a Galois Field of characteristic 2; therefore they are linear over addition, which in this field is ©

    • Hunh?

    • Function c has the following property:

      • c(x © y) = c(x) © c(y)

    • This property lets us modify any ciphertext such that the WEP integrity check will still pass

      • C = RC4(v, k) © (M, c(M))

      • We want to change M to M’

Altering wep ciphertext l.jpg
Altering WEP Ciphertext Problem

  • Suppose we want M’ = M © instead of M

    • Compute C’ = C © (, c())

    • Let’s check:

      • C’ = C © (, c())

        = RC4(v, k) © (M, c(M)) © (, c())

        = RC4(v, k) © (M ©, c(M) © c())

        = RC4(v, k) © (M’, c(M ©))

        = RC4(v, k) © (M’, c(M’))

    • Note: we don’t need to know what M is to do this; we can blindly modify M as we desire

Defeating the wep access mechanism l.jpg
Defeating the WEP Access Mechanism Problem

  • Recall that mal-formed WEP packets are discarded (optional feature)

    • If we know one plaintext and its corresponding ciphertext, we are able to inject arbitrary traffic into the network

    • Suppose we know M, v, and C = RC4(v, k) © (M, c(M))

      • Then we know c(M) // c() is public and unkeyed

      • So we know RC4(v, k)

      • Now we can produce C’ = RC4(v, k) © (M’, c(M’))

    • Note: we are re-using an IV, but that’s ok according to the WEP specification

Summary wep is no good l.jpg
Summary: WEP is no good Problem

  • A tenet of security protocol design: “don’t do it”

  • And after all this, I actually recommend running WEP

    • It does create a barrier to the casual hacker

    • It doesn’t add much of a performance hit

    • It does give you legal recourse