The definitive guide to form-based website authentication

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Form-based authentication for websites  We believe that Stack nbsp Overflow should not just be a resource for very specific technical questions  but also for general guidelines on how to solve variations on common problems   Form based authentication for websites  should be a fine topic for such an experiment   It should include topics such as    How to log in How to log out How to remain logged in Managing cookies  including recommended settings  SSL HTTPS encryption How to store passwords Using secret questions Forgotten username password functionality Use of nonces to prevent cross-site request forgeries  CSRF  OpenID  Remember me  checkbox Browser autocompletion of usernames and passwords Secret URLs  public URL protected by digest  Checking password strength E-mail validation and much more about form based authentication      It should not include things like    Roles and authorization HTTP basic authentication   Please help us by    Suggesting subtopics Submitting good articles about this subject Editing the official answer

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I just thought I d share this solution that I found to be working just fine   I call it the Dummy Field  though I haven t invented this so don t credit me    In short  you just have to insert this into your  lt form gt  and check for it to be empty at when validating    lt input type  text  name  email  style  display none    gt    The trick is to fool a bot into thinking it has to insert data into a required field  that s why I named the input  email   If you already have a field called email that you re using you should try naming the dummy field something else like  company    phone  or  emailaddress   Just pick something you know you don t need and what sounds like something people would normally find logical to fill in into a web form  Now hide the input field using CSS or JavaScript jQuery - whatever fits you best - just don t set the input type to hidden or else the bot won t fall for it   When you are validating the form  either client or server side  check if your dummy field has been filled to determine if it was sent by a human or a bot   Example   In case of a human  The user will not see the dummy field  in my case named  email   and will not attempt to fill it  So the value of the dummy field should still be empty when the form has been sent   In case of a bot  The bot will see a field whose type is text and a name email  or whatever it is you called it  and will logically attempt to fill it with appropriate data  It doesn t care if you styled the input form with some fancy CSS  web-developers do it all the time  Whatever the value in the dummy field is  we don t care as long as it s larger than 0 characters   I used this method on a guestbook in combination with CAPTCHA  and I haven t seen a single spam post since  I had used a CAPTCHA-only solution before  but eventually  it resulted in about five spam posts every hour  Adding the dummy field in the form has stopped  at least until now  all the spam from appearing    I believe this can also be used just fine with a login authentication form   Warning  Of course this method is not 100  foolproof  Bots can be programmed to ignore input fields with the style display none applied to it  You also have to think about people who use some form of auto-completion  like most browsers have built-in   to auto-fill all form fields for them  They might just as well pick up a dummy field   You can also vary this up a little by leaving the dummy field visible but outside the boundaries of the screen  but this is totally up to you    Be creative

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I d like to add one suggestion I ve used  based on defense in depth  You don t need to have the same auth amp auth system for admins as regular users  You can have a separate login form on a separate url executing separate code for requests that will grant high privileges  This one can make choices that would be a total pain to regular users  One such that I ve used is to actually scramble the login URL for admin access and email the admin the new URL  Stops any brute force attack right away as your new URL can be arbitrarily difficult  very long random string  but your admin user s only inconvenience is following a link in their email  The attacker no longer knows where to even POST to

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Definitive Article  Sending credentials  The only practical way to send credentials 100  securely is by using SSL  Using JavaScript to hash the password is not safe  Common pitfalls for client-side password hashing    If the connection between the client and server is unencrypted  everything you do is vulnerable to man-in-the-middle attacks  An attacker could replace the incoming javascript to break the hashing or send all credentials to their server  they could listen to client responses and impersonate the users perfectly  etc  etc  SSL with trusted Certificate Authorities is designed to prevent MitM attacks  The hashed password received by the server is less secure if you don t do additional  redundant work on the server    There s another secure method called SRP  but it s patented  although it is freely licensed  and there are few good implementations available   Storing passwords  Don t ever store passwords as plaintext in the database  Not even if you don t care about the security of your own site  Assume that some of your users will reuse the password of their online bank account  So  store the hashed password  and throw away the original  And make sure the password doesn t show up in access logs or application logs  OWASP recommends the use of Argon2 as your first choice for new applications  If this is not available  PBKDF2 or scrypt should be used instead  And finally if none of the above are available  use bcrypt   Hashes by themselves are also insecure  For instance  identical passwords mean identical hashes--this makes hash lookup tables an effective way of cracking lots of passwords at once  Instead  store the salted hash  A salt is a string appended to the password prior to hashing - use a different  random  salt per user  The salt is a public value  so you can store them with the hash in the database  See here for more on this   This means that you can t send the user their forgotten passwords  because you only have the hash   Don t reset the user s password unless you have authenticated the user  users must prove that they are able to read emails sent to the stored  and validated  email address    Security questions  Security questions are insecure - avoid using them  Why  Anything a security question does  a password does better  Read PART III  Using Secret Questions in  Jens Roland answer here in this wiki   Session cookies  After the user logs in  the server sends the user a session cookie  The server can retrieve the username or id from the cookie  but nobody else can generate such a cookie  TODO explain mechanisms    Cookies can be hijacked  they are only as secure as the rest of the client s machine and other communications  They can be read from disk  sniffed in network traffic  lifted by a cross-site scripting attack  phished from a poisoned DNS so the client sends their cookies to the wrong servers  Don t send persistent cookies  Cookies should expire at the end of the client session  browser close or leaving your domain    If you want to autologin your users  you can set a persistent cookie  but it should be distinct from a full-session cookie  You can set an additional flag that the user has auto-logged in  and needs to log in for real for sensitive operations  This is popular with shopping sites that want to provide you with a seamless  personalized shopping experience but still protect your financial details  For example  when you return to visit Amazon  they show you a page that looks like you re logged in  but when you go to place an order  or change your shipping address  credit card etc    they ask you to confirm your password   Financial websites such as banks and credit cards  on the other hand  only have sensitive data and should not allow auto-login or a low-security mode   List of external resources   Dos and Don ts of Client Authentication on the Web  PDF  21 page academic article with many great tips    Ask YC  Best Practices for User Authentication Forum discussion on the subject   You re Probably Storing Passwords Incorrectly Introductory article about storing passwords Discussion  Coding Horror  You re Probably Storing Passwords Incorrectly Forum discussion about a Coding Horror article  Never store passwords in a database  Another warning about storing passwords in the database  Password cracking Wikipedia article on weaknesses of several password hashing schemes  Enough With The Rainbow Tables  What You Need To Know About Secure Password Schemes Discussion about rainbow tables and how to defend against them  and against other threads  Includes extensive discussion

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I do not think the above answer is  wrong  but there are large areas of authentication that are not touched upon  or rather the emphasis is on  how to implement cookie sessions   not on  what options are available and what are the trade-offs    My suggested edits answers are   The problem lies more in account setup than in password checking  The use of two-factor authentication is much more secure than more clever means of password encryption Do NOT try to implement your own login form or database storage of passwords  unless  the data being stored is valueless at account creation and self-generated  that is  web 2 0 style like Facebook  Flickr  etc     Digest Authentication is a standards-based approach supported in all major browsers and servers  that will not send a password even over a secure channel     This avoids any need to have  sessions  or cookies as the browser itself will re-encrypt the communication each time  It is the most  lightweight  development approach   However  I do not recommend this  except for public  low-value services  This is an issue with some of the other answers above - do not try an re-implement server-side authentication mechanisms - this problem has been solved and is supported by most major browsers  Do not use cookies  Do not store anything in your own hand-rolled database  Just ask  per request  if the request is authenticated  Everything else should be supported by configuration and third-party trusted software   So      First  we are confusing the initial creation of an account  with a password  with the re-checking of the password subsequently  If I am Flickr and creating your site for the first time  the new user has access to zero value  blank web space   I truly do not care if the person creating the account is lying about their name  If I am creating an account of the hospital intranet extranet  the value lies in all the medical records  and so I do care about the identity     of the account creator   This is the very very hard part  The only decent solution is a web of trust  For example  you join the hospital as a doctor  You create a web page hosted somewhere with your photo  your passport number  and a public key  and hash them all with the private key  You then visit the hospital and the system administrator looks at your passport  sees if the photo matches you  and then hashes the web page photo hash with the hospital private key  From now on we can securely exchange keys and tokens  As can anyone who trusts the hospital  there is the secret sauce BTW   The system administrator can also give you an RSA dongle or other two-factor authentication   But this is a lot of a hassle  and not very web 2 0  However  it is the only secure way to create new accounts that have access to valuable information that is not self-created    Kerberos and SPNEGO - single sign-on mechanisms with a trusted third party - basically the user verifies against a trusted third party   NB this is not in any way the not to be trusted OAuth  SRP - sort of clever password authentication without a trusted third party  But here we are getting into the realms of  it s safer to use two-factor authentication  even if that s costlier  SSL client side - give the clients a public key certificate  support in all major browsers - but raises questions over client machine security     In the end  it s a tradeoff - what is the cost of a security breach vs the cost of implementing more secure approaches  One day  we may see a proper PKI widely accepted and so no more own rolled authentication forms and databases  One day

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A good article about realistic password strength estimation is   Dropbox Tech Blog  raquo  Blog Archive  raquo  zxcvbn  realistic password strength estimation

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I dont t know whether it was best to answer this as an answer or as a comment  I opted for the first option   Regarding the poing PART IV  Forgotten Password Functionality in the first answer  I would make a point about Timing Attacks    In the Remember your password forms  an attacker could potentially check a full list of emails and detect which are registered to the system  see link below     Regarding the Forgotten Password Form  I would add that it is a good idea to equal times between successful and unsucessful queries with some delay function    https   crypto stanford edu  dabo papers webtiming pdf

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I would like to add one very important comment  -    In a corporate  intra- net setting   most if not all of the foregoing might not apply    Many corporations deploy  internal use only  websites which are  effectively   corporate applications  that happen to have been implemented through URLs  These URLs can  supposedly      only be resolved within  the company s internal network    Which network magically includes all VPN-connected  road warriors      When a user is dutifully-connected to the aforesaid network  their identity   authentication   is  already       conclusively known   as is their permission   authorization   to do certain things     such as      to access this website    This  authentication   authorization  service can be provided by several different technologies  such as LDAP  Microsoft OpenDirectory   or Kerberos   From your point-of-view  you simply know this  that anyone who legitimately winds-up at your website must be accompanied by  an environment-variable magically containing      a  token    i e  The absence of such a token must be immediate grounds for 404 Not Found    The token s value makes no sense to you  but  should the need arise   appropriate means exist  by which your website can   authoritatively  ask someone who knows  LDAP    etc    about any and every    question that you may have  In other words  you do not avail yourself of any  home-grown logic   Instead  you inquire of The Authority and implicitly trust its verdict   Uh huh     it s quite a mental-switch from the  wild-and-wooly Internet

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My favourite rule in regards to authentication systems  use passphrases  not passwords  Easy to remember  hard to crack  More info  Coding Horror  Passwords vs  Pass Phrases

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First  a strong caveat that this answer is not the best fit for this exact question  It should definitely not be the top answer   I will go ahead and mention Mozilla   s proposed BrowserID  or perhaps more precisely  the Verified Email Protocol  in the spirit of finding an upgrade path to better approaches to authentication in the future   I   ll summarize it this way    Mozilla is a nonprofit with values that align well with finding good solutions to this problem  The reality today is that most websites use form-based authentication Form-based authentication has a big drawback  which is an increased risk of phishing  Users are asked to enter sensitive information into an area controlled by a remote entity  rather than an area controlled by their User Agent  browser   Since browsers are implicitly trusted  the whole idea of a User Agent is to act on behalf of the User   they can help improve this situation  The primary force holding back progress here is deployment deadlock  Solutions must be decomposed into steps which provide some incremental benefit on their own  The simplest decentralized method for expressing an identity that is built into the internet infrastructure is the domain name  As a second level of expressing identity  each domain manages its own set of accounts  The form    account domain    is concise and supported by a wide range of protocols and URI schemes  Such an identifier is  of course  most universally recognized as an email address  Email providers are already the de-facto primary identity providers online  Current password reset flows usually let you take control of an account if you can prove that you control that account   s associated email address  The Verified Email Protocol was proposed to provide a secure method  based on public key cryptography  for streamlining the process of proving to domain B that you have an account on domain A  For browsers that don   t support the Verified Email Protocol  currently all of them   Mozilla provides a shim which implements the protocol in client-side JavaScript code  For email services that don   t support the Verified Email Protocol  the protocol allows third parties to act as a trusted intermediary  asserting that they   ve verified a user   s ownership of an account  It is not desirable to have a large number of such third parties  this capability is intended only to allow an upgrade path  and it is much preferred that email services provide these assertions themselves  Mozilla offers their own service to act like such a trusted third party  Service Providers  that is  Relying Parties  implementing the Verified Email Protocol may choose to trust Mozilla s assertions or not  Mozilla   s service verifies users    account ownership using the conventional means of sending an email with a confirmation link  Service Providers may  of course  offer this protocol as an option in addition to any other method s  of authentication they might wish to offer  A big user interface benefit being sought here is the    identity selector     When a user visits a site and chooses to authenticate  their browser shows them a selection of email addresses     personal        work        political activism     etc   they may use to identify themselves to the site  Another big user interface benefit being sought as part of this effort is helping the browser know more about the user   s session     who they   re signed in as currently  primarily     so it may display that in the browser chrome  Because of the distributed nature of this system  it avoids lock-in to major sites like Facebook  Twitter  Google  etc  Any individual can own their own domain and therefore act as their own identity provider    This is not strictly    form-based authentication for websites     But it is an effort to transition from the current norm of form-based authentication to something more secure  browser-supported authentication

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Use OpenID Connect or User-Managed Access   As nothing is more efficient than not doing it at all

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When hashing  don t use fast hash algorithms such as MD5  many hardware implementations exist    Use something like SHA-512   For passwords  slower hashes are better   The faster you can create hashes  the faster any brute force checker can work  Slower hashes will therefore slow down brute forcing  A slow hash algorithm will make brute forcing impractical for longer passwords  8 digits

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PART I  How To Log In We ll assume you already know how to build a login password HTML form which POSTs the values to a script on the server side for authentication  The sections below will deal with patterns for sound practical auth  and how to avoid the most common security pitfalls  To HTTPS or not to HTTPS  Unless the connection is already secure  that is  tunneled through HTTPS using SSL TLS   your login form values will be sent in cleartext  which allows anyone eavesdropping on the line between browser and web server will be able to read logins as they pass through  This type of wiretapping is done routinely by governments  but in general  we won t address  owned  wires other than to say this  Just use HTTPS  In essence  the only practical way to protect against wiretapping packet sniffing during login is by using HTTPS or another certificate-based encryption scheme  for example  TLS  or a proven  amp  tested challenge-response scheme  for example  the Diffie-Hellman-based SRP   Any other method can be easily circumvented by an eavesdropping attacker  Of course  if you are willing to get a little bit impractical  you could also employ some form of two-factor authentication scheme  e g  the Google Authenticator app  a physical  cold war style  codebook  or an RSA key generator dongle   If applied correctly  this could work even with an unsecured connection  but it s hard to imagine that a dev would be willing to implement two-factor auth but not SSL   Do not  Roll-your-own JavaScript encryption hashing Given the perceived  though now avoidable  cost and technical difficulty of setting up an SSL certificate on your website  some developers are tempted to roll their own in-browser hashing or encryption schemes in order to avoid passing cleartext logins over an unsecured wire  While this is a noble thought  it is essentially useless  and can be a security flaw  unless it is combined with one of the above - that is  either securing the line with strong encryption or using a tried-and-tested challenge-response mechanism  if you don t know what that is  just know that it is one of the most difficult to prove  most difficult to design  and most difficult to implement concepts in digital security   While it is true that hashing the password can be effective against password disclosure  it is vulnerable to replay attacks  Man-In-The-Middle attacks   hijackings  if an attacker can inject a few bytes into your unsecured HTML page before it reaches your browser  they can simply comment out the hashing in the JavaScript   or brute-force attacks  since you are handing the attacker both username  salt and hashed password   CAPTCHAS against humanity CAPTCHA is meant to thwart one specific category of attack  automated dictionary brute force trial-and-error with no human operator  There is no doubt that this is a real threat  however  there are ways of dealing with it seamlessly that don t require a CAPTCHA  specifically properly designed server-side login throttling schemes - we ll discuss those later  Know that CAPTCHA implementations are not created alike  they often aren t human-solvable  most of them are actually ineffective against bots  all of them are ineffective against cheap third-world labor  according to OWASP  the current sweatshop rate is  12 per 500 tests   and some implementations may be technically illegal in some countries  see OWASP Authentication Cheat Sheet   If you must use a CAPTCHA  use Google s reCAPTCHA  since it is OCR-hard by definition  since it uses already OCR-misclassified book scans  and tries very hard to be user-friendly  Personally  I tend to find CAPTCHAS annoying  and use them only as a last resort when a user has failed to log in a number of times and throttling delays are maxed out  This will happen rarely enough to be acceptable  and it strengthens the system as a whole  Storing Passwords   Verifying logins This may finally be common knowledge after all the highly-publicized hacks and user data leaks we ve seen in recent years  but it has to be said  Do not store passwords in cleartext in your database  User databases are routinely hacked  leaked or gleaned through SQL injection  and if you are storing raw  plaintext passwords  that is instant game over for your login security  So if you can t store the password  how do you check that the login password combination POSTed from the login form is correct  The answer is hashing using a key derivation function  Whenever a new user is created or a password is changed  you take the password and run it through a KDF  such as Argon2  bcrypt  scrypt or PBKDF2  turning the cleartext password   quot correcthorsebatterystaple quot   into a long  random-looking string  which is a lot safer to store in your database  To verify a login  you run the same hash function on the entered password  this time passing in the salt and compare the resulting hash string to the value stored in your database  Argon2  bcrypt and scrypt store the salt with the hash already  Check out this article on sec stackexchange for more detailed information  The reason a salt is used is that hashing in itself is not sufficient -- you ll want to add a so-called  salt  to protect the hash against rainbow tables  A salt effectively prevents two passwords that exactly match from being stored as the same hash value  preventing the whole database being scanned in one run if an attacker is executing a password guessing attack  A cryptographic hash should not be used for password storage because user-selected passwords are not strong enough  i e  do not usually contain enough entropy  and a password guessing attack could be completed in a relatively short time by an attacker with access to the hashes  This is why KDFs are used - these effectively  quot stretch the key quot   which means that every password guess an attacker makes causes multiple repetitions of the hash algorithm  for example 10 000 times  which causes the attacker to guess the password 10 000 times slower  Session data -  quot You are logged in as Spiderman69 quot  Once the server has verified the login and password against your user database and found a match  the system needs a way to remember that the browser has been authenticated  This fact should only ever be stored server side in the session data   If you are unfamiliar with session data  here s how it works  A single randomly-generated string is stored in an expiring cookie and used to reference a collection of data - the session data - which is stored on the server  If you are using an MVC framework  this is undoubtedly handled already   If at all possible  make sure the session cookie has the secure and HTTP Only flags set when sent to the browser  The HttpOnly flag provides some protection against the cookie being read through XSS attack  The secure flag ensures that the cookie is only sent back via HTTPS  and therefore protects against network sniffing attacks  The value of the cookie should not be predictable  Where a cookie referencing a non-existent session is presented  its value should be replaced immediately to prevent session fixation  Session state can also be maintained on the client side  This is achieved by using techniques like JWT  JSON Web Token   PART II  How To Remain Logged In - The Infamous  quot Remember Me quot  Checkbox Persistent Login Cookies   quot remember me quot  functionality  are a danger zone  on the one hand  they are entirely as safe as conventional logins when users understand how to handle them  and on the other hand  they are an enormous security risk in the hands of careless users  who may use them on public computers and forget to log out  and who may not know what browser cookies are or how to delete them  Personally  I like persistent logins for the websites I visit on a regular basis  but I know how to handle them safely  If you are positive that your users know the same  you can use persistent logins with a clean conscience  If not - well  then you may subscribe to the philosophy that users who are careless with their login credentials brought it upon themselves if they get hacked  It s not like we go to our user s houses and tear off all those facepalm-inducing Post-It notes with passwords they have lined up on the edge of their monitors  either  Of course  some systems can t afford to have any accounts hacked  for such systems  there is no way you can justify having persistent logins  If you DO decide to implement persistent login cookies  this is how you do it   First  take some time to read Paragon Initiative s article on the subject  You ll need to get a bunch of elements right  and the article does a great job of explaining each   And just to reiterate one of the most common pitfalls  DO NOT STORE THE PERSISTENT LOGIN COOKIE  TOKEN  IN YOUR DATABASE  ONLY A HASH OF IT  The login token is Password Equivalent  so if an attacker got their hands on your database  they could use the tokens to log in to any account  just as if they were cleartext login-password combinations  Therefore  use hashing  according to https   security stackexchange com a 63438 5002 a weak hash will do just fine for this purpose  when storing persistent login tokens    PART III  Using Secret Questions Don t implement  secret questions   The  secret questions  feature is a security anti-pattern  Read the paper from link number 4 from the MUST-READ list  You can ask Sarah Palin about that one  after her Yahoo  email account got hacked during a previous presidential campaign because the answer to her security question was     quot Wasilla High School quot   Even with user-specified questions  it is highly likely that most users will choose either   A  standard  secret question like mother s maiden name or favorite pet  A simple piece of trivia that anyone could lift from their blog  LinkedIn profile  or similar  Any question that is easier to answer than guessing their password  Which  for any decent password  is every question you can imagine   In conclusion  security questions are inherently insecure in virtually all their forms and variations  and should not be employed in an authentication scheme for any reason  The true reason why security questions even exist in the wild is that they conveniently save the cost of a few support calls from users who can t access their email to get to a reactivation code  This at the expense of security and Sarah Palin s reputation  Worth it  Probably not  PART IV  Forgotten Password Functionality I already mentioned why you should never use security questions for handling forgotten lost user passwords  it also goes without saying that you should never e-mail users their actual passwords  There are at least two more all-too-common pitfalls to avoid in this field   Don t reset a forgotten password to an autogenerated strong password - such passwords are notoriously hard to remember  which means the user must either change it or write it down - say  on a bright yellow Post-It on the edge of their monitor  Instead of setting a new password  just let users pick a new one right away - which is what they want to do anyway   An exception to this might be if the users are universally using a password manager to store manage passwords that would normally be impossible to remember without writing it down    Always hash the lost password code token in the database  AGAIN  this code is another example of a Password Equivalent  so it MUST be hashed in case an attacker got their hands on your database  When a lost password code is requested  send the plaintext code to the user s email address  then hash it  save the hash in your database -- and throw away the original  Just like a password or a persistent login token    A final note  always make sure your interface for entering the  lost password code  is at least as secure as your login form itself  or an attacker will simply use this to gain access instead  Making sure you generate very long  lost password codes   for example  16 case-sensitive alphanumeric characters  is a good start  but consider adding the same throttling scheme that you do for the login form itself  PART V  Checking Password Strength First  you ll want to read this small article for a reality check  The 500 most common passwords Okay  so maybe the list isn t the canonical list of most common passwords on any system anywhere ever  but it s a good indication of how poorly people will choose their passwords when there is no enforced policy in place  Plus  the list looks frighteningly close to home when you compare it to publicly available analyses of recently stolen passwords  So  With no minimum password strength requirements  2  of users use one of the top 20 most common passwords  Meaning  if an attacker gets just 20 attempts  1 in 50 accounts on your website will be crackable  Thwarting this requires calculating the entropy of a password and then applying a threshold   The National Institute of Standards and Technology  NIST  Special Publication 800-63 has a set of very good suggestions   That  when combined with a dictionary and keyboard layout analysis  for example   qwertyuiop  is a bad password   can reject 99  of all poorly selected passwords at a level of 18 bits of entropy   Simply calculating password strength and showing a visual strength meter to a user is good  but insufficient   Unless it is enforced  a lot of users will most likely ignore it  And for a refreshing take on user-friendliness of high-entropy passwords  Randall Munroe s Password Strength xkcd is highly recommended  Utilize Troy Hunt s Have I Been Pwned API to check users passwords against passwords compromised in public data breaches  PART VI  Much More - Or  Preventing Rapid-Fire Login Attempts First  have a look at the numbers  Password Recovery Speeds - How long will your password stand up If you don t have the time to look through the tables in that link  here s the list of them   It takes virtually no time to crack a weak password  even if you re cracking it with an abacus  It takes virtually no time to crack an alphanumeric 9-character password if it is case insensitive  It takes virtually no time to crack an intricate  symbols-and-letters-and-numbers  upper-and-lowercase password if it is less than 8 characters long  a desktop PC can search the entire keyspace up to 7 characters in a matter of days or even hours   It would  however  take an inordinate amount of time to crack even a 6-character password  if you were limited to one attempt per second    So what can we learn from these numbers  Well  lots  but we can focus on the most important part  the fact that preventing large numbers of rapid-fire successive login attempts  ie  the brute force attack  really isn t that difficult  But preventing it right isn t as easy as it seems  Generally speaking  you have three choices that are all effective against brute-force attacks  and dictionary attacks  but since you are already employing a strong passwords policy  they shouldn t be an issue    Present a CAPTCHA after N failed attempts  annoying as hell and often ineffective -- but I m repeating myself here   Locking accounts and requiring email verification after N failed attempts  this is a DoS attack waiting to happen   And finally  login throttling  that is  setting a time delay between attempts after N failed attempts  yes  DoS attacks are still possible  but at least they are far less likely and a lot more complicated to pull off     Best practice  1  A short time delay that increases with the number of failed attempts  like   1 failed attempt   no delay 2 failed attempts   2 sec delay 3 failed attempts   4 sec delay 4 failed attempts   8 sec delay 5 failed attempts   16 sec delay etc   DoS attacking this scheme would be very impractical  since the resulting lockout time is slightly larger than the sum of the previous lockout times   To clarify  The delay is not a delay before returning the response to the browser  It is more like a timeout or refractory period during which login attempts to a specific account or from a specific IP address will not be accepted or evaluated at all  That is  correct credentials will not return in a successful login  and incorrect credentials will not trigger a delay increase   Best practice  2  A medium length time delay that goes into effect after N failed attempts  like   1-4 failed attempts   no delay 5 failed attempts   15-30 min delay  DoS attacking this scheme would be quite impractical  but certainly doable  Also  it might be relevant to note that such a long delay can be very annoying for a legitimate user  Forgetful users will dislike you  Best practice  3  Combining the two approaches - either a fixed  short time delay that goes into effect after N failed attempts  like   1-4 failed attempts   no delay 5  failed attempts   20 sec delay  Or  an increasing delay with a fixed upper bound  like   1 failed attempt   5 sec delay 2 failed attempts   15 sec delay 3  failed attempts   45 sec delay  This final scheme was taken from the OWASP best-practices suggestions  link 1 from the MUST-READ list  and should be considered best practice  even if it is admittedly on the restrictive side   As a rule of thumb  however  I would say  the stronger your password policy is  the less you have to bug users with delays  If you require strong  case-sensitive alphanumerics   required numbers and symbols  9  character passwords  you could give the users 2-4 non-delayed password attempts before activating the throttling   DoS attacking this final login throttling scheme would be very impractical  And as a final touch  always allow persistent  cookie  logins  and or a CAPTCHA-verified login form  to pass through  so legitimate users won t even be delayed while the attack is in progress  That way  the very impractical DoS attack becomes an extremely impractical attack  Additionally  it makes sense to do more aggressive throttling on admin accounts  since those are the most attractive entry points PART VII  Distributed Brute Force Attacks Just as an aside  more advanced attackers will try to circumvent login throttling by  spreading their activities    Distributing the attempts on a botnet to prevent IP address flagging  Rather than picking one user and trying the 50 000 most common passwords  which they can t  because of our throttling   they will pick THE most common password and try it against 50 000 users instead  That way  not only do they get around maximum-attempts measures like CAPTCHAs and login throttling  their chance of success increases as well  since the number 1 most common password is far more likely than number 49 995  Spacing the login requests for each user account  say  30 seconds apart  to sneak under the radar   Here  the best practice would be logging the number of failed logins  system-wide  and using a running average of your site s bad-login frequency as the basis for an upper limit that you then impose on all users  Too abstract  Let me rephrase  Say your site has had an average of 120 bad logins per day over the past 3 months  Using that  running average   your system might set the global limit to 3 times that -- ie  360 failed attempts over a 24 hour period  Then  if the total number of failed attempts across all accounts exceeds that number within one day  or even better  monitor the rate of acceleration and trigger on a calculated threshold   it activates system-wide login throttling - meaning short delays for ALL users  still  with the exception of cookie logins and or backup CAPTCHA logins   I also posted a question with more details and a really good discussion of how to avoid tricky pitfals in fending off distributed brute force attacks PART VIII  Two-Factor Authentication and Authentication Providers Credentials can be compromised  whether by exploits  passwords being written down and lost  laptops with keys being stolen  or users entering logins into phishing sites   Logins can be further protected with two-factor authentication  which uses out-of-band factors such as single-use codes received from a phone call  SMS message  app  or dongle  Several providers offer two-factor authentication services  Authentication can be completely delegated to a single-sign-on service  where another provider handles collecting credentials  This pushes the problem to a trusted third party  Google and Twitter both provide standards-based SSO services  while Facebook provides a similar proprietary solution  MUST-READ LINKS About Web Authentication  OWASP Guide To Authentication   OWASP Authentication Cheat Sheet Dos and Don   ts of Client Authentication on the Web  very readable MIT research paper  Wikipedia  HTTP cookie Personal knowledge questions for fallback authentication  Security questions in the era of Facebook  very readable Berkeley research paper

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