Lets say a user is logging into a typical site, entering their username and password, and they mistype one of their inputs. I have noticed that most, if not all, sites show the same message something along the lines of, "Invalid username or password" despite only one input being wrong. To me, it seems easy enough to notify the user of which input was wrong and this got me wondering why sites do not do it. So, is there a security reason for this or is it just something that has become the norm? asked Jul 30, 2012 at 840 20 If a malicious user starts attacking a website by guessing common username/password combinations like admin/admin, the attacker would know that the username is valid is it returns a message of "Password invalid" instead of "Username or password invalid". If an attacker knows the username is valid, he could concentrate his efforts on that particular account using techniques like SQL injections or bruteforcing the password. answered Jul 30, 2012 at 844 13 As others have mentioned, we don't want you to know whether or not it was the username or password that was wrong so that we are not as susceptible to brute-force or dictionary attacks.. If some websites wanted to let their users know which one failed while still being in the green security-wise, they could implement "honeypot" usernames such as Administrator, admin, etc. that would alert website admins that someone is snooping around their website. You could even setup some logic to ban their IP address if they were to attempt to login with one of those "honeypot" usernames. I know one person who actually had a website and put in their source code an HTML comment such as "Since you keep forgetting Richard Username cheese Password Burger123" near the login box with the intent to monitor any IP address that attempted to use that username/password. Adding monitoring logic like that is a lot of fun when you're developing a website. Of course, logging invalid login attempts and adding appropriate logic to deal with those IP addresses works too. I know some would disagree with me, but depending on the type of website, I don't think it is too big of a deal to let the user know as long as you add additional security measures in preventing different kinds of attacks. answered Jul 30, 2012 at 1312 GaffGaff3513 silver badges5 bronze badges 4 My favorite secure implementation of this is done by a bank I use. If I type in my username correctly, it will say "Welcome Jimbob!" and then prompts me to answer security questions if I have never logged in from this browser on this computer, wait for me to answer the security questions correctly, and then will let me see my security image/caption and input my password. If I type in the wrong username, I will see something like "Welcome Bessie/Kareem/Randal!" where the displayed name is very uncommon — though you will always be the same name for a same username I'm usually not sure between one or two usernames; and the wrong one consistently calls me Frenshelia. I assume its implemented as some sort of non-cryptographic hash applied to any inputted username that uniquely map to one username on a long list of fairly uncommon names. This lets legitimate users know if they typed in the wrong username as even if you have an uncommon name like Bessie; its very unlikely that the wrong username you randomly guessed maps back to your specific uncommon name, without making it obvious to people trying to find random accounts that the username doesn't exist. As an aside I'm not particularly fond of the security questions/security image part, which seems to border on security theater. A sophisticated attacker doing a man-in-the-middle MITM attack after installing fake certificates in your web-browser; and DNS/ARP spoofing to point to their IP address could wait until you try logging into the site, then have an automated script sign in on their computer to the real site, get the security questions, display the chosen security questions back to you, send back the answers to the site themselves from their browser, wait to get the security image, serve back the security image to you, and then wait for you to input the password from their end at which point they use the password to log in as you and do malicious things. Granted the questions+image makes the process more difficult than having all the time in the world to collect all the security images for a variety of attacked usernames by turning it into an attack that must be done in real-time and possibly leaves a suspicious signature. answered Jul 30, 2012 at 1948 dr jimbobdr gold badges93 silver badges163 bronze badges 2 Other answers provide good insight on security reasons behind this behavior. Although they are correct, I'm pretty sure that at least some websites just have the authorization routine programmed the way it's impossible to tell what was wrong - login or password. Sample query SELECT COUNT* FROM users WHERE login = 'john' AND hash = '2bbfcdf3f09ae8d700402f36913515cd' This will return 1 on successful logging attempt and 0 if there is no user with such name or this user has different password. There is no way to tell which part of the condition failed. So when it comes to displaying error message programmer just honestly tells you that something is wrong and he isn't really sure what exactly it is. I personally saw similar queries in few PHP-based websites so I'm pretty sure that part of the reason comes from the way the authentication process is coded, really. Rory Alsop♦ gold badges117 silver badges321 bronze badges answered Jul 30, 2012 at 1954 DypplDyppl2312 silver badges4 bronze badges 8 The security reason behind it is otherwise it becomes a lot easier to find valid usernames. answered Jul 30, 2012 at 842 Lucas KauffmanLucas gold badges115 silver badges196 bronze badges In addition to all great answers already given, there's a generic security principle which says you shouldn't provide unsolicited information to unauthorized users. if you have a choice to answer either "your authentication data is not valid" or explaining which part is not valid - you should always choose the former. Some very nasty cryptographic attacks are based on the tiniest amount of error information provided by implementation trying to be "helpful" - see Padding oracle attack. So it is a good idea to always opt for the littlest possible bit of information disclosed to the unauthorized entity - if his username/password combo is not good, you always answer "username/password not good", and never disclose any more data. Even if in a specific case like gmail where username is public it's not important, it's a good practice to adopt by default. answered Jul 30, 2012 at 2341 StasMStasM1,8812 gold badges15 silver badges23 bronze badges Let's say you enter a random username and an equally random passwordjust note what password you enter . Now the passwords can be common among the n users. So, if the website says the password is correct... then you know what follows next.. mayhem among the genuine users as getting login names are quite easy. answered Jul 30, 2012 at 1420 Providing ambiguous answer is useful to prevent user enumeration attack. In some cases attacker doesn't need to compromise user account. Information that user has account is sufficient without any other action. For example it's valuable information for commerce that their customer has account on competitive web shop. answered Jul 30, 2012 at 1902 I appreciate various answers above as they say the most but sometimes Applications are also unaware what is wrong UserName or Password. In case of a token based authentication specially to implement SSO Single Sign On IBM Tivoli Access Manager your application either receives a successful token or gets an error back. answered Jul 30, 2012 at 2331 if the login is an email address, it's easy to find out that a person is registered at a website - I might not want that >> Sometimes people use email account real password for websites they register when they use email id as login id answered Jul 30, 2012 at 1922 You must log in to answer this question. Not the answer you're looking for? Browse other questions tagged .