If I get a JWT and I can decode the payload, how is that secure? Couldn't I just grab the token out of the header, decode and change the user information in the payload, and send it back with the same correct encoded secret?
I know they must be secure, but I just would really like to understand the technologies. What am I missing?
JWTs can be either signed, encrypted or both. If a token is signed, but not encrypted, everyone can read its contents, but when you don't know the private key, you can't change it. Otherwise, the receiver will notice that the signature won't match anymore.
Answer to your comment: I'm not sure if I understand your comment the right way. Just to be sure: do you know and understand digital signatures? I'll just briefly explain one variant (HMAC, which is symmetrical, but there are many others).
Let's assume Alice wants to send a JWT to Bob. They both know some shared secret. Mallory doesn't know that secret, but wants to interfere and change the JWT. To prevent that, Alice calculates Hash(payload + secret) and appends this as signature.
When receiving the message, Bob can also calculate Hash(payload + secret) to check whether the signature matches.
If however, Mallory changes something in the content, she isn't able to calculate the matching signature (which would be Hash(newContent + secret)). She doesn't know the secret and has no way of finding it out.
This means if she changes something, the signature won't match anymore, and Bob will simply not accept the JWT anymore.
Let's suppose, I send another person the message {"id":1} and sign it with Hash(content + secret). (+ is just concatenation here). I use the SHA256 Hash function, and the signature I get is: 330e7b0775561c6e95797d4dd306a150046e239986f0a1373230fda0235bda8c. Now it's your turn: play the role of Mallory and try to sign the message {"id":2}. You can't because you don't know which secret I used. If I suppose that the recipient knows the secret, he CAN calculate the signature of any message and check if it's correct.
You can go to jwt.io, paste your token and read the contents. This is jarring for a lot of people initially.
The short answer is that JWT doesn't concern itself with encryption. It cares about validation. That is to say, it can always get the answer for "Have the contents of this token been manipulated"? This means user manipulation of the JWT token is futile because the server will know and disregard the token. The server adds a signature based on the payload when issuing a token to the client. Later on it verifies the payload and matching signature.
The logical question is what is the motivation for not concerning itself with encrypted contents?
The simplest reason is because it assumes this is a solved problem for the most part. If dealing with a client like the web browser for example, you can store the JWT tokens in a cookie that is secure (is not transmitted via HTTP, only via HTTPS) and httpOnly (can't be read by Javascript) and talks to the server over an encrypted channel (HTTPS). Once you know you have a secure channel between the server and client you can securely exchange JWT or whatever else you want.
This keeps thing simple. A simple implementation makes adoption easier but it also lets each layer do what it does best (let HTTPS handle encryption).
JWT isn't meant to store sensitive data. Once the server receives the JWT token and validates it, it is free to lookup the user ID in its own database for additional information for that user (like permissions, postal address, etc). This keeps JWT small in size and avoids inadvertent information leakage because everyone knows not to keep sensitive data in JWT.
It's not too different from how cookies themselves work. Cookies often contain unencrypted payloads. If you are using HTTPS then everything is good. If you aren't then it's advisable to encrypt sensitive cookies themselves. Not doing so will mean that a man-in-the-middle attack is possible--a proxy server or ISP reads the cookies and then replays them later on pretending to be you. For similar reasons, JWT should always be exchanged over a secure layer like HTTPS.
Let's discuss from the very beginning:
JWT is a very modern, simple and secure approach which extends for Json Web Tokens. Json Web Tokens are a stateless solution for authentication. So there is no need to store any session state on the server, which of course is perfect for restful APIs.
Restful APIs should always be stateless, and the most widely used alternative to authentication with JWTs is to just store the user's log-in state on the server using sessions. But then of course does not follow the principle that says that restful APIs should be stateless and that's why solutions like JWT became popular and effective.
So now let's know how authentication actually works with Json Web Tokens. Assuming we already have a registered user in our database. So the user's client starts by making a post request with the username and the password, the application then checks if the user exists and if the password is correct, then the application will generate a unique Json Web Token for only that user.
The token is created using a secret string that is stored on a server. Next, the server then sends that JWT back to the client which will store it either in a cookie or in local storage.
Just like this, the user is authenticated and basically logged into our application without leaving any state on the server.
So the server does in fact not know which user is actually logged in, but of course, the user knows that he's logged in because he has a valid Json Web Token which is a bit like a passport to access protected parts of the application.
So again, just to make sure you got the idea. A user is logged in as soon as he gets back his unique valid Json Web Token which is not saved anywhere on the server. And so this process is therefore completely stateless.
Then, each time a user wants to access a protected route like his user profile data, for example. He sends his Json Web Token along with a request, so it's a bit like showing his passport to get access to that route.
Once the request hits the server, our app will then verify if the Json Web Token is actually valid and if the user is really who he says he is, well then the requested data will be sent to the client and if not, then there will be an error telling the user that he's not allowed to access that resource.
All this communication must happen over https, so secure encrypted Http in order to prevent that anyone can get access to passwords or Json Web Tokens. Only then we have a really secure system.
So a Json Web Token looks like left part of this screenshot which was taken from the JWT debugger at jwt.io. So essentially, it's an encoding string made up of three parts. The header, the payload and the signature Now the header is just some metadata about the token itself and the payload is the data that we can encode into the token, any data really that we want. So the more data we want to encode here the bigger the JWT. Anyway, these two parts are just plain text that will get encoded, but not encrypted.
So anyone will be able to decode them and to read them, we cannot store any sensitive data in here. But that's not a problem at all because in the third part, so in the signature, is where things really get interesting. The signature is created using the header, the payload, and the secret that is saved on the server.
And this whole process is then called signing the Json Web Token. The signing algorithm takes the header, the payload, and the secret to create a unique signature. So only this data plus the secret can create this signature, all right?
Then together with the header and the payload, these signature forms the JWT,
which then gets sent to the client.
Once the server receives a JWT to grant access to a protected route, it needs to verify it in order to determine if the user really is who he claims to be. In other words, it will verify if no one changed the header and the payload data of the token. So again, this verification step will check if no third party actually altered either the header or the payload of the Json Web Token.
So, how does this verification actually work? Well, it is actually quite straightforward. Once the JWT is received, the verification will take its header and payload, and together with the secret that is still saved on the server, basically create a test signature.
But the original signature that was generated when the JWT was first created is still in the token, right? And that's the key to this verification. Because now all we have to do is to compare the test signature with the original signature.
And if the test signature is the same as the original signature, then it means that the payload and the header have not been modified.
Because if they had been modified, then the test signature would have to be different. Therefore in this case where there has been no alteration of the data, we can then authenticate the user. And of course, if the two signatures
are actually different, well, then it means that someone tampered with the data.
Usually by trying to change the payload. But that third party manipulating the payload does of course not have access to the secret, so they cannot sign the JWT.
So the original signature will never correspond to the manipulated data.
And therefore, the verification will always fail in this case. And that's the key to making this whole system work. It's the magic that makes JWT so simple,
but also extremely powerful.
The contents in a json web token (JWT) are not inherently secure, but there is a built-in feature for verifying token authenticity. A JWT is three hashes separated by periods. The third is the signature. In a public/private key system, the issuer signs the token signature with a private key which can only be verified by its corresponding public key.
It is important to understand the distinction between issuer and verifier. The recipient of the token is responsible for verifying it.
There are two critical steps in using JWT securely in a web application: 1) send them over an encrypted channel, and 2) verify the signature immediately upon receiving it. The asymmetric nature of public key cryptography makes JWT signature verification possible. A public key verifies a JWT was signed by its matching private key. No other combination of keys can do this verification, thus preventing impersonation attempts. Follow these two steps and we can guarantee with mathematical certainty the authenticity of a JWT.
More reading: How does a public key verify a signature?
I would explain this with an example.
Say I borrowed $10 from you, then I gave you an IOU with my signature on it. I will pay you back whenever you or someone else bring this IOU back to me, I will check the signature to make sure that is mine.
I can't make sure you don't show the content of this IOU to anyone or even give it to a third person, all I care is that this IOU is signed by me, when someone shows this IOU to me and ask me to pay it.
The way how JWT works is quite the same, the server can only make sure that the token received was issued by itself.
You need other measures to make it secure, like encryption in transfer with HTTPS, making sure that the local storage storing the token is secured, setting up origins.
Ref - JWT Structure and Security
It is important to note that JWT are used for authorization and not authentication.
So a JWT will be created for you only after you have been authenticated by the server by may be specifying the credentials. Once JWT has been created for all future interactions with server JWT can be used. So JWT tells that server that this user has been authenticated, let him access the particular resource if he has the role.
Information in the payload of the JWT is visible to everyone. There can be a "Man in the Middle" attack and the contents of the JWT can be changed. So we should not pass any sensitive information like passwords in the payload. We can encrypt the payload data if we want to make it more secure. If Payload is tampered with server will recognize it.
So suppose a user has been authenticated and provided with a JWT. Generated JWT has a claim specifying role of Admin. Also the Signature is generated with
This JWT is now tampered with and suppose the
role is changed to Super Admin
Then when the server receives this token it will again generate the signature using the secret key(which only the server has) and the payload. It will not match the signature
in the JWT. So the server will know that the JWT has been tampered with.
Only JWT's privateKey, which is on your server will decrypt the encrypted JWT. Those who know the privateKey will be able to decrypt the encrypted JWT.
Hide the privateKey in a secure location in your server and never tell anyone the privateKey.
I am not a cryptography specialist and hence (I hope) my answer can help somebody who is neither.
There are two possible ways of using cryptography in programming:
Signing / verifying
Encryption / decryption
We use Signing when we want to ensure that data comes from a trusted source.
We use Encryption when we want to protect the data.
Signing / verifying uses asymmetrical algorithms i.e. we sign with one key (private) and the data receiver uses the other (public) key to verify.
A symmetric algorithm uses the same key to encrypt and decrypt data.
The encryption can be done using both symmetric and asymmetric algorithms.
relatively simple article on subject
The above is common knowledge below is my opinion.
When JWT is used for simple client-to-server identification there is no need for signing or asymmetric encryption. JWT can be encrypted with AES which is fast and supersecure. If the server can decrypt it, it means the server is the one who encrypted it.
Summary: non-encrypted JWT is not secure. Symmetric encryption can be used instead of signing in case no third party is involved.
I have one site - account.mysite.com which only register/login users in my system.
I want to check user's email and passwords in one site and send something to another site, which definitely determines who login.
Can I use FormsAuthentication?
FormsAuthenticationTicket ticket = new FormsAuthenticationTicket(name, true, 60);
string encrypted = FormsAuthentication.Encrypt(ticket);
return "mysite.com?" + FormsAuthentication.FormsCookieName + "=" + encrypted;
Or do I need to use some features from aspNet.Identity? If yes, how I can do this?
Thanks.
If you're using MVC 5, then you should also use Identity. However, whether you use ASP.NET Membership (forms auth) or Identity has no bearing on any of this.
Simply, in either scheme, an auth cookie will be set. This is what determines a user's "logged in" status. With each request, the user's browser will send the cookie back to the server, and the server will validate it and either consider the user authenticated or not as a result.
So, this all boils down to making sure that auth cookie is shared between your different sites and can be read and validate by each of your sites.
On the first part, cookies are restricted by domain. If the cookie is set on account.mysite.com then it will only be sent to account.mysite.com. However, you can set it as a wildcard, so that it will be sent to anything on *.mysite.com. That then would allow account.mysite.com to set the cookie and still have it be sent to something like subdomain.mysite.com. If you have completely different domains in play though, like account.mysite.com and you want that to log you in at foo.com, that's not possible. The only way to handle that situation is set up a SSO implementation which is non-trivial and far beyond the scope of what can be reasonably answered here. There's entire companies devoted only to setting up SSO for organizations.
So, assuming you're not dealing with different domains and you can at least get each site to get the same cookie, then next part is unencrypting that cookie. The auth cookie is encrypted before it is set, so it can only be read by a site that knows how to decrypt it. The encryption is based on the machine key, so essentially, all your sites need to share the same machine key. For more information, see: https://technet.microsoft.com/en-us/library/cc755177(v=ws.10).aspx
I read the documentation of Nancy Forms Authentication. As far as I can tell, the approach recommended there leads to lifelong session IDs:
"The identifier is the token that will be put in the authentication
cookie which will be used to re-establish the identity of the user
that is performing the request, so that you do not need to enter your
credentials for each request."
As far as I understand, that "identifier" is what most people call a session ID.
It is also important to know that the identifier should be treated as
permanent for the user that it was generated for and will be reused
across requests and application sessions.
Is this really the recommended approach? If I understand correctly, this means that session IDs never change and never expire. So the session ID is the equivalent of a password, which
is retransmitted in a cookie with every request
is probably stored in clear-text in the DB, if you follow the docs to the end
I know that I could implement this differently with Nancy, but my point is that such an approach should not be explained in the docs as reference.
So if an attacker ever succeeds in stealing that session ID, e.g. by an XSS attack, he gains lifelong access to the system.
Please correct me and show me the mistake in my thoughts!
The identifier you are referring to isn't a session id, it's an unpredictable user identifier, which is then mapped (if necessary) to the real user identifier in the back end. This is so if someone logs in as user X, and somehow manages to decrypt, re-encrypt and re-sign the cookie they can't just change the user ID to "admin" or something similar and gain admin access (which is how the ASP.Net Oracle attack worked). It's also HttpOnly, so not really capturable via XSS, although technically it could be captured using XST.
Creating and expiring a session (and deleting the auth cookie if necessary) is a different task altogether - how and when you determine if an auth cookie should be accepted, removed, or confirmed with an additional password request is application specific. This is a common pattern now where a site will consider you "logged in" eternally, until you do something "secure", in which case it will ask you to revalidate if you haven't done so recently.
Hope that makes sense.
that "identifier" is what most people call a session ID.
It's not. It's something like UserId. as the documentation states:
We have chosen to use a GUID as the identifier. The reason for this is that using something like the username of id is a potential vulnerability, because, if the cookie encryption was somehow compromised, it would be easy to spoof the identity of another user by guessing their username or id.
They're just using a GUID assigned to the user for more security. Of course, (cookie based) FormsAuthentication has all the disadvantages of cookies. If someone can get access to your auth cookie, they can authenticate themselves as you. But session cookies and forms authentication cookies are completely different things, This answer states the differences pretty clearly.
First off, I am not using Forms Authentication.
I found a great tutorial that almost does what I want:
http://www.codeproject.com/Questions/358434/Keep-me-signed-in-until-Loggged-out
The only problem is that it does not seem like a good idea. It stores the username in the cookie. That seems very bad.
How could I do something like this tutorial but in a safe way?
I essentially just want this basic flow:
if user logged in then show page
User can have the option of being logged in for the session (30 mins of inactivity) or until they choose to explicitly logout.
I have a feeling I will need a session table in my db for this, but I am not sure.
It doesn't have to be top of the line security since this is for an intranet, but I do still want it to be somewhat safe.
Thanks
Easiest way is in your Login function, once you have verified them just add:
FormsAuthentication.SetAuthCookie(theUserName, persistCookieBoolean);
Now you have an authentication cookie set. No encrypting or decrypting needed. Get the username like:
HttpContext.Current.User.Identity.Name
And see if they are logged in:
HttpContext.Current.User.Identity.IsAuthenticated
And now you can set authorization easily in the web.config too. Related post: Manual Access control in ASP .Net
Encrypt your data (username, etc) before storing in the cookie.
Read the cookie, decrypt the data & then validate.
Check this:
Encrypting & Decrypting Data in .NET Applications
Encrypt/Decrypt string in .NET
You can use Sessions.
Example Code VB:
On login do:
Session("Username") = "username"
Then each time on page load check if Session("Username") has any data/is not null.
If it's null or empty then the Session has expired and you can kick them out of the page.
How can I remember a user that is logged in without having a security issue? Currently, I am storing the id and a guid into two different cookies and compare them when there is no session alive. If it match then I re-create the session. Btw, both id and guid are nore encrypted.
Is this method safe enough, or should I follow a rather distinct way?
Since cookies can be easily stolen by use of XSS, does not matter if you place information in just one cookie or two, because a theft will take them all.
I have solved with a more complex procedure: I place one encrypted cookie divided in 2 parts (but 2 cookies will work as well), one fixed and the other variable each session. Both are saved also on server side to check: they are only session identifiers, no sensible information contained in the cookie, the correspondence with the user id is saved on the server.
If a fake user enters the account with a stolen cookie, the variable part (or cookie) will change, so when real user connects again he will not be able to enter and you will have the proof that an unauthorized access occurred: then you can delete the permanent session on server side (defined by the fixed part or cookie) in order to avoid any further access by the theft. The real user will re-login and create a new permanent session with a new cookie. You can also warn him that you saw a security flaw, suggesting him to reset password (operation that should never be accessible directly from cookie) or to use another browser for unsafe navigation.
If you save additional user information (user-agent, IP by location) you can check cookie validity also on them in order to avoid even the first entrance of the fake user.
I would recommend you using forms authentication for tracking logged in users in an ASP.NET application.